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[Sticky] Radio Receiver Intermediate Frequencies

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Synchrodyne
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Posted by: @synchrodyne

The second loop was quite complex, involving 37.5 and 3.75 MHz feeds from harmonic generators and a VFO.

Actually, the second Wadley Loop involved two VFOs, as shown in this diagram, although the interpolation VFO (200 to 300 kHz) was, strictly speaking, not within the loop proper.

Marconi Hydrus ISB Block Schematic

The main loop is highlighted in mauve, the second loop in yellow. The latter, unlike the main loop, does not have any of its legs overlapping with the IF path. One may imagine that with that complex of frequencies, very good screening was required for the so-called synthesizer compartment. Both loops were of the premixer type, with the output of a narrow-range interpolation VFO added to the main drift-cancelling feed.

Another Marconi receiver of the same era as the Hydrus was the N2020 naval unit of 1968. Like the MST models, this was intended to use the same synthesizer as its counterpart transmitters, and had self-tuning of the RF circuits. Thus it also followed the integer IF “rule”. In this case, the required frequency coverage was 240 to 525 kHz and 1.5 to 28 MHz. Here a 2 MHz 1st IF could not be used, given that it would have been in-band. That left 1 MHz. Evidently that was viewed as not being sufficiently high for good image rejection at the higher HF frequencies. So for frequencies above 8 MHz, an initial IF of 4 MHz was used. Thus, above 8 MHz, the receiver was triple conversion, with IFs of 4 MHz, 1 MHz, and 100 kHz. Below 8 MHz, it was double-conversion, with IFs of 1 MHz and 100 kHz. The 100 kHz final IF was, at the time, normal for SSB/ISB HF receivers.

Marconi N2020 Block Schematic

 

Different again was the Marconi N2050/Mimco Apollo marine main receiver of 1970. This was double conversion in the range 1.4 to 28 MHz, with IFs of 1.1 MHz and 100 kHz, and single-conversion (100 kHz) IF in the range 15 kHz to 1.4 MHz, except for the 65 to 140 kHz range, which was double conversion (1.1 MHz and 100 kHz IFs). The need for continuous “search” manual tuning capability, plus fully tracked, tight selectivity front end tuning likely pointed to the use of a traditional valve-era configuration, with 10 switched bands covering the whole range. The extra high stability required for marine SSB was provided, on eight marine HF bands only, by a simple synthesizer coupled with a relatively narrow-span interpolation VFO. In a way, the 8 HF marine bands were somewhat analogous to bandspread bands on a conventional receiver.

The 1.1 kHz 1st IF was unusual. Previously, where the choice was not otherwise constrained, Marconi had used 1.6 MHz for receivers tuning down to 2.0 MHz, and 1.2 MHz for receivers tuning down to 1.5 MHz. In both cases the 1st IF was 80% of the lowest tuning frequency. Applying the same ratio to a 1.4 MHz tuned frequency points to 1.12 MHz, which could be rounded down to 1.1 MHz. So on that basis, it was a reasonable number. More than that though, the second mixer used a 1 MHz oscillator input to mix down from 1.1 MHz to 100 kHz. That 1 MHz came from the synthesizer, simply divided down from the 8 MHz master oscillator. That would have been another reason to use a 1.1 MHz 1st IF. There was evidently some flexibility in the 1st IF choice, unlike the N2020 case, where 1 MHz was otherwise determined, and so it was necessary to synthesize a 1.1 MHz feed for the 2nd mixer. The synthesizer is highlighted in yellow in this diagram:

Marconi Apollo Block Schematic

Released at the same time as the N2050/Apollo was the Marconi Nebula, more-or-less as a “junior” partner. The Nebula was a rebadged Eddystone EC958/5, the marine version of the EC958. It was also a 10-band, tracking-tuned receiver, covering 10 kHz to 30 MHz. A high stability mode in the range 1.6 to 30 MHz was provided by a Wadley Loop. It was triple-conversion above 1.6 MHz, with IFs of 1335 nominal (range 1335 to 1235 kHz), 250 kHz and 100 kHz. Below 1.6 MHz, it was either double-conversion (250 and 100 kHz IFs) or single-conversion (100 kHz IF).

The 1335 kHz nominal 1st IF appears to have derived from established Eddystone practice. The earlier 830/7 valved receiver had a 1st IF of 1350 kHz, tunable over the range 1250 to 1450 kHz, this providing for incremental tuning on the ranges above 1.5 MHz. In the case of the EC958, it seems likely that the 1335 kHz number, with one-sided adjustment to 1235 kHz, was chosen to suit the specifics of the Wadley Loop. But whereas the 830/7 went direct from 1350 to 100 kHz, the EC958 had an intermediate step of 250 kHz. The Wadley Loop specifics might have pointed to this approach, in terms of generating the feed to the 2nd mixer with minimum clashes. Also, it allowed sideband inversion in the 250-to-100 kHz conversion by having both infradyne and supradyne options. That would have been difficult in say a 1335-to-100 kHz conversion, given the nature of the 2nd mixer feed. 250 kHz was evidently suitable for the purpose, and not only that, it was an established IF, albeit mostly used for final processing in SSB/ISB receivers where very compact filters were desired.

The 2nd mixer feed was generated either by a 935 kHz crystal oscillator mixed with the output of a narrow-range VFO, or, in the high-stability mode, by a 935 kHz signal from the Wadley Loop, similarly mixed. In that mode, the 1st VFO was “locked” in 100 kHz increments. This was a conventional Wadley Loop in that one of its legs overlapped the 1st IF path. It was also of the pre-mixer type, like both in the Hydrus.

Marconi Nebula Block Schematicx

The loop is highlighted in yellow. A subsidiary AFC function acting on the 1st VFO, highlighted in green, kept the 935 kHz signal on-frequency, needed because the Wadley Loop was narrow-band.

The AFC system was a true feedback loop. The Wadley Loop I should imagine was so-described because its key components do form a loop. But the signal flow within that loop is not unidirectional, so it is not a true loop in the narrower definition of the term.

The four Marconi receivers mentioned, the Hydrus, the N2020, the Apollo and the Nebula all had a 100 kHz final IF, at the time the customary number for final processing in commercial/industrial SSB/ISB receivers. But their earlier IFs were largely circumstantial, dictated in part by choices made for the overall circuit configurations, and to some extent by established maker practice. The available information gives some pointers to the reasons for their selection. Otherwise, it is necessary to derive plausible (but not necessarily correct) explanations by working backwards, as it were.

