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[Sticky] Radio Receiver Intermediate Frequencies

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Sundog
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Thank you Steve. You have worked tirelessly to present your findings. I'm sure many of the (rather few) members are very grateful for your efforts. Certainly I am.

More generally, this thread and some others that Steve has posted are a unique set of carefully curated and compiled information. I feel that these data (a pretentious attempt to drag in some Latin grammar) should hopefully outlive this forum and many of us.

As an archivist I feel the need to preserve these in a better way than just the archive.com's Wayback Machine which, while wonderful, could have the rug pulled from under it at any time by copyright owners.

Unfortunately the archive for which I work does not collect or preserve the printed word, nor do I think the British Library would be interested - though I'd be very pleased to be proved wrong.

I'd welcome any ideas how we may preserve this information for our descendants who might show interest.

 

 
Posted : 16/08/2022 9:07 pm
Cathovisor
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A good place to preserve it would be on Radiomuseum.org as I know plans are in place to preserve it long-term. 

 
Posted : 17/08/2022 12:22 pm
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Synchrodyne
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Sundog, thanks vey much for the kind words.
 
On the archiving issue, I am not sure that I can add anything material.  A thought that did come to mind in a case like this is would one archive the whole forum, or just selected threads, or just selected information.
 
The information in the intermediate frequency threads certainly could be condensed just to that which has been verified or is supported by strong circumstantial evidence and/or plausible “reverse engineering”.  But then the threads themselves, like many others, also represent a journey of discovery, not of anything new, but of recent history that is not so easy to find, at least from the viewpoint of an “armchair researcher”.  And perhaps the journey of discovery is as much a representation of what this kind of forum is about as is the “hard” information contained in the threads.
 
Given that internet forums have been extant for around 25 years, I wonder if much thought has been given to the archiving thereof at a professional level?
 
I must admit that I had not expected that the intermediate frequency threads would run for ten years.  In the TV case, I thought that I would gather up the missing items in the list of standard IFs, and that would be that.  The radio case was more of an offshoot, initially discussing the standard numbers such as 455 kHz family and 10.7 MHz, and then quasi-standard frequencies such as 100 kHz and 1.4 MHz.  There are still some information gaps around those.  But beyond that, items such as the diversity of IF approaches for HF receivers called for at least selective treatment.  These days I have a “now-and-again” approach to writing new postings, perhaps a couple or three times a year.  For both the TV and radio IF cases, and also for the intercarrier sound thread, I still have a list of items for future postings, including some recently found double-conversion IF numbers for TV.
 
 
Cheers,
 
Steve
 
Posted : 21/08/2022 11:39 pm
Synchrodyne
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Not yet covered in any detail are the intermediate frequencies used in “converter” type HF receivers.  Here the topology was a big factor in determining the IF numbers.
 
Broadly speaking, a converter type receiver divided the whole or part(s) of the HF spectrum into manageable (from a tuning precision viewpoint) bands of a given size, often, but not necessarily 1 MHz in width.
 
Each band was subject to the customary RF tuning and amplification to provide the desired front end selectivity performance.  The first conversion for each band was done with a crystal-controlled oscillator input to the mixer, so that the whole band was transposed to another designated band of the same width, this being the 1st IF band.  This was typically placed somewhere in the lower HF/upper MF region, although usually above the MW broadcast band.
 
If the part of the HF range that was occupied by the 1st IF band was part of the receiver’s tuning range, then that was exempt from the 1st conversion, and was handled directly by the second part of the receiver.
 
The part of the HF range below the 1st IF band, and also any parts of the MF band if covered, were first upconverted, either to the 1st IF band, or to a higher band in the HF range, in that case with subsequent down conversion.
 
The second part of the receiver was essentially a single-conversion superhet that tuned across the 1st IF band using a VFO.  In general, the 2nd IF could be any suitable value, including but not limited to standard numbers such as 455 kHz or 100 kHz.  But there were situations where it was determined by the topology.
 
This kind of receiver clearly required very good isolation between its various sections.  Also, on the face of it, it could require a large number of crystals for the 1st oscillator, for example 30, if there were 30 1 MHz tuning bands to cover the whole HF range.
 
The crystal count could be reduced if the 1st oscillator used integer MHz frequencies, as then it would have been possible to use harmonics for some of the bands.  This implied the use of 1 MHz wide bands, or slightly more than 1 MHz allowing for edge overlaps.  If the use of two 1st IF bands was accepted, then the crystal count could be further reduced as a given 1st oscillator frequency could in general be used for two adjacent bands.
 
With two 1st IF bands, each could have its own VFO range to convert down to the desired 2nd IF.  But if the two 1st IF bands were adjacent, with their nominal centres 1 MHz apart, then a single VFO could be used, with supradyne conversion on the lower IF band and infradyne conversion on the upper, this giving a 2nd IF of 500 kHz.  This is how the 500 kHz IF came about.  Otherwise, 500 kHz, an international marine distress frequency from the early days, is one that was studiously avoided by a reasonable margin for IF use.
 
As far as I know, this technique, including the use of two adjacent 1st IF bands and the 500 kHz 2nd IF, was pioneered by Collins, with its 51J HF receiver of c.1949.
 
Accepting the possibility of some mechanical complexity, this kind of receiver lent itself to tracked RF tuning by ganging the VFO and the RF tuning devices.
 
Overall, this type of receiver provided crystal oscillator stability first conversion across the whole of the tuning range, and coupled this with tuning ranges of manageable width in terms of frequency readout and resetting with conventional slide-rule scales.
 
An example of a converter-type HF receiver from late in the valve era was the Eddystone 880 series, dating from 1959.
 
This covered the range 0.5 to 30.5 MHz in 30 bands, each nominally 1 MHz wide (plus 100 kHz edge overlaps), and each centred on a MHz integer from 1 to 30 MHz.
 
There were two 1st IF bands, again each nominally 1 MHz wide (plus edge overlaps), respectively centred on the integer values 3 and 4 MHz.
 
The VFO tuned from 3 to 4 MHz (plus edge overlaps).  Thus by supradyne conversion for the 3 MHz 1st IF band (2.5 to 3.5 MHz) and infradyne conversion for the 4 MHz 1st IF band (3.5 to 4.5 MHz), the 500 kHz 2nd IF was generated.
 
Except for the 4 MHz band, the even MHz tuning bands, centred on 2, 6, 8 MHz, etc., were converted to the 4 MHz 1st IF band.  The 4 MHz tuning band corresponded with the 4 MHz 1st IF band, so was not converted.
 
Except for the 1 and 3 MHz bands, the odd MHz tuning bands, centred on 5, 7, 9 MHz, etc., were converted to the 3 MHz 1st IF band.  The 3 MHz tuning band corresponded with the 3 MHz 1st IF band, so was not converted.
 
The 1 MHz band was converted to the 4 MHz 1st IF band, using 5 MHz injection to the 1st mixer.  Here one may deduce that it was preferable to use an oscillator frequency that was outside of the two 1st IF bands; if so, the 4 MHz was outruled.
 
The first oscillator required 10 crystals to achieve this.
 
Details of the tuning bands and 1st IF bands are shown in this table:
 
Eddystone 880 Tuning Ranges and IFs
 
 
And this table shows how the 10 oscillator crystals were used:
 
Eddystone 880 Crystal Frequencies and Deployment
 
 
Looking at the premise that the topology determined the IFs, it is reasonable to assume that two adjacent 1st IF ranges would need to be in the 1.5 to 5 MHz region, above the MW broadcast band but below that part of the HF spectrum where relatively high powered transmitters were found.  Given that simplicity in terms of crystal count pointed to integer-centred 1 MHz bands, the available bands were centred on 2, 3 and 4 MHz.  Collins did in fact use the 2 MHz (1.5 to 2.5 MHz) band on some receivers, but perhaps Eddystone thought it was better to be further away from the MW band.  If so, that determined the use of the 2 and 3 MHz bands.  And as already noted, the preference for using a single 2nd conversion VFO determined the 500 kHz 2nd IF.
 
The Eddystone 880 reputedly had an extremely low level of oscillator radiation.  This may have been an independent design objective, given that one end use was in Embassy buildings, but I imagine a large part of the required performance along that vector was inherent from the level of intercompartment screening required.  For example, the 1st oscillator section was in a double-screened box within the main box.
 
 
Cheers,
 
Steve
 
Posted : 02/09/2022 11:48 pm
Synchrodyne
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Marconi also used a modified version of the Eddystone 880 from 1964, this being known as the H2301.  It would appear to have been developed to provide an economic (relative to full-scale point-to-point equipment) single-box SSB receiver.  It probably sat under the HR120, also “single-box”, in the Marconi range.  The latter dated from 1960, and was also of the converter type.
 
Here the tuning range was 2.1 to 30 MHz, in 1 MHz bands, selected by the MHz x 10 and MHz x 1 rotary switched controls.  The first IF band was 1 to 2 MHz, so below the reception range of interest.  Thus all reception frequencies were subject to double conversion.
 
The second IF was 100 kHz, this being the norm at the time for the final processing of point-to-point signals, particularly of the SSB type.  Thus the second conversion oscillator operated over the range 1.1 to 2.1 MHz.  It was continuously variable, controlled by the kHz x 100 and kHz x 1 controls.  Specifying the lower limit of the coverage range as 2.1 MHz rather than the expected 2 MHz would have been done to remove any overlap with the second oscillator range.
 
The crystal controlled first oscillator was switched to operate in 1 MHz increments, but further details do not seem to be available.  Each 1 MHz tuning range would appear to have been centred on a half-MHz point, e.g. 29.5 MHz for the 29 to 30 MHz.  But it is conceivable that integer 1st oscillator frequencies were used.  For example, a 31 MHz oscillator frequency would convert the 29 to 30 MHz range to 2 to 1 MHz.  Or 28 MHz would convert it to 1 to 2 MHz.  At the bottom end, the 2 to 3 MHz range would require either 1 MHz or 3 MHz.  The supradyne option would have kept the 1st oscillator clear of the 1st IF band, so looks to have been preferable.  Given that there was only one 1st IF band, then each 1 MHz band required its own oscillator frequency.  A 1st oscillator range stepped from 3 to 31 MHz would have allowed some use of harmonics to minimize the crystal count, but that is speculation on my part.
 
RF tuning was separate, and appears to have been done in four ranges, although with the range switching ganged with the MHz switching.
 
Here, one could say that the putative need for crystal 1st oscillator stability on all HF ranges (bearing in mind that the Eddystone 880/Marconi H2301 did not have crystal stability across the 2.5 to 4.5 MHz section) determined that the 1st IF be placed below the desired 2 to 30 MHz tuning range.  (The alternative of upconversion simply would have recreated the initial problem of converting down from HF (or so) with crystal stability.)  The resultant 1 to 2 MHz IF band was probably not ideal in that it overlapped the MW broadcast band, but that was no doubt addressed by a high level of screening.  Anyway, anything that went much higher than 2 MHz might have created difficulties with the VFO down conversion to 100 kHz, perhaps pointing to the need for an intermediate IF.  Some SSB equipment did the 3.1 MHz to 100 kHz conversion directly, but this was with crystal controlled oscillators, not VFOs, and usually with balanced mixers, as well.)
 
 
Cheers,
 
Steve
 
Posted : 03/09/2022 1:53 am
Synchrodyne
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The Rode & Schwarz (R&S) EK-070 professional HF receiver dated from c.1979.  It was upconversion, with a 1st IF of 81.4 MHz and a second IF of 1.4 MHz.
 
In respect of the second IF, it was said:
 
“The IF choice of 1.4 MHz was made because of filter availability.  At this frequency, filters with excellent response characteristics and good stability over a wide temperature range may be obtained at relatively low cost.”
 
The 81.4 MHz first IF was explained as follows:
 
“As a rule of thumb, this first IF should be selected to be at least 2.1 times greater than the highest operating frequency.  However, if too high a frequency is selected, it becomes difficult to obtain a stable filter with low loss.  The 81.4-MHz filter used in the EK-070 was selected to provide a good compromise.”
 
The “2.1 x” rule implies a minimum 1st IF of 63 MHz for HF receivers.  R&S did not justify it further, but one possibility is that it kept third order mixing products out of the HF band.
 
By the late 1970s, synthesizer design probably easily accommodated a 1st IF above 63 MHz.  In the earlier days of upconversion though, synthesizer limitations evidently dictated lower 1st IF numbers.  Here is a summary of some previously discussed examples:
 
1967 Plessey PR155 37.3 MHz
1968 Marconi Hydrus 40.0 MHz
1969 Redifon R550 38.0 MHz
1970 Marconi H2900 80.8 ± 1.0 MHz
1972 Racal RA1772 35.4 MHz
1976 Marconi H2540 68.6 MHz
1978 Redifon R1000 38.0 MHz
1979 Plessey PR2250 65.0 MHz
1979 Racal R1792 40.455 MHz
1984 Eddystone 1650 46.205 MHz
 
Before the Marconi H2540, it looks as if economic synthesizer realization limited the 1st IF to significantly less than 63 MHz.  The Marconi H2900 was something of an outlier, but it did use a set of crystals rather than full synthesis for the 1st conversion.
 
Both the Marconi H2540, and the Plessey PR2250 conformed to the “2.1 x“ rule, evidently deliberately so in the Marconi case, to keep 3rd order products out-of-band.  Plessey’s choice of 65.0 MHz may well have been made for the same reason.
 
However, it would appear that Redifon (R1000), Racal (R1792) and Eddystone (1650) were less concerned about that rule, and felt that they could meet the requisite performance requirements without it.
 