Cheers,

Steve

 
Posted : 16/06/2021 8:46 am
Synchrodyne
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There was a departure from the 100 kHz final IF in the Marconi H2900 receiver of 1970, which chronologically was in the same bracket as the above-mentioned 100 kHz cluster. The H2900 was described by Marconi as a “breakthrough” receiver, with a synthesizer that tuned in 1.0 Hz steps. Basically it was a one-box, dual diversity point-to-point ISB receiver, with a tuning range of 1.25 to 30 MHz. It was of the triple-conversion, upconversion type, with a fully-tuned front-end. The 1st IF was 79.8 to 81.8 MHz, or 80.8 ± 1.0 MHz. The 2nd was 30 MHz, and the 3rd and final IF was 2 MHz, an unusual number at which to do the sideband and carrier filtering.

With upconversion HF receivers, the 1st IF appears to have been a matter of individual maker/designer choice, inter alia related to synthesizer design. For example, the higher the IF, the smaller the synthesizer swing ratio. Given that the 1st IF would be above 30 MHz by a lesser or greater amount, then if the final IF were to be 100 kHz, direct conversion to that low number might have given rise to image issues, indicating that an intermediate, 2nd IF was desirable. In that case, the choice of the 2nd IF also was also an individual maker/designer issue. In the case of Wadley Loop receivers, the possibilities were predetermined by the loop characteristics. And in some cases, the 1st and 2nd IFs were related by the need to obtain from the synthesizer a convenient oscillator feed to the 2nd conversion mixer.

In the H2900 case, one might suppose that direct conversion from 80.8 to 2 MHz was probably feasible. However, it would seem that its triple-conversion nature was an integral part of the tuning process, in that the 1st IF was tunable over a ±1 MHz range. That the 2nd IF was a relatively high number, 30 MHz, as 2nd IFs go, may well have been required by synthesizer considerations. The oscillator feed to the 1st mixer was stepped in 2 MHz intervals, the smaller intervals down to 1.0 kHz being covered by the oscillator feed to the 2nd mixer. Conversion from 30 MHz direct to 100 kHz may not have been considered favourably, indicating that a higher final IF was required. At the time that the H2900 was designed, the later standard 1.4 MHz number was embryonic, so was probably not an obvious choice, so Marconi evidently chose its own number to suit, namely 2 MHz. This may also have been related to synthesizer design, bearing in mind that the oscillator feed to the 2nd mixer also came from the synthesizer, as did the 2 MHz carrier insertion oscillator feed for the SSB demodulators. Given the one-box nature of this receiver, a frequency for which compact selectivity filters could be used was also likely to have been a desideratum.

The H2900 though did not set a precedent when it came to following Marconi receivers. It may be noted though that the 2 MHz final IF of the H2900 had a precedent in the 2 MHz 1st IF of the MST receivers. And the 30 MHz 2nd IF later turned up as the 1st IF of the McKay Dymek HF receivers, starting with the DR22 circa 1977. These were triple conversion, upconversion, with 2nd and 3rd IFs of 10.7 MHz and 455 kHz, respectively.

In 1976 the H2540 arrived, being a one-box MF/HF General Purpose and point-to-point ISB receiver, covering the range 15 kHz to 30 MHz, and intended for remote as well as local control. This was of the double conversion, upconversion type, with a 1st IF of 68.6 MHz and a 2nd IF of 1.4 MHz, the latter by then the new norm for HF SSB/ISB receivers. The exact number for the 1st IF, 68.6 MHz, derived simply from the choice to use an existing synthesizer, namely that used in the contemporary H1540 transmitter drive. In the H2540 case, the synthesized oscillator input to the 1st mixer moved in 1 kHz increments, so there was no need for an incrementally tuned 1st IF.

The H1540 transmitter drive does provide some insights to the H2540 IF choices. In this case, the SSB/ISB modulation was done at 1.4 kHz, indicating that the “new” receiver final IF was also used at the transmitting end. This was then translated to 68.6 MHz, and from there to an output in the 1.5 to 30 MHz range. The Marconi commentary in respect of the 1.4 MHz generation frequency was:

“The choice of 1.4MHz for the sideband filters is complex, and is governed in part by the following considerations:

“The frequency of the filters should be outside the h.f band, and as high as possible, consistent with obtaining an adequate carrier rejection. Generally the higher the frequency, the easier is the design of the first mixer and i.f amplifier in the frequency translator. It is also important that the heterodyne frequency for the modulator is easily derived from the master oscillator. The chosen frequency should be well within the range of the crystal filter manufacturers' current techniques, so that high-quality filters with an adequate passband phase characteristic can be manufactured for high-speed data transmissions.”

For many practical purposes, the HF band began at 1.5 MHz (all though formally it began at 3 MHz), so 1.4 MHz was about as high as one could go.

Then in respect of the 68.6 MHz number, Marconi said:

“It will be seen that an intermediate frequency of 68.6 MHz has been chosen. This frequency is dictated primarily by the needs of the second mixer which has an output frequency in the range 1.5-30 MHz. The choice of input frequency is limited by a number of factors. The upper frequency limitation is that of the synthesized local oscillator which has been taken as 99.999 MHz in order to avoid the complication of introducing a 100 MHz decade into the synthesizer control loops. The lower frequency limit was chosen so that the unwanted third order mixing products appearing at the output would be greater than about 38MHz which is well outside the HF band.

“A further constraint is that the intermediate frequency should be low enough to permit the use of high selectivity bandpass filter having good attenuation characteristics either side of its centre frequency. This is to ensure that noise generated in circuits before the filtering, does not contribute significantly to the wide-band noise content of the drive output.

“The first mixer has a signal input frequency of 1.4 MHz, as discussed earlier in the section dealing with the modulator. It is convenient if the heterodyne frequency is a multiple of the 5MHz standard frequency to which the whole drive system is locked and in fact, 70 MHz has been chosen. The output frequency is, of course, the intermediate frequency referred to above.”

This does show clearly how the seemingly strange number of 68.6 MHz came about. And essentially the same arguments may be marshalled in reverse in respect of the H2540 receiver.

It may be added that the 70 MHz frequency used for the heterodyne frequency also easily divided down to provide the 1.4 MHz number required for SSB carrier reinsertion at demodulation.