Having postulated the “2.1 x” rule, R&S then had to explain why they did not adhere to it in their ESM-1000 VHF-UHF receiver,
 
“The IF selection in this receiver does not follow the rule of thumb given for the HF receiver.  A first IF of 810.7 MHz is used because of filter availability.  At this frequency, a surface acoustic wave (SAW) filter can be obtained with low insertion loss and high selectivity (about 2.0 MHz).  The ultimate rejection of such a filter, with 4-dB insertion loss and less than 1-dB ripple in the passband is about 80 dB.  The preferred choice of a first IF, above 2500 MHz, would have resulted in a filter with too wide a bandwidth and excessive insertion loss.  The second IF is 10.7 MHz.  A drawback of the first IF frequency selection is that at IF/2 (405.35 MHz) there is a frequency window where the balanced mixer shows poor image rejection.  This band is about ± 500 kHz wide since the first IF bandwidth has about 2.0-MHz bandwidth.  This design indicates that the availability of components sometimes limits design so that general rules must be abandoned and a performance compromise accepted.”
 
This receiver tuned the range 20 to 1000 MHz.  Not explained though were the consequences of having an in-band 1st IF, namely 810.7 MHz.  I think it may be assumed that the 1st mixer had excellent isolation between its input and output ports.
 
 
Cheers,
 
Steve
 
Posted : 09/12/2022 10:05 pm
Synchrodyne
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Regarding the 1.4 MHz de facto standard final IF for HF receivers, a fairly casual review leads to a couple of observations, as follows:
 
(1) That it was a British and European receiver maker choice, not often used by American and Japanese makers.
 
(2) That it was mostly used in professional receivers, and less so those in the semi-professional and upper consumer class.
 
In respect of the first observation, American professional receiver practice in the upconversion era seems to have been to favour the long established 455 kHz final IF, although 9 MHz was also used (and sometimes found as an intermediate IF in triple conversion receivers).  The established wide availability of IF filters, at all price and performance levels may have been a significant factor in the retention of 455 kHz.  To a lesser extent, the same probably applied to 9 MHz.
 
In that context, it may be noted that Racal had adopted the 1.4 MHz final IF for its RA1772 HF receiver in the early 1970s, but at the end of that decade, switched to 455 kHz for its RA1792 “Anglo-American" receiver, which in large part was aimed at the American market.  (The 455 kHz final IF choice may well have been an American customer/potential customer requirement or preference.)
 
The second observation may reflect the fact that the wider use of 1.4 MHz from the late 1960s was in professional receivers, so that the range of standard filters developed for it were of professional performance and price, and so less suitable for consumer level equipment.
 
As an example, the Lowe HF-125 of the mid-1980s was released at a time when the 1.4 MHz IF was well established for professional equipment.  But it combined a 45 MHz 1st IF with a 455 kHz final IF, the latter said to be chosen for the ready availability of suitable filters, with the implication being “suitable filters whose cost was appropriate for a consumer receiver”.
 
 
There might have been a somewhat similar differentiation for wide coverage VHF-UHF communications receivers.  Professional equipment typically used 10.7 MHz (or sometimes 21.4 MHz) as a final IF for all modes, including NBFM, AM and SSB.  On the other hand, semi-professional/consumer equipment sometimes used 10.7 MHz as the final IF for WBFM, but had another conversion to 455 kHz for NBFM, AM and SSB, an example being the ICOM R7000.  Presumably, the narrow band 455 kHz filters were significantly more economical than 10.7 MHz narrow band filters.
 
 
Cheers,
 
Steve
 
Posted : 09/12/2022 10:08 pm
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turretslug
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I wonder how much the US professional market was influenced by the success of the Collins mechanical filter series, I'm sure I recall seeing somewhere that the type was suited to frequencies in the range 50kHz to 600kHz, presumably outside this range made them unwieldy below the LF end and demanding to machine with sufficient precision beyond the HF end? I assume that one of the first widely produced examples of this series were those for Collins' R-388 receiver of the early '50s whose dual-conversion/LO1 crystal allocation pretty much dictated 500kHz second IF. Without witnesses/dependable literature of the era, there's a risk of slipping into chicken-and-egg misconception, but maybe later settling on 455kHz was influenced by a desire for commonality with mass-market IFT manufacture plus the existance of professional accessories that functioned at IF such as RTTY/SSB decoders, panadapters and so on.

With less existing background in "package" filters, maybe European manufacturers chose an IF suited to a popular professional application, i.e. marine receivers, i.e. outside the 300-500kHz and 1.6MHz+ regions- albeit needing competent screening and filtering to avoid MW broadcasting breakthrough, admittedly this would have been a concern with pretty much any chosen part of the by-then busy sub-30MHz spectrum of the era.

 
Posted : 10/12/2022 5:33 pm
Synchrodyne
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As you say, determining cause and effect from this distance is not so easy.
 
Nonetheless, the Collins mechanical filters were probably a significant factor in the retention of 455 kHz as a final IF for American HF receivers at a time when the European makers were moving to 1.4 MHz.
 
An undated Collins brochure that I just found on the internet, at:
suggests that apart from use in its own receivers, Collins was primarily concerned with the military and professional market, including SSB multiplexing filters for long-distance telephone networks.  The use of Collins filters in semi-professional and amateur/domestic equipment might have been something of a trickledown effect.

Cheers,

Steve

P.S.  There is a 1969 December Collins filter catalogue at:  http://www.peakbagging.com/Electronic/CollinsMechFilters.pdf.

 
Posted : 19/12/2022 10:57 pm
Synchrodyne
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Re Collins mechanical filters, I also found the following, dated 2015 August 13:
 
 
 
To quote therefrom:
 
>>>>>>>>>>
 
Rockwell Collins makes two different types of mechanical filters, many of which have found their way into Amateur Radio products and applications. In a mechanical filter, input and output transducers convert the electrical signal to and from resonant mechanical vibrations, respectively.
 
“For frequencies between 100 kHz and 700 kHz, we create filters made from rods resonating in a torsion mode,” the company explained on its website. “For frequencies below 100 kHz, we use flexure mode bar resonators.”
 
Collins has made mechanical filters for more than 6 decades, and their initial application was in telephone circuits. The filters gained favor for Amateur Radio use because of their excellent selectivity, especially in IF applications. It is said to take about 12 weeks to manufacture a single unit.
 
>>>>>>>>>>
 
 
That confirms that the mechanical filters were first developed for telephone applications.  Previously, I too had thought that they were developed by Collins for its SSB receivers.  It might have been the other way around.  Perhaps the fact that Collins started out making mechanical filters for telephone SSB multiplex purposes created the interest in SSB HF radio, and thence the applications of its filters thereto.
 
 
Cheers,
 
Steve
 
Posted : 20/12/2022 9:11 pm
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Synchrodyne
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An HF receiver that did use mechanical (presumably Collins) 455 kHz IF filters for USB, LSB and other narrow band applications was the McKay Dymek DR33C of circa 1979.  At the time, this was described as being a professional model, basically similar to the DR22 of 1977, the first in that series, and intended more as a consumer product.  The DR33C looks as if had been aimed at the lower end of the professional market.
 
These and the following models all had the same basic triple conversion, with initial upconversion, structure.  Here is the block schematic for the DR33C:
 
McKay Dymek DR33C Block Schematic
 
 
 
The IFs were 30 MHz, 10.7 MHz and 455 kHz.
 
The 30 MHz 1st IF was as low as could be used for an upconversion receiver that covered the full HF range.  In fact there was slight truncation, as the upper tuning limit was 29.7 MHz, the lower limit being 50 kHz.
 
Possibly this “low” high IF was chosen in the interests of synthesizer economy.  The latter had 5 kHz steps, so was quite coarse, and which probably also helped keep the cost down.  The 5 kHz steps were interpolated by providing the 2nd oscillator with a ±5 kHz range.
 
With a 455 kHz final IF, it was not essential to have an intermediate IF.  For example, in about the same timeframe, Japan Radio Corporation (JRC) adopted a 70.455 MHz, 455 kHz IF combination for its HF receivers, and used this across several models.
 
In this case though, conversion from 30 MHz to 455 kHz would have required 2nd local oscillator nominal frequency of 30.455 MHz, assuming supradyne conversion.  In turn this would have an in band image.at 29.545 MHz, in a receiver that had very little in the way of front end selectivity.  So it might have been seen as undesirable.
 
Obtaining a higher 2nd oscillator frequency in order to avoid an image problem, whilst retaining the 30 MHz 1st and 455 kHz final IFs, inevitably required the use of an intermediate IF.  The 10.7 MHz number used was logical.  Being a very widely used standard IF, there was an abundant range of standard filters.  Not only that, if receiver internal screening was such that there was something of a dead spot in the tuning range at the 2nd IF, it didn’t matter very much in the 10.7 MHz case, given that its standard IF status meant that it wasn’t used as an HF transmission frequency.  The 2nd conversion was supradyne, with a nominal oscillator frequency of 40.7 MHz.
 
The 3rd conversion was supradyne with an oscillator frequency of 11.155 MHz.  The two supradyne conversions means that at 455 kHz, the sidebands were correctly oriented.
 
As noted upthread, a 10.7 MHz “intermediate” IF had also been used by Plessey for its PR155 HF receiver of about a decade earlier.  In that case though, an intermediate step between the 37.3 MHz 1st and the relatively low 100 kHz 3rd IFs was probably unavoidable.
 
 
Cheers,
 
Steve
 
Posted : 21/12/2022 9:42 pm
Synchrodyne
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Another HF receiver form that had its own fairly complex set of IF requirements was the premixer type.
 
This was somewhat like the converter type, in that the initial conversion was done in discrete frequency steps by crystal controlled oscillators.  But whereas in the converter type, the interpolation between crystal frequency steps was done in the 2nd conversion, in the premixer type it was done in the initial conversion stage.  This was done by “premixing” the output of an interpolation VFO with the crystal bank output, and using the mix as the 1st oscillator feed.  Effectively it was an early form of synthesis.
 
The Drake SPR-4 of 1969 provides a suitable worked example:
 
Drake SPR4 Block Schematic
 
In this case the tuning range was from 150 kHz to 30 MHz in 0.5 MHz ranges.  The lowest range was truncated at 150 kHz at its lower and did not require a crystal.  1st conversion was done using the VFO output alone.
 
The nominal VFO range was 4955 to 5455 kHz, and the 1st IF was 5645 kHz.
 
For conversion of the 150 to 500 kHz tuning range up to 5645 kHz, a VFO range of 5495 to 5145 kHz was required.  In fact that required use of a 50 kHz margin at the upper end of the VFO range to get down to 150 kHz.
 
Each of the other ranges, from 1.0 to 1.5 MHz through 29.5 to 30.0 MHz required its own crystal, although the receiver could not accommodate the total number of crystals available, 23 ranges being the maximum.  (The receiver operator would need to make some choices as to which ranges were covered).
 
The crystal frequencies were all at 11.09 MHz above the bottom edge of the band to which they applied (not taking account of band edge overscans).  Thus they were 12.09 MHz for the 1.0 to 1.5 MHz band, 12.59 MHz for the 1.5 to 2.0 MHz band, and so on through to 40.59 MHz for the 29.5 to 30.0 MHz band.
 
The feed to the 1st mixer, taken from the premixer, was the difference between the crystal frequency and the VFO frequency.  Thus, taking the 10.0 to 10.5 MHz band as an example, the crystal frequency was 21.09 MHz.  Thus gave a premixer output of 15 635  to 16 135 kHz.  Thus, with appropriate VFO tuning, incoming signals in the 10.0 to 10.5 MHz range were converted to a 1st IF of 5645 kHz.  The 1st IF was taken as the difference frequency generated by the 1st mixer.
 
All bands had the same tuning sense, moving from the low to the high end as the VFO moved from its high to its low end.  It may be seen that in respect of the 150 to 500 kHz band, this required the VFO range to be below the 1st IF, and for all other bands, for the VFO to be subtracted from the crystal frequency, with the net 1st oscillator feed from the premixer above the 1st IF.
 
This the 1st IF had to be between the upper end of the VFO range, and the lower end of the lowest 1st oscillator feed (premixer output) range.  Also, given that the VFO alone was used for the 150 to 500 kHz band, the 1st IF was going to be not far above the top of the VFO range.
 
For stability, the VFO swing probably needed to be but a small fraction of its mean frequency, say around 5 MHz for a 500 Hz swing.  So it was going to be in the HF range.  Evidently Drake chose 5645 kHz as providing the best trade-off, and presumably clear of any major HF activities.  In particular, it was underneath the 49 metre SW broadcast band, which started at 5950 kHz.  
 
With the VFO range and the 1st IF determined, the required crystal frequencies for the ranged from 1.0 to 1.5 MHz and upwards could be derived.
 
Effectively, the lowest, non-crystal tuning range determined the required VFO range.  The VFO range indicated, at least approximately, where the 1st IF should lie.  Then the combination of the VFO range and the 1st IF determined the required crustal frequencies.
 
Given that 5645 Hz was probably seen as too high for final processing, at least in a consumer/amateur market receiver, there was a second conversion, in this case down to 50 kHz.  This number had been in the Drake lexicon from the start, with its initial 1-A SSB receiver of 1957.  It allowed for the provision of adequate selectivity using L-C filters.
 
The 2nd conversion was done with a crystal oscillator with selectable frequencies of 5595 and 5695 kHz.  This provide for sideband selection.
 
From the signal path viewpoint, the SPR-4 was simply a double conversion receiver with IFs of 5645 and 50 kHz.  The complications were a sideshow as it were, in the premixer arrangement.
 
Drake had introduced the premixer form to its product line with its R-4 model in 1964, having previously used the converter form.  The SW-4 of 1966 was somewhat of a derivative for broadcast SW purposes, with the same premixer form and 5645 kHz 1st IF, but with a 455 kHz 2nd IF, using a 5190 kHz crystal oscillator for the 2nd conversion.
 
 
Cheers,
 
Steve
 
Posted : 22/12/2022 12:04 am
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Synchrodyne
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Drake also used the 5645, 50 kHz IF combination in its R-7 receiver of 1978.  This was of the synthesized, upconversion type with a 1st IF of 48.05 Mhz.  5645 and 50 kHz were used for the 2nd and 3rd IFs respectively.
 