An observation that could be made about the 1.4 MHz final IF is that for dual-conversion receiver purposes at least, it did not necessarily have to be outside of the HF band. Final IFs that were in-band were not that unusual, and for example the H2900 had a 2 MHz final IF. But 1.4 MHz was also used for single-conversion spot-frequency receivers, in which case it did have to be outside the HF band. As discussed upthread, whilst 1.4 MHz did have a prior history of use, its arrival as a de facto standard appears to have coincided with the impending adoption of SSB for marine communications purposes, with Redifon being an early user, and with the filter makers offering a full range for that frequency. The use of the same number for transmitter drives, and single-conversion and double-conversion receivers was certainly quite rational.

Cheers,

Steve

 
Posted : 27/06/2021 1:25 am
irob2345
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Receivers in VHF land mobiles (i.e. taxi, police) in the 1960s commonly used a double conversion design with a 1st IF of 10.7MHz followed by a 2nd IF on 455KHz. Later, with the availability of crystal filters and silicon transistors, they became single conversion to 10.7MHz followed by untuned IF gain. These radios worked extremely well, pulling signals in the 0.2uV range out of the noise. I used them for many years. Most VHF air radios still use this design.

 
Posted : 27/06/2021 4:46 am
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Synchrodyne
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Thanks for that!

10.7 MHz (dating from 1945) and 455 kHz (standardized in 1937, but in use earlier than that) were probably the most widely used broadcast receiver standard IFs, and were frequently appropriated for non-broadcast applications.

I suspect too that the 10.7 MHz, 455 kHz pairing was probably the most common IF combination for double-conversion receivers of all types. Even when regular AM receivers drifted from 455 kHz et al to 450 kHz, the double-conversion combination mostly stayed at 10.7 MHz, 455 kHz.

Regarding land-mobile equipment, the Pye R401 (AM) and R402 (FM) receivers were interesting cases, in that both had PLL synchronous demodulators. They were described as single-conversion, with 10.7 MHz IF. But perhaps one could also describe them as double-conversion, with a 10.7 MHz 1st IF and a zero Hz 2nd IF!

Cheers,

Steve

 
Posted : 27/06/2021 10:25 pm
Synchrodyne
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Further to my recent post on the Marconi H2900 and H2540 receiver IF systems, here are their respective block schematics: 

Marconi H2900 Block Schematic

 

Marconi H2540 Block Schematic

 

Cheers,

Steve

 
Posted : 27/06/2021 10:31 pm
Synchrodyne
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Returning to the Eddystone EC958/Marconi Nebula, I suspect that one reason for its triple conversion layout was that its sub-HF coverage extended down to 10 kHz. Thus the desired final IF of 100 kHz would be in-band for the tuning range that included 100 kHz, so that an alternative number would be required for single-conversion in that tuning range at least. The 1335 kHz 1st IF was associated with the incremental tuning facility and the Wadley loop available for the HF band 1.6 to 30 MHz, so was probably not convenient to use for one of the LF bands. That indicated the need for a middle IF, between 1335 and 100 kHz, and adequately above the top of the tuning range containing 100 kHz, namely 53 to 126 kHz. 250 kHz fitted the bill. Concomitantly, the range containing 250 kHz, namely 125 to 295 kHz, was converted directly to 100 kHz.

Also, the use of a middle IF kept the Wadley loop clear of the final IF, and as previously mentioned, allowed the final conversion to be switched between infradyne and supradyne for sideband inversion. For the drift correction to work in this case, the second conversion needed to be supradyne. The second conversion oscillator feed was in the range 1485 to 1585 kHz, converting a 1st IF range of 1235 to 1335 kHz to 250 kHz. One may see that the 250 kHz choice also ensured that the second oscillator was kept (just) below the 1.6 MHz bottom of the HF range. In fact it looks as if Eddystone’s established 1350 kHz IF number was shifted downwards slightly to provide some margin under 1.6 MHz. The one-sided incremental tuning, downwards from 1335 kHz, rather than the customary two-sided approach may also have been part of keeping the second oscillator feed below 1.6 MHz.

I think that some validation of the foregoing reasoning is found by looking at the following Eddystone 1830 model, a simpler receiver using some of the EC958 modules, and which was the direct replacement for the 830. It was double conversion above 1.5 MHz with IFs of 1350 ± 50 kHz and 100 kHz, with infradyne second conversion. From 1.5 MHz down to its lowest tunable frequency of 120 kHz, it was single-conversion, with 100 kHz IF. Thus, absent the complication of the high-stability Wadley loop, Eddystone had returned to the pattern used with the 830, which was double-conversion above 1.5 MHz, with 1350 ± 100 kHz 1st IF followed by infradyne conversion to 100 kHz, and single-conversion, to 100 kHz for the range 300 to 1500 kHz.

Before the 830 was the 910/1 marine receiver, which was double conversion with IFs of 1400 ± 50 kHz and 85 kHz, the latter also an established final IF for communications work, although one which faded out in favour of 100 kHz quite early on. The 910/1 tuned 1.5 to 30 MHz and 375 to 525 kHz. It looks as if the 1400 kHz nominal 1st IF was moved to 1350 kHz nominal for the 830 in order to allow for a ±100 kHz incremental range instead of ±50 kHz, whilst still keeping the range safely below 1.5 MHz. As mentioned a long way upthread, the 1400 kHz 1st IF had been used previously for the Pye/Rees Mace CAT marine receiver.

The 1350 kHz/100 kHz combination was used again by Eddystone in the 1837/1838 receivers of 1977. In this case the incremental tuning range was quite small, 1350 ± 10 kHz. One could say that there was an inertial effect here, in which 1350 kHz or thereabouts was carried over through several receiver generations. Thus it is sometimes desirable to look at the history in any retroactive “outside-in” analysis of receiver IF choices.

Cheers,

Steve

 
Posted : 22/07/2021 8:05 pm
Synchrodyne
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As mentioned previously, one area in which IF numbers appeared to be somewhat random was in respect of the 1st IF of upconversion HF receivers, where they usually varied by make and sometimes amongst different models from the same makers. In the early days of synthesized receivers, say the later 1960s and early 1970s, the available synthesizer technology favoured keeping the 1st IF as low as reasonably possible, albeit adequately clear of the 30 MHz upper end of the HF band. Thus numbers around 35 MHz or so were found. With improving synthesizer technology, the numbers tended to move upwards. Perhaps the fact that a higher 1st IF meant a lower 1st oscillator range, in geometric terms, was considered advantageous. With these higher numbers, the reasons for the choice is probably not easily deducible by inspection; designer comment might be necessary. With the early synthesizer designs, though, a certain amount of “outside-in” analysis is possible.

One early example is the Plessey PR155 group, dating from 1966, and including the later PR155n variants. These were all triple conversion, with IFs of 37.3 MHz, 10.7 MHz, and 100 kHz. They probably just predated the advent of the 1.4 MHz final IF for SSB processing, hence the use of the well-established 100 kHz number. That being the case, the step from somewhere around 35 MHz to 100 kHz was probably too large, so a middle IF was needed. 10.7 MHz was the obvious choice, being a widely used standard number with a very wide range of filters available, including for HF applications. I suspect that there would have to have been a very good reason to do otherwise.

The block schematic for the PR1553 is shown, as it happened to be on hand:

Plessey PR1553 Block Schematic

In the PR155 group, the 1st, 2nd, and 3rd mixer oscillator frequencies were synthesized from a 1 MHz harmonic comb derived from a 1 MHz master oscillator. In the case of the 1st oscillator, interpolation between the 1 MHz points was done by a nominally 1 MHz range VFO, whose output was mixed with the selected 1 MHz harmonic and then used to control the 1st VCO via a PLL.

The harmonic range used was 35 to 64 MHz. Given that the harmonic comb as generated had to be filtered to suppress those members within the HF band (30 MHz and lower), I think we may suppose that 35 MHz was the lowest usable harmonic.

Then in the interests of stability, the interpolating VFO would have to be kept to a reasonable range, say around 1.5:1. A reference point is the Racal Wadley loop receivers, where the interpolating oscillator covered 2.1 to 3.1 MHz. So 2 to 3 MHz is a reasonable approximation. The VFO output could be added to or subtracted from the 1 MHz harmonics, but for the 35 MHz harmonic, subtraction probably moved the output uncomfortably close to 30 MHz. That pointed to addition, suggesting that the 1st IF was going to be somewhere around 37 MHz.

Conversion from the 1st IF to the 2nd IF of 10.7 MHz using a 1 MHz harmonic as the 2nd oscillator frequency implied that the 1st IF was going to be non-integer. Infradyne conversion was outruled, since this would have required the 27 MHz harmonic, which was necessarily suppressed by the harmonic comb filter. Thus supradyne conversion was necessary, and using the 48 MHz harmonic produced the 37.3 MHz 1st IF for a 2nd IF of 10.7 MHz.

In turn that established the required range for the interpolation VFO, nominally 2.3 to 3.3 MHz, but with some “overscan” at 2.2 to 3.4 MHz, giving a tuning ratio of 1.55:1. Thus was provided a 1st VCO range of (35 + 2.2) = 37.2 MHz to (64 + 3.4) = 67.4 MHz).

The 3rd conversion, 10.7 MHz to 100 kHz, was done by dividing down from an appropriate 1 MHz harmonic. For the original PR155, it could be switched between infradyne and supradyne in order to effect sideband inversion, using 10.6 and 10.8 MHz oscillator feeds respectively divided down from the 53 and 54 MHz harmonics. The PR1553 looks to have been slightly different.

One could say that given the synthesis scheme employed and the choices of the 2nd and 3rd IFs, the 37.3 MHz 1st IF was the lowest that reasonably could be employed.

The following Plessey upconversion HF receiver, the PR2250 of 1979, was double conversion, with IFs of 65 and 1.4 MHz. It was also fully synthesized. One may deduce that by then, there was no great difficulty in synthesizing a 65 to 95 MHz 1st VCO output. Possibly that range was selected because it was a little under the 100 MHz point, and so avoided the need to move from one “hectade” as it were, to another. Also, synthesizing a non-integer 2nd oscillator output, in this case 63.6 MHz, was evidently not a problem, either.

Cheers,

Steve

 
Posted : 22/07/2021 9:57 pm
Synchrodyne
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Somewhat more difficult to analyze than the Plessey PR155 was the Redifon R550/R551 HF receiver of 1969. However, a recent discussion on UKVRR has provided some helpful background.

The R550/R551 was an upconversion receiver with double conversion. The 1st IF was 38.0 MHz, and the 2nd IF was 1.4 MHz. It was probably one of the first receivers to use the 1.4 MHz final IF for SSB processing.

Being a little later than the Plessey PR155 receiver, it had a more complex synthesizer. The oscillator feed to the 1st mixer was synthesized in 10 MHz, 1 MHz, and 100 kHz steps, with a VFO interpolating between the 100 kHz points. The reference for the synthesizer was a 1.4 MHz master oscillator that also served as the carrier insertion oscillator.

Apparently in the interests of easing the difficulties of synthesizer design, the 1st VCO output was not fed directly into the main PLL, but was first mixed down to a much lower frequency, in this case 3 to 32 MHz. That range, just over a decade, might well have been the lowest reasonably possible.

Redifon R551 Block Schematic

One may see that the 1st VCO and the mixed down feed to the PLL would both occupy 30 MHz blocks of spectrum. The mix-down would need to be infradyne in order to preserve the directional sense, so there would need to be room between the two blocks for the mix-down signal. Thus the mix-down signal was going to be adequately above 32 MHz, and in turn the 1st IF adequately above that.

Although the mix-down signal could come from an independent crystal oscillator, by using the same crystal oscillator for the second conversion, its output could be included in a drift-correcting loop. That though required that the second conversion be supradyne, in turn implying that the oscillator needed to operate at 1.4 MHz above the 1st IF and so also 1.4 MHz above the lowest 1st VCO frequency. That made it too high in its raw state for the mix-down function, where it needed to be lower than the lowest 1st VCO frequency. Thus it needed some mixing down for this purpose, but done in a way that did not introduce another opportunity for drift.

That was done by separating the synthesizer into two parts, one doing the 10 and 1 MHz steps, and the other doing the 100 kHz steps and the interpolation between them. The output of the latter, covering a nominally 1 MHz range was mixed with the crystal oscillator output, their difference being used for the mix-down.

The mix-down signal was chosen to be 35 to 36 MHz. Presumably this was the lowest reasonable range given that the mixed-down output was 3 to 32 MHz. This determined the 1st VCO range, which was (3 + 35) = 38 MHz) to (32 + 36) = 68 MHz. Thus the 1st IF was 38.0 MHz. And the 2nd conversion crystal oscillator frequency was (38.0 +1.4) = 39.4 MHz.