Drake R 7 Block Schematic
 
If the 50 kHz final IF was a given, basis Drake’s long history of using it, then an intermediate IF was required.  But even if that did precondition not obtain, then there was another reason for having three IFs, and that was to facilitate the use of passband tuning.  In this case that was done by synchronous fine adjustment of the 2nd and 3rd oscillator frequencies.
 
That being the case, it could have been that Drake simply chose the same 5645 and 50 combination kHz that it had previously used successfully in its “4” series receivers.
 
The 48.05 MHz 1st IF looks as if it were chosen to suit the synthesis scheme used, which had 500 Hz steps with VFO interpolation.  Conceivably the need to synthesize the appropriate 2nd and 3rd oscillator feeds also had some influence on the 1st IF choice.  The 2nd and 3rd oscillator feeds were respectively the passband tuning VFO plus 40 MHz and the passband tuning VFO less 8 MHz.  So the two feeds were 48 MHz apart.  That in turn meant that the 1st IF was 48 MHz above the (50 kHz) 3rd IF, which gave the 48.08 MHz number.  But that does not necessarily determine cause and consequence.
 
Certainly the 1st IF for upconversion HF receivers appears to have been very much an individual manufacturer choice.  This produced a diversity of numbers, with commonality between manufacturers uncommon.
 
 
Cheers,
 
Steve
 
 
Posted : 22/12/2022 11:14 pm
Synchrodyne
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Posted by: @synchrodyne

If the 50 kHz final IF was a given, basis Drake’s long history of using it, then an intermediate IF was required.  But even if that did precondition not obtain, then there was another reason for having three IFs, and that was to facilitate the use of passband tuning.  In this case that was done by synchronous fine adjustment of the 2nd and 3rd oscillator frequencies.

 

My previous assertion that conversion direct from a high (i.e. above 30 MHz) 1st IF to a very low (say somewhere below 455 kHz) 2nd and final IF turns out to have been incorrect.
 
I have just been looking at the data for the Drake R8 HF receiver.  This was of the double conversion upconversion type, with a 1st IF of 45 MHz, and a 2nd IF of 50 kHz.
 
This was achieved by the use of an image rejection mixer (IRM) for the second conversion.  In this, the incoming 1st IF was split into two streams with a quadrature phase relationship, each down converted in its own mixer.  The two mixer outputs were then combined in such a way as to cancel (or significantly diminish) the unwanted image.  This was akin to the phasing method for SSB generation.
 
A reasonable inference is that had 45 MHz to 50 kHz been attempted with a conventional mixer, there would have been an image problem.
 
The Drake R8 receiver had passband tuning.  This was achieved by varying the second oscillator frequency up to ±3 kHz, and synchronously varying the frequency of the SSB carrier insertion oscillator.
 
There was a separate sidechain for NBFM.  This used a Motorola MC3362 IC, in which the incoming 45 MHz 1st IF was first downconverted to 10.7 MHz, and then further downconverted to 455 kHz.  Both conversions were infradyne.
 
Drake R8 Block Schematic
 
Not directly connected with its IF system, but tangential to it, it may be observed that the R8 included a fully synchronous AM demodulator.  The AM envelope demodulator was of the quasi-synchronous type with a broadband reference, using the same Motorola MC1496 IC as for SSB demodulation.  On the IF side, the limited 50 kHz reference, developed for envelope demodulation, was doubled to 100 kHz for use by the fully synchronous demodulator.  This 100 kHz signal was fed to a PLL, whose output was divided by two to produce a clean 50 kHz reference for demodulation.  Whilst Drake did not appear to have given a reason for the frequency doubling, one may be derived.  The doubling process actually recreates the carrier, even if such were absent from the incoming feed.  (Frequency doubling was one method, known as the 2F method for demodulating suppressed carrier DSB signals.)  Thus even in very deep fades that seriously reduced the carrier level, the PLL would still retain lock due to the carrier recreated in the doubling process.
 
Returning to the IF structure of the Drake R8, its 45 MHz 1st IF looks to have been an ab initio choice, whereas the 50 kHz 2nd IF was evidently a carryover from long-established practice.  On the NBFM side, the 10.7 MHz and 455 kHz IFs were industry standard practice.
 
 
Cheers,
 
Steve
 
 
Posted : 23/12/2022 2:32 am
Synchrodyne
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Another HF receiver that was fairly complex in IF terms was the JRC NRD-535 of circa 1991, so of about the same time as the Drake R8.
 
Up to and including the NRD-525, the JRC receivers in the NRD-5n5 series had used a fairly straightforward IF configuration of 70.455 MHz nominal 1st IF and 455 kHz 2nd IF.
 
The NRD-535 was nominally triple conversion, with IFs of 70.455 MHz, 455 kHz and 97 kHz.
 
But there was an optional side-loop, used as a bandwidth control facility.  This involved conversion from 455 to 400 kHz (nominal), then conversion back to 455 kHz after passing through a filter centred on 400 kHz.  Both conversions were supradyne.  With this facility in operation, the NRD-535 became a quintuple conversion receiver with four IFs.  Varying the actual frequency away from 400 kHz effected bandwidth control by changing the overlap with the preceding 455 kHz filters.
 
Presumably 400 kHz was seen as a convenient IF for the purpose, and one that did not cause any problems elsewhere in the circuit.
 
The final conversion to 97 kHz, infradyne, was apparently to accommodate a variable notch filter.  In fact the notch itself was fixed, but its position in the passband was varied by moving the passband, this being done by varying the oscillator frequency and so the exact final IF.
 
One may ask why 97 kHz, and not say 100 kHz.  It might have been because 100 kHz harmonics could be problematical in respect of the bandwidth control unit, with its 400 kHz IF.  The demodulation process included limiting of the 97 kHz IF, which would produce abundant harmonics.  AM demodulation was done quasi-synchronously with a wideband reference, using the same IC as for SSB and CW.
 
The fact that the 97 kHz IF was in fact variable by a small amount required a similar variation in the BFO/CIO inputs to the product demodulator.  This was obtained by starting with a set of BFO/CIO frequencies that were suitable for use with 455 kHz, and then downconverting these using the same oscillator frequency as used to convert the IF from 455 kHz to 97 kHz.  Thus the BFO/CIO set moved synchronously with the final IF.
 
Fully synchronous PLL AM synchronous demodulation was done in a separate module fed with the 97 kHz final IF, with all processing done at that frequency.
 
NBFM was processed in a sidechain evidently operating at 455 kHz, with split-off after the main 455 kHz filter bank.
 