From this the required output from the 100 kHz step synthesizer and VCO could be calculated as being between (39.4 – 36.0) = 3.4 MHz to (39.4 – 35.0) = 4.4 MHz. That was achieved by having the 100 kHz step synthesizer operate between 4.1 and 5.0 MHz, and the interpolating VFO operate between 600 and 700 kHz, and mixing to form their difference, (4.1 - 0.7) = 3.4 MHz to (5.0 – 0.6) = 4.4 MHz. Presumably these ranges were chosen as providing the best trade-off when it came to stability and the avoidance of potentially harmful spurs.

That I think shows that given the choice of a mix-down synthesizer and the 1.4 MHz 2nd IF, then 38.0 MHz was about the lowest 1st IF reasonably possible.

The following Redifon upconversion HF receiver was the R1000 of 1978. This was fully synthesized, but retained the same 38.0 and 1.4 MHz IFs as the R550/R551. Give that by then, other makers of similar-class receivers were using – apparently without difficulty – 1st IFs in the 60 MHz range, Redifon’s choice to stay with 38 MHz does look as if it contained an element of inertia. In this case, the 2nd conversion was infradyne, using a synthesized 36.6 MHz oscillator feed.

Cheers,

Steve

 
Posted : 22/07/2021 10:22 pm
Synchrodyne
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By the time that the Racal RA1772 HF receiver arrived on the scene, c.1972, synthesizer design had evidently advanced to the point where the “frequency gymnastics” seen in the Plessey PR155 and Redifon R550/551 were no longer needed, and the 1st VCO output could be fully synthesized. The RA1772 was double conversion, with IFs of 35.4 and 1.4 MHz. The second conversion was infradyne, with a synthesized oscillator feed of 34.0 MHz.

Racal RA1772 Block Schematic

Nonetheless, one may infer that at that time, it was still desirable, from the cost and “doability” perspectives, to keep the 1st IF as low as reasonably possible, and that it was still preferable to generate an integer 2nd oscillator second feed, meaning that the 1st IF would be non-integer. If we take 35 MHz as being about the lowest 1st IF that was adequately clear of 30 MHz, then the RA1772 was close to it, offset slightly upwards to accommodate the need for the integer 2nd oscillator. That it got closer to 35 MHz than say the Plessey PR155 and Redifon R550/R551 could have been because it did not require a larger margin for the “gymnastics”.

The following Racal receiver, the RA1792 “Anglo-American” of 1979, had IFs of 40.455 MHz and 455 kHz. The latter was probably to suit American requirements, given that the American market was the primary target. That the 1st IF showed only a moderate upward movement might have been because this was something of a seriously cut-cost receiver. Apparently, it had quite a different synthesizer design as compared with the RA1772, but there might have still been some cost advantage in not going too high for the 1st IF. It also looks as if offsetting the 1st IF to allow for an integer 2nd oscillator output (40 MHz) might have provided some cost advantage, at a time when others did not see that as necessary.

Racal RA1792 Block Schematic

Eddystone was a late entrant to the professional upconversion HF receiver field, with its 1650 series of 1984. That lateness may have had something to do with the success of its EC958 series. The 1650 was double conversion, with IFs of 46.205 and 1.4 MHz. That 46.205 MHz number does look rather odd. A closer inspection shows that the main synthesizer had operated with 2 kHz steps, and that the interpolation down to 5 Hz steps was done by the second oscillator, which was thus adjustable over a 2 kHz range. Possibly it was done this way to facilitate the use of a single PLL in the 1st oscillator synthesizer.

Eddystone 1650 7 Block Schematic

But by itself that does not explain the oddity of the 46.205 MHz 1st IF number. For that, a closer look at the second oscillator synthesizer is needed. The VCO output was mixed down with a reference signal to provide a 4 to 6 kHz input to the controlling PLL. Presumably this was the best way to provide 5 Hz steps over a 2 kHz range. The reference signal used was the 8th harmonic, 44.8 MHz, of the 5.6 MHz synthesizer reference oscillator. The latter choice appears to have been made on the basis that simple division by 4 resulted in the required 1.4 MHz carrier insertion frequency.

Eddystone 1650 7 Synthesizer

Returning to the 2nd oscillator mixdown, for the 4 to 6 kHz output with a 44.8 MHz reference frequency, the VCO output must range from 44.804 to 44.806 MHz, with a mean of 44.805 MHz. Given the 1.4 MHz 2nd IF, this in turn implied a nominal 46.205 MHz 1st IF for infradyne conversion, which was used. (Supradyne conversion would have resulted in a 1st IF of 43.205 MHz.

Thus the apparent oddity of the 46.205 MHz 1st IF is accounted for, or, in other words, why it was that and not say 46.0 MHz exactly. But its actual placement is not explained. The use of different 5.6 MHz harmonics for the 2nd oscillator mix-down would, for the infradyne case, have allowed a range of different 1st IF numbers, for example from 35.005 MHz for the 6th harmonic to 68.605 MHz for the 12th harmonic. One may infer that there were other reasons, not readily apparent, for the actual choice.

This rather small sample survey of synthesized upconversion professional HF receivers has shown the following range of IFs:

Plessey PR155 et al:     37.3 MHz      10.7 MHz     100 kHz
Redifon R550, R551:    38.0 MHz       1.4 MHz
Racal RA1772              35.4 MHz       1.4 MHz
Marconi H2540            68.6 MHz       1.4 MHz
Redifon R1000            38.0 MHz       1.4 MHz
Plessey PR2250           65.0 MHz       1.4 MHz
Racal RA1792              40.455 MHz   455 kHz
Eddystone 1650           46.205 MHz   1.4 MHz

The 1st IFs vary by make and by model, QED. The 2nd (and where used, 3rd) IFs are all standard numbers.

Cheers,

Steve

 
Posted : 23/07/2021 3:06 am
Synchrodyne
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The Marconi H2900 point-to-point HF communications receiver was mentioned upthread. I have since found an article (*) which provided some detail about the choice of the triple-conversion approach and the IF values, something not always included in HF receiver descriptions. It is worth including some of that in this thread, , and it is simplest to simply quote from the pertinent part of the Marconi article, with comments inserted after each paragraph.

“Choice of Intermediate Frequencies

“The choice of intermediate frequencies needs justification where one must consider the advantages and disadvantages of single, double and triple superheterodyne. Advances in the design of crystal filters allows the construction of sideband filters working at 2 MHz. Choosing the final intermediate frequency (IF3) of 2 MHz eliminates the necessity of additional conversion to the region of 100 kHz.”

That the need to for a final IF of 100 kHz (or some other similarly low frequency) to facilitate sideband filtering was disappearing was demonstrated by the widespread adoption of the 1.4 MHz final IF, with Redifon having been one of the first so to do. At the time that the H2900 was in design, it may not have been clear that 1.4 MHz would be the future norm. Be that as it may, Marconi did not give a rationale for the 2 MHz number as such. Perhaps it represented the upper limit of what was possible with the newer sideband filters. Or perhaps it was because 2 MHz was already in the Marconi lexicon, for example as the 1st IF for the MST point-to-point receivers. Marconi did though choose 1.4 MHz for the later H2540 model.

“In a receiver with a single i.f. the radio frequency is converted directly to a relatively low intermediate frequency. Even if the latter is as high as 2 MHz, which will inhibit reception round about 2 MHz, in order to avoid image reception the requirement on the input filtering is impossible. This scheme must be rejected outright.”

That position does seem to be a bit severe. For example, the established MST receivers had a 2 MHz 1st IF. The Marconi N2050 (Apollo), which was more-or-less a contemporary of the H2900, had a 1.1 MHz 1st IF. And the Eddystone EC958, which just preceded it, had a 1.335 MHz (nominal) 1st IF. In the latter case, Eddystone had considered whether its design objectives could be met with a sub-1.6 MHz 1st IF before proceeding on that basis. Perhaps with the H2900, Marconi was looking for even better image rejection than any of those provided. On the other hand, the marine requirements for front-end selectivity, met by both the N2050 and the EC958/5, were regarded as being more severe than most of those applicable to HF receivers.

“The next, less simple arrangement would be to use a high first i.f. above the highest receiver frequency, followed by a relatively low second i.f. The situation is considerably improved in this case but closer examination indicates that the amount of isolation between the first and second frequency changer is again almost impossible to obtain in practice. Lack of this isolation will cause image receptions at twice the second i.f. Reception of two input signals spaced at the second i.f. will also be present. This arrangement also requires the heterodyne oscillator to be continuously variable over the 30 MHz band from a frequency synthesizer. Although it is possible to combine the two high-frequency signals generated in the synthesizer into one continuously variable signal, the mixing process involved would be detrimental to the reciprocal-mixing performance of the receiver. This is another reason for avoiding a dual frequency-conversion system.”

Dual-conversion of the type rejected became the norm, and in fact was adopted by Marconi for the H2540. Thus it looks as if Marconi was being very careful about isolation between the 1st and 2nd IFs. That the H2900 was doing in one box what had previously required a 7 ft rack may have been a factor, also that the H2900 was a dual-diversity receiver, whereas the other single-box units of the period were simply single receivers. Synthesizer performance was a valid concern at the time. Redifon had used a quite complex process in its R550/R551 upconversion receiver, and that was probably lower down the overall performance ladder. But that changed quite quickly, as evidenced by the Racal RA1772 of the early 1970s.

“Triple-frequency conversion is thus necessary. Restraints on the if. and heterodyne frequencies can now be placed in the usual manner taking into account harmonics, particularly harmonics of the heterodyne signals to the high orders. The analysis led to the five possible selections (Table 2).”

Table 2

“The selected schemes were then investigated in greater detail. I.p.s up to the 15th order were plotted and weighted according to their order and fractional separation relative to the intermediate frequencies. They were also checked by a computer. No particular scheme is outstandingly advantageous but generally of the similar pairs 2 is preferred to 1, and 4 to 5 as the lowest-order spurious frequencies are further off-tune. On the remaining possibilities 2, 3, 4, scheme 3 is an inverting mode which has some slight disadvantages for a drive application and 2 requires the highest frequency for the variable oscillator. Scheme 4 is therefore used as the best compromise. This scheme also has the possible advantage that all the intermediate and oscillator frequencies employed are either outside or near the extreme limits of the h.f. band.”

The second IF, at 30 MHz, was at the upper edge of the HF band, although probably most band activity was below around 28 MHz. The 2 MHz 3rd IF was in-band, given the 1.25 to 30 MHz tuning range. So would have been the emerging 1.4 MHz de facto standard. In general, an in-band final IF does not seem to have been an issue for professional HF receivers of that generation. The use of a crystal-controlled first oscillator appears to have dispended with the issue of its having a frequency range that crossed the 100 MHz point, something evidently preferably avoided with some synthesized approaches.

Marconi H2900 Signal Path
Marconi H2900 Frequency Control

Cheers,

Steve

(*) B.M. Sosin; H.F. Communication Receiver Performance Requirements and Realization; The Radio & Electronic Engineer; 1971 July.

 
Posted : 18/11/2021 6:49 am
Nuvistor
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@synchrodyne 

Thanks Steve.

Another excellent post.

Frank

 
Posted : 18/11/2021 8:27 am
Synchrodyne
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Thanks for the feedback. It was fortunate that Marconi did publish its rationale for the H2900 receiver IF choices. That one would have been very difficult to work out by “reverse engineering”.

Cheers,

Steve

 
Posted : 28/11/2021 6:31 am
Synchrodyne
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Very early on in this sequence, in fact before ‘Radio IFs’ was split off from the ‘TV IFs’ thread, “Anonymous” mentioned the use of a 6.75 MHz FM IF for some French and German receivers (valved) of the early 1960s. (See: https://www.radios-tv.co.uk/community/black-white-tvs/television-receiver-intermediate-frequencies/#post-41483.)

I have recently found another, earlier reference to FM IFs in the 6 MHz range. This was in a 1957 March British IRE paper “Principles of Design of Battery Operated Frequency Modulation Receivers”, by R.A. Lampitt and J.F. Hannifan, both Ever Ready staffers. With the available battery valves, getting enough IF gain on FM was something of a challenge, particularly for portable receivers where keeping the valve count down was paramount. One option shown was double conversion for FM, with a 10.7 MHz 1st IF, and a 2nd IF of around 6 MHz, where more gain was possible. In the example circuit, the actual 2nd IF was 6.5 MHz.

Ever Ready 8 Valve Double Conversion FM AM

 

Thus, the “6 MHz” IF was chosen to facilitate higher gain with a given valve complement, presumably with the exact chosen number to be above the top end of the relatively 49-metre SW broadcasting band. A logical question is “why double conversion”. One possible reason is that with the simple front end, lacking an RF amplifier, it may have been thought better to stay with the standard 10.7 MHz number for the 1st IF, which ensured that all images were not just out-of-band, but well out-of-band in respect of the then UK FM band of 87.5 to 100 MHz. Another reason is that the DK92 valve, included as the AM frequency changer, also served as the 2nd frequency changer on FM. In other FM-AM implementations shown in the paper, a DK96 was used as the AM frequency changer, and bypassed on FM. Thus double conversion resulted in better valve utilization.