JRC NRD 535 Block Schematic
 
 
Cheers,
 
Steve
 
 
Posted : 28/12/2022 1:46 am
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As previously said, with upconversion HF receivers, the 1st IF appears to be an individual manufacturer choice, with little or no commonality or standardization.  In some cases the reasons for specific choices were given, whilst in others they may be deduced.  But there are many where very little may be gleaned as to the reasons for the specific  choice.  That said, the existence of “offsets” to allow for the 2nd IF is usually readily apparent in numbers such as 35.4, 70.455 MHz, etc,
 
One case that initially looks to have an “anonymous” 1st IF is that of the Sony ICF-2001D/2010 portable radio receiver of c.1985.  On the AM side, it was of the synthesized double conversion type with initial upconversion, the IFs being of 55.845 and 455 kHz, and the frequency coverage 150 kHz through 30 MHz.  It also had an extended FM band, 76 to 108 MHz, covering both the Japanese band as it then was, 76 to 90 MHz, and the “international” band 87/87.5/88 to 108 MHz.  In this mode, single conversion with the standard 10.7 MHz IF was used.
 
An additional feature was the inclusion of the VHF air band, 116 to 136 MHz.  Although this band had its own RF amplifier, thereafter, the signal followed the AM double conversion path, with initial down conversion to 55.845 MHz.  This down conversion was infradyne, requiring a local oscillator range of 60.155 to 80.155 MHz.  It may be observed that this oscillator range was within the range required for AM, namely 55.995 to 85.845 MHz.
 
I suspect that this nesting of the air band local oscillator range within the AM oscillator range was an initial condition.  If so, then it placed a quite significant constraint on the choice of the 1st IF.  This may be shown by a few simple calculations.  To simplify, we may take the AM tuning range as 0 to 30 MHz.  The two limiting case are: (1) where 30 MHz and 136 MHz share the same oscillator frequency, and (2) where 0 MHz and 116 MHz share the same oscillator frequency.
 
In case (1), the shared oscillator frequency must be 83 MHz,  and the 1st IF is then 53 MHz.
 
In case (2), the shared oscillator frequency must be 58 MHz, and the 1st IF is then 58 MHz.
 
That means that the 1st IF must be within the range 53 to 58 MHz.  The exact choice will determine where in the AM oscillator range the air band oscillator range is nested.  As a practical matter, 58 MHz itself is probably unusable, the available range would be smaller.
 
Still using the above numbers, if the air band oscillator range were centred within the AM oscillator range, then 15 MHz and 126 MHz would share the same oscillator frequency of 70.5 MHz, and the 1st IF would be 55.5 MHz, which is fairly close to the actual 55.875 MHz.
 
One may redo the calculations for the 150 kHz to 30 MHz AM case, which would shift the boundary numbers inwards slightly and asymmetrically, the differences though are not material to the basic premise, which is that in this case the 1st IF was selected to allow the use of the AM oscillator for the air band without alteration.
 
In general, for a VHF air band receiver, one might normally expect the 10.7 MHz IF to be used, perhaps with a second conversion to 455 kHz.  In this case though, Sony may well have chosen to route the air band through the AM pathway, not only because the air band is AM, but also to take advantage of the much finer tuning thus available.  That way, the FM synthesizer would not have required finer than usual tuning steps.
 
The conversion from 58.845 MHz to 455 kHz was infradyne, with an oscillator frequency of 55.390 MHz.
 
The ICF-2001D/2010 was also notable for including a PLL fully synchronous AM demodulator with selectable sidebands.  This used the same IC set that Sony had developed for multisystem AM stereo decoding, and which had been deployed in the SRF-A100 portable receiver of c.1983.  This IC set used an “8 times” oscillator for the PLL, so in the ICF-2001D/2010 required a 3.64 MHz crystal.  On the other hand, the SRF-A100, which had analogue tuning, had a 450 kHz AM IF, and so had used a 3.60 MHz crystal.
 
At the time, the 450 kHz AM IF was relatively new for domestic receivers, and was significantly associated with the synthesized type.  But Sony was evidently following the general trend when it came to the SRF-A100.  On the other hand, the use of 455 kHz in the slightly later ICF-2001D/2010 tends to cement the notion that notwithstanding the emergence of 450 kHz for MW purposes, it [455 kHz] was seen as the “right and proper” number for consumer level HF receivers.  Filter availability was a part of it, and I recall that third party filter kits were made available for this model.
 
Sony ICF 2001D, 2010 Block Schematic
 
 
Cheers,
 
Steve
 
Posted : 30/12/2022 10:23 pm
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As is fairly well known, a new LF and MF AM broadcasting frequency allocation plan for ITU Regions 1 and 3 was implemented in 1978 November.  In the European case, this replaced the ITU Copenhagen Plan of 1948.  In the African case, it had replaced the ITU Geneva Plan of 1966, which incidentally used the Copenhagen channelling.  Region 3, Asia and Oceania, had hitherto mostly used 10 kHz channelling.  However, alignment of Regions 1 and 3 was a key objective.
 
The ITU and associated CCIR documents related to this major change made some comments about receiver IFs, albeit without assuming a single standard IF for planning purposes.
 
The starting position was:
 
“No single value of intermediate frequency is, at present, satisfactory for the whole European area,
 
“Nevertheless, studies have shown that when a frequency plan is being established for a large area, two values of intermediate frequency may be recommended to minimize interference and to permit all available channels to be used.”
 
 
The “two IF” case was well illustrated by the BREMA advice following the Copenhagen plan, that 422 and 470 kHz were suitable for use in the UK.  In France, 455 and 480 kHz were in use, with 455 and 468 kHz recorded for Germany.
 
The definitive position was:
 
“When choosing the carrier frequencies, channel spacing and also the intermediate frequencies to be used in receivers, it is important that they should be chosen to minimize interference from the local oscillators of the receivers in use or of nearby receivers, either by the fundamental or a harmonic frequency; harmonics of a transmitted  frequency, or other possible intermodulation products.
 
“If both the carrier frequencies and the intermediate frequency are an integral multiple of the carrier spacing, then all interfering products will also be integral multiples of the carrier spacing. Theoretically, therefore, maximum protection could then be obtained because the frequency difference between any interfering signal of this kind and the wanted carrier frequency would be zero or a multiple of the channel spacing.
 
“If these requirements are to be met in a particular broadcasting band it would be essential for the channel spacing to be uniform throughout the band. It would be more advantageous, moreover, if this condition could be met in both bands 5 [LF] and 6 [MF] or better still throughout bands 5, 6 and 7 [HF]. On the other hand, this condition should be satisfied on a world-wide scale or at least in those areas, where a single frequency assignment plan exists or will be established.”
 
 
Consistent with the foregoing, 9 kHz channelling with carrier frequencies as integral multiples of 9 kHz was established for the MF band under the new plan.  But the same change for the LF band was deferred, apparently because it would also have affected non-broadcast applications at the shared edge portions of the band.
 
That paved the way for the introduction of “new” IFs that were integral multiples of 9 kHz, such as 450, 459 and 468 kHz, although in some cases these had seen previous use.  The specific best IF(s) would probably vary somewhat by geography, according to the frequencies actually used or receivable therein.
 
That the LF channels were not changed at the same time may have inhibited somewhat IF change in Europe, but for most of Africa and all of Region 3, that was a non-issue.
 