In the French and German receivers mentioned above were both valved, and both were single-conversion. Presumably 6.75 MHz was chosen over 10.7 MHz for IF gain reasons, and evidently their front ends, almost certainly including RF amplifiers, were considered to be robust enough on the selectivity front for the lower IF not to cause any undue problems.

Cheers,

Steve

 
Posted : 28/11/2021 9:34 pm
Synchrodyne
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As previously shown, by the late 1970s, a popular format for professional HF receivers was initial upconversion to a 1st IF of individual choice, followed by downconversion to a 2nd IF of 1.4 MHz for SSB, ISB, etc., processing. Bucking that trend was the Redifon R500 of 1981, which reverted to single-conversion with a 1.4 MHz IF. Redifon argued that good front end selectivity was an essentially element of overall satisfactory performance, and that with it, a single-conversion receiver was not only possible but also more economic. The details were covered in a paper by R.A. Barrs, “A Reappraisal of H.F. Receiver Selectivity”, published in Radio & Electronic Engineer, 1982 July, p.315ff.

Of course, Redifon had previously used single-conversion with a 1.4 MHz IF for its R499 receiver, but this was crystal-controlled, with plug-in fixed-tuned front end selectivity units. In the case of the R500, RF tuning was done by a motor-driven four-gang variable capacitor, controlled by a microprocessor, which kept it on-track with the synthesized oscillator.

Four gangs, in this case a bandpass input and a bandpass interstage, ahead of the mixer, was more than had typically been used for earlier HF receivers with 1st IFs in the 1 to 2 MHz range. Apparently, the cost-benefit trade-off precluded the use of another gang.

Cheers,

Steve

 
Posted : 28/11/2021 9:39 pm
Synchrodyne
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Here are a few examples of non-standard and unusual IFs, evidently chosen to suit specific circumstances.
 
Firstly, a look at a couple of aviation LF/MF/HF receivers from the valve era, namely the Marconi AD108 and AD118.
 
The usual frequency coverage of the AD108 was 260 to 510 kHz and 2 to 18.5 MHz, whilst the AD118 was 150 to 510 kHz and 2 to 18.5 MHz.  Both were single conversion.
 
The AD108 had an IF of 600 kHz, whereas the AD118 was 1 MHz.
 
Clearly, with single conversion, the IFs needed to be in the band 510 kHz and 2 MHz, and adequately away from the edges.  Evidently the positioning within that band was not overly critical, in that both had IFs that were multiples of 100 kHz.  In the AD108 case, possibly 600 kHz was chosen as being as close as reasonably possible to the lower edge of the available band in order to obtain the best IF gain and selectivity.  The AD108 had a single RF stage, with three-gang tuning, and image rejection at 18.5 MHz was quoted as 25 dB.
 
On the other hand, the AD118 had two RF stages with four-gang tuning, and an image rejection at 18.5 MHz of 60 dB.  Both the extra RF stage and gang, and the higher IF would have contributed to the improvement.  Perhaps here the upward IF movement was calculated to provide the increment of image rejection beyond that contributed by the extra tuning as required to meet the target.  If so, it appears that placing the IF at about the geometric midpoint of the available 510 kHz to 2 MHz band did the job.
 
Secondly, also from Marconi, some valve-era marine LF/MF/HF receiver examples.
 
The Mimco Mercury Type 1017 (MWT NS601) and Electra Type 1018 (MWT NS301) formed a complementary pair.
 
The Mercury covered 15 to 40 kHz and then 100 kHz to 4 MHz in four ranges.
 
For the 15 to 40 kHz range and for the two ranges covering 100 kHz to 640 kHz, it was single conversion, with an IF of 85 kHz.  For the two ranges covering 640 kHz to 4 MHz, it was dual-conversion, with IFs of 4.5 MHz and 85 kHz.
 
85 kHz was a standard (although perhaps not standardized) number, and logical given that it fitted into the 40 to 100 kHz coverage gap.
 
4.5 MHz was an ad hoc number though, probably chosen as the being the lowest that was adequately above the 4 MHz top end of the tuning range.  At the time, upconversion was not so common at HF, although it was commonplace, although not much commented upon for being such, at LF, particularly for the LW band in domestic receivers.  The “breakpoint” of 640 kHz may have been chosen, from the feasible tuning range distributions (each band had a tuning ratio of around 2.5:1) as about as high as was reasonable for use with a single conversion to 85 kHz whilst still providing adequate image rejection.  At the time this receiver pair was released (PMG approvals recorded in WW 1949 July), 4.5 MHz was emerging as the intercarrier sound frequency for US TV receivers, but it seems unlikely that this had any effect on Marconi’s choice of that number.
 
The Electra covered 250 to 510 kHz and then 1.5 to 25 MHz in four ranges.  It was single-conversion on all ranges with an IF of 690 kHz.  That looks as if it were an ad hoc number.  And the implication is that for example, rounding to 700 kHz, the nearest hectade break point, would not have been satisfactory.  One suspects that was done to avoid some deleterious spurs.  Given that the Electra and Mercury were usually installed as a pair, with minimal physical separation, there may have been a preference to avoid a multiple of 85 kHz, although 690 kHz was fairly close to the 8th harmonic, 680 kHz.  Similarly submultiples of 4.5 MHz may have been avoided.  That 690 kHz was towards the lower end of the available 510 kHz to 1.5 MHz band suggests that in a general sense, it was chosen to be no higher than needed to provide the desired image rejection, 40 dB at 25 MHz.  Both the Electra and the Mercury had four-gang front ends with two-stage RF amplifiers.
 
The Marconi Atalanta Type 2207C (MWT NS702) replaced the Mercury/Electra pair c.1956.  This covered 15 kHz to 28 MHz in 10 ranges, using a mix of single- and double-conversion.
 
It was single-conversion, to an IF of 85 kHz, for the range 15 to 25 kHz, and for the three ranges covering 100 to 800 kHz.
 
It was double conversion, with a 1st IF of 700 kHz and a 2nd of 85 kHz, for the range 25 to 100 kHz, and for the five ranges covering 800 kHz to 28 MHz.
 
Again, 85 kHz was a standard IF, and logical as it fitted between the 15 to 25 and 100 to 200 kHz ranges.
 