In particular, Japan, in region 3, made the change from 10 kHz to 9 kHz MF channelling.  Hitherto its standard AM IF had been 455 kHz.  One may assume with reasonable probability that it was found that 450 kHz, an integral multiple of 9 kHz, was suitable in its new situation, and thus a change from 455 to 450 kHz was prompted.  Furthermore, evidently it was also found that 450 kHz would also be suitable for use with the 10 kHz channeling in Region 2 (Americas), where hitherto 455 kHz had been the norm.  Certainly, 450 kHz was an integral multiple of 10 kHz, and for the most part Region 2 channels were also integral multiples of 10 kHz.  Thus the “new” IF of 450 kHz was probably seen as suitable for all Japanese domestic and exported AM-MF equipment.
 
At least that provides a plausible, albeit unverified explanation for the progressive adoption of the 450 kHz number for MF equipment.  
 
It was also stated by the CCIR:
 
“However, it must be noted that the disturbance caused by an interfering signal increases rapidly as its frequency difference from the wanted signal increases from zero.”
 
“Under present-day conditions the frequency differences might have any possible value and this may require an additional protection ratio of up to 17.5 dB. With the adoption of the proposed arrangement, the maximum frequency difference would depend on the accuracy with which the local oscillator frequency and the centre frequency of intermediate-frequency pass-band can be controlled. To achieve an improvement close to the maximum possible it would be necessary to achieve stabilities of the order of 100 Hz. As far as the intermediate-frequency stability is concerned this could be achieved by using ceramic or mechanical filters rather than using conventional intermediate-frequency coils. The control of the initial tuning operation and the frequency drift of the local oscillator may require special techniques in which automatic frequency control may be required. The adoption of the proposal would, therefore, give little improvement in the short term, with existing receivers, but would offer the chance of substantial improvement in the future without any disadvantages under present-day conditions.”
 
 
Thus, the use of the “integral multiple” channels and IFs also required enabling technology.  By 1978, ceramic IF filters were routine in consumer equipment, and synthesized tuning, already well-established in professional equipment, was making its entrance into the consumer field.  So the timing of the MF channeling change was “right”.  Previously noted was that the advent of the 450 kHz in consumer equipment seemed to have been associated somewhat with synthesized tuning.  The foregoing does provide some causal linkage beyond happenstance.
 
With earlier consumer level technology, the reasonably realizable precision levels for receiver tuning and IF stability indicated avoiding the “integral multiple” IFs.  Rather, in the 10 kHz channelling case for example, the “safe” place to be was an odd integral multiple of 5 kHz (half the channel separation), hence numbers like 455 and 465 kHz.
 
Although there is some speculation and assumption in the foregoing, I think that it does provide a better background for the 450 kHz case.
 
Incidentally, the CCIR analysis and prognosis was based in part upon work done earlier (1966) by SCART, the industry body in France that inter alia developed and promulgated standard IFs for that country.
 
 
Cheers,
 
Steve
 
 
Posted : 23/02/2023 1:18 am
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Synchrodyne
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Quite early in this thread I noted that I had found only one datapoint relating to the IF used for Eastern European FM receivers for the OIRT FM band, 66 to 73 MHz, or thereabouts.  That was in this posting:  https://www.radios-tv.co.uk/community/radio/radio-receiver-intermediate-frequencies/paged/2/#post-81371; the apparent standard IF was 8.4 MHz.
 
This number has been confirmed in a recent posting on UKVRR by a Ukrainian correspondent, who said:  “The intermediate frequency was also 8.4 MHz, then 6.5 was used, 6.8 rarely, and 10.7.”
 
The earlier reference mentioned 6.5 MHz as an FM 2nd IF in TV-FM receivers, wherein the 8.4 MHz 1st IF was converted to align with the TV system D intercarrier frequency.  But it is certainly plausible that 6.5 MHz was also used as solitary FM IF, bearing in mind that IFs in the 6 MHz range were also used in a minority of Western European receivers.  There was some mention of this in an earlier posting, https://www.radios-tv.co.uk/community/radio/radio-receiver-intermediate-frequencies/paged/5/#post-108673
 
The 6.8 MHz number might have been an effort to say in the 6 MHz range, but far enough through it that all local oscillator frequencies were (just) out -of-band.  Early use of the OIRT FM band appeared to cover allocated frequencies from 66.5 to 73 MHz, a range of 6.5 MHz.
 
The eventual migration to the international norm of 10.7 MHz was probably inevitable.
 
 
Cheers,
 
Steve
 
Posted : 05/04/2023 10:13 pm
Synchrodyne
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Another facet of the IF story was the use of relatively low final IFs for some FM receivers that used phase-locked loop (PLL) demodulation.
 
A couple of examples are provided by the Signetics NE563 and Plessey SL6600 integrated circuits.
 
The NE563 was an IF subsystem for higher-performance broadcast FM receivers, offering essentially the same facilities as the established RCA CA3089 and its derivatives, but using PLL rather than quadrature demodulation.
 
Input to the NE563 was the standard 10.7 MHz IF.  This was amplified and then downconverted to 900 kHz, using a 9.8 MHz local oscillator.
 
Signetics NE563
 
 
The advantages claimed for double conversion were higher gain at any given stability level, due to distribution of the gain over two frequencies, and better demodulated signal-to-noise ratio, due to the higher deviation-to-carrier ratio obtained with the relatively low final IF.  PLL demodulation was also claimed to provide better linearity.
 
Notwithstanding its claimed advantages, the NE563 did not seem to make significant inroads into the market established by the CA3089 et al.  Timing may have been a factor.  The CA3089 was announced in 1971, and described in a 1971 June IEEE paper “Advances in FM Receiver Design”, by Jack Avins.  The NE563 was announced during 1974, the earliest reference I have found being in “Electronics Australia” 1974 August, and described in a 1975 August IEEE paper “A Monolithic IF Subsystem for the demodulation of FM Signals”, by Werner Hoeft and Ira Sigmond.
 
Earlier, Signetics had proposed that its NE561B PLL IC could be used as a PLL demodulator for both FM and AM.  This was described in a 1971 January IEEE paper “An Integrated Frequency-Selective AM/FM Demodulator”, by Alan B. Grebene.  Data were shown for incoming IFs of 10.7 MHz (FM), 4.5 MHz (system M TV sound), 455 kHz (AM) and 67 kHz (FM SCA).  The same IC could be used to demodulate FM at 10.7 MHz and AM at 455 kHz.
 
One imagines that to be competitive, Signetics found that it had to offer an FM IF subsystem, and that the second conversion to a low (sub-1 MHz) final IF was necessary in order to provide a competitive/advantageous demodulated signal-to-noise ratio.
 
 
Plessey offered the SL6600C IC for NBFM communications applications.  This down converted, either infradyne or supradyne, from a 1st IF that could be up to 50 MHz, although was normally 10.7 or 21.4 MHz.  The 2nd IF was not fixed, but was set case-by-case as the greater of 100 kHz and ten times the maximum deviation, but less than 1 MHz.  I think that its main rationale was that it was a low-consumption device that did not require a quadrature coil, so suited compact portable equipment.  Plessey also offered communications FM ICs with quadrature demodulation.  The SL6600C could also be used in single-conversion mode, with an input IF of 800 kHz maximum.
 
Plessey SL6600C
 
 
The Plessey “10 x deviation” rule would mean a minimum final IF of 750 kHz for broadcast reception (±75 kHz deviation).  The 900 kHz number used by Signetics was consistent with that, allowing for some margin.  One could perhaps generalize and suggest that for PLL FM demodulation with a low final IF, the latter would be in the range 100 kHz to 1 MHz, but at least 10 times the deviation.
 
 
PLL demodulation, whether AM or FM, may also be seen as another frequency conversion, but a special case in which the output “IF” is of zero frequency.  It would appear that Pye may have viewed it that way in respect of some of its VHF communications base station receivers of the 1970s, including the R401 (AM) and R402 (FM).  These were single-conversion, with an IF of 10.7 MHz, and had PLL demodulation at that frequency.  The data sheet for the R402 included the following wording:  “The single superhet receiver design with phase-locked loop discriminator results in simpler circuits with improved reliability and a substantial reduction in the number of spurious responses, compared with double conversion designs.”  One can infer that the benefits were not due to the use of single-conversion as against double-conversion, but from its combination with PLL demodulation, or effectively a zero frequency 2nd IF.  (Conventional demodulators, both AM and FM, typically produce abundant harmonics, etc., whereas PLLs are much better in this regard.)
 
 
Cheers,
 
Steve
 
 
Posted : 06/04/2023 3:27 am
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Synchrodyne
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Some interesting comments on broadcast radio receiver IFs were made in a couple of SAE papers, namely:
 
 
SAE 840089  World Radio Receiver:  Reality or Myth, by Carlos A. Altgelt, Ford Motor Company.
 
SAE 900040  World Radio Receiver Revisited  Still a Myth? by Carlos A. AItgeIt and Richard L. Summerville, Ford Motor Company
 
 
Neither paper had receiver IFs as its primary subject, but both addressed it in context.  Thus they provide a professional viewpoint from a maker who necessarily had a worldwide purview.
 
 
To quote from the 1984 paper in respect of the AM case:
 
“In the AM band(s), and for historical reasons, there are a multitude of intermediate frequencies used throughout the world.  For car radios, these typically are:
 
Market                        AM IF (kHz)
 
U.S.A., Canada            262.5, 450
 
W. Europe                   450, 452, 455, 460, 465, 465, 465, 470
 
Latin America              262.5, 450, 455
 
Australasia                  450, 455
 
 
“The reason for this variety of frequencies has to do with the interference created by the first harmonic of the IF (or twice the IF), and station frequency allocation.  Intrinsically, and by the very nature of its operation, the superheterodyne receiver produces a multitude of spurious responses, or interference, one of which is related to the IF harmonics.  The IF interference is heard through the loudspeakers as "tweets," or whistles.  For instance, there is a popular BBC station at Brookmans Park, near London, that is located at 909 kHz (Radio 4 ).  If the IF utilized in England were to be 455 kHz, the first IF harmonic generated within the receiver would be 455 kHz x 2 = 910 kHz, and thus interfere with the desired signal only 1 kHz away, which is well within the desired passband of the receiver.  To avoid this situation, the specified IF in England is typically 465 kHz or 470 kHz, since there are no stations located at — or near — 930 kHz or 940 kHz.
 
“Some broadcasting authorities in W. Europe have proposed the standardization to 459 kHz across the European Broadcasting Area, and to move to another frequency those few stations currently operating at 918 kHz, thus creating the least disruption to the established transmitters in that area.
 
“In Buenos Aires, Argentina, to give an identical example, but with a different solution, there is also a very popular station — Radio Excelsior — located at 910 kHz.  To avoid first-harmonic IF interference, the IF which is typically utilized is 450 kHz; no major stations are located at 900 kHz.
 
“In most of the Americas, where there are no LW stations, it has been customary to use 262.5 kHz as the IF, since its first harmonic — 525 kHz — falls outside the AM band.  This solution does not work in W. Europe, since 262.5 kHz falls in the middle of the popular LW band (150 kHz to 285 kHz).”
 
 
From the tabulation, one may deduce that by 1984, 450 kHz, as the general successor to 455 kHz, was well-established, although by no means universal.  And that 262.5 kHz, established particularly for car radios (with predecessors 262 and 260 kHz) was still active in the Americas.  In Europe, the move to IFs that were multiples of 9 kHz had started, but older numbers were still in use.  The Brookman’s Park case was evidently widely known.
 
The 1990 paper essentially repeated the above, with some additional comments:
 
 
“Further complicating the problem on AM is the choice of the intermediate frequency itself.  Long ago car radio IFs around 262 kHz were selected because of the availability of transformers capable of providing the proper bandwidths and because the first harmonic was outside of the medium wave band.  Obviously this would be a problem in Europe where the long wave band extends from 153 kHz to 279 kHz putting the IF in the middle of the band.  Even the choice of the popular 455 kHz IF is a problem in Britain where for example a popular program is transmitted from Brookman's Park outside of London at 909 kHz which would beat against the first harmonic of the IF at 910 kHz to produce a 1 kHz note.
 
“Until recently, some manufacturers produced radios with a 455 kHz IF for the European continent and 465 kHz for Great Britain.  This fact prompted some European authorities to propose standardization of a single 459 kHz IF and a change in frequency of the few stations operating at 918 kHz.
 
“In this situation microprocessor based tuning helps.  If an IF such as 450 kHz is chosen, the first harmonic will be at 900 kHz which happens to be zero beat with both a 9 kHz and 10 kHz channel.  Since the user cannot tune between the channels, the beat note frequency is controlled only by the accuracy of the transmitter and receiver reference frequencies and can be conveniently maintained as subaudible.”
 
 
In fact, electronic tuning had been used for AM receivers since the early 1980s at least, and it appears that the device makers had catered for both 9 and 10 kHz AM channels in conjunction with the 450 kHz IF.
 
 
Turning to FM, this was said in the 1984 paper:
 
 
“With regards to the FM band, most receivers throughout the world use 10.7 MHz as the standard IF, for reasons which have to do with receiver bandwidth, IF interference, gain, stability, and selectivity.
 
“The usage of 10.7 MHz for the FM IF is not mandatory in any country, with the exception of South Africa, where it must be 10.7 MHz +/- 0.2 MHz.  Somewhat related to this requirement, is that in South Africa, the FM local oscillator frequency must be 10.7 MHz below the frequency of the desired station; in the rest of the world, it is a standard practice — but not mandatory — to set the local oscillator frequency at 10.7 MHz above the incoming signal.”
 
 
One may infer that by 1984, Eastern Europe had largely abandoned the original OIRT FM IF of 8.4 MHz, and had moved to 10.7 MHz.  The South African mandate is curious, but I suspect not always observed.  The 1990 paper essentially repeated the same commentary.
 
 
 
Cheers,
 
Steve
 
Posted : 07/04/2023 1:59 am
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