The 800 kHz top frequency for which single-conversion to 85 kHz was used was a convenient breakpoint in the general vicinity of a reasonable maximum.
 
The 700 kHz number looks as if it were ad hoc.  It was close to, but not the same as the 690 kHz IF of the Electra, but as it was part of a double-conversion system, there would have been a different set of spurs to consider.  Presumably it needed to have been comfortably away from any 85 kHz harmonics, since in this case the two frequencies co-existed in the same receiver.  Conversion from 700 to 85 kHz was infradyne, with an oscillator frequency of 615 kHz.  Also, of course, the 2nd IF needed to be adequately below the highest frequency at which single conversion was used, in this case 800 kHz.
 
Thirdly, an example of an LF receiver, the Eddystone 850 series.
 
It covered 10 kHz to 600 kHz in six ranges.  It was single-conversion and upconversion, with an IF of 720 kHz.  That was presumably an ad hoc number, chosen to be sufficiently above the highest tuning frequency that any IF signal getting back to the input would be well-rejected by the front end selectivity circuitry.
 
All of the receivers mentioned in this posting had at least an element of upconversion from the LF and lower MF (say the 1st octave, 300 to 600 kHz) bands:
 
Marconi AD108 260 to 510 kHz up to 600 kHz
Marconi AD118 150 to 510 kHz up to 1000 kHz
Marconi Mercury 15 to 40 kHz up to 85 kHz
Marconi Electra 250 to 510 kHz up to 690 kHz
Marconi Atalanta 15 to 25 kHz up to 85 kHz, and 25 to 100 kHz up to 700 kHz
Eddystone 850 10 to 600 kHz up to 720 kHz
 
Neglecting for the time being the 85 kHz IF case, then the upconversion IFs range from 600, through 690, 700 and 720, to 1000 kHz, with say (700 ±20) kHz being modal.
 
Eddystone also used the 720 kHz IF in some of its later receivers.
 
The EC10A2 was a marine derivative of the EC10 single-conversion MF/HF receiver, modified to have tuning ranges 300 to 550 kHz and 1.5 to 30 MHz.  Basis the 850 experience, the established 720 kHz IF was a logical choice for the MF segment, and evidently it was also unproblematical for the HF range.  (The EC10 itself had an IF of 465 kHz.)
 
The 1004 was the marine variant in the single-conversion 1000 series.  The latter normally used a 455 kHz IF for LF/MF/HF.  The 1004 tuned 150 to 535 kHz and 1.6 to 30 MHz.  Again, one could reasonably infer that the existing 720 kHz IF was seen as a logical and satisfactory choice.  In this case a ceramic IF filter was used, presumably not a standard catalogue item.
 
 
Cheers,
 
Steve
 
Posted : 11/08/2022 5:23 am
Synchrodyne
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As mentioned in the previous post, the Marconi Mercury used upconversion to 4.5 MHz for the range 640 kHz to 4 MHz.  It was probably possible to have used a 1st IF below 640 kHz, perhaps even 465 kHz, and still have obtained satisfactory image rejection results.  Perhaps though a reason for not doing so was to keep the 1st IF, and also the 2nd conversion oscillator frequency, out of any of the receiving frequency ranges, in which case upconversion was required.  Alternatively, the both the 1st IF and the 2nd conversion oscillator might have been placed somewhere above 530 kHz, on the basis that the range 530 to 1.5 MHz was not used for marine purposes, so was less critical.  But there might not have been a workable pair of frequencies, separated by 85 kHz, in the space from 530 to 640 kHz, with the higher of the two also sufficiently below 640 kHz.
 
In the case of the Marconi Atalanta, the notional less critical tuning range ran from 530 to 800 kHz, so there was room therein or both a suitable 1st IF (700 kHz) and the 2nd conversion oscillator (615 kHz).
 
Marine receivers in general were somewhat difficult propositions, in that the required frequency coverage went from approximately 15 kHz to 28 MHz, albeit with some gaps in the LF and MF ranges, although even so, there appeared to be a preference to cover at least the MF gap.  At the same time, there were fairly tight front end selectivity specifications that typically required some form of tunable bandpass facilities ahead of the first mixer.  As an alternative to single-conversion on the lower bands, and double conversion on the upper bands, another approach was to have single-conversion with a double-frequency IF strip, with the operating frequency switched according to the band in use.
 
An example was provided by the Redifon R50 receiver from c.1947.  The IF strip was switched to operate at either 110 or 465 kHz, both standard numbers.  110 kHz was used for the 13.5 to 26 kHz and 240 to 600 kHz ranges.  465 kHz was used for the 95 to 250 kHz range, and for 5 ranges covering 585 kHz to 32 MHz.  (There was a 26 to 95 kHz gap in coverage). 
 
 
Cheers,
 
Steve

 

 
Posted : 12/08/2022 3:01 am
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I have a vague recollection that 690kHz kept IF harmonics largely clear of the principal marine HF allocations, or something like that and that a 690kHz calibrator crystal had something to do with falling at band edges. Must check what these allocations actually were, and do a bit of maths!

Eddystone's gargantuan 700 marine receiver, another that offered no-gaps general coverage, also featured switchable-IF single conversion with 110kHz and 455kHz.

 
Posted : 13/08/2022 1:10 pm
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Beg pardon- the Eddystone 700 (15kHz to 31MHz) featured a higher IF of 465, rather than 455kHz- so even more like the Redifon.

 
Posted : 13/08/2022 2:45 pm
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It does seem to have been the case that some UK makers used the 465 kHz IF, standard for UK domestic broadcast receivers in the pre-Copenhagen Plan era, for some of their commercial/industrial HF receivers, in much the same way as American makers used 455 kHz.
 
Another user of 465 kHz was Marconi.  Its CR150 HF receiver was dual-conversion, with 1.6 MHz 1st and 465 kHz 2nd IFs.  This arrangement was also carried over to the later CR150/3 and CR150/6 versions.  The CRD150/4 diversity version had 1.2 MHz and 465 kHz IFs.  In this case the tuning range went down to 1.5 MHz rather than 2.0 MHz, hence the lower 1st IF.  The CRD150/4+SSR2 combination was Marconi’s early point-to-point SSB receiver; there was a 3rd conversion to 100 kHz in the SSR2 “add-on” part.
 
Eddystone was a “mixed bag” when it came to IFs in the vicinity of 465 kHz, as follows:
 
450 kHz 670 series, 680, 680X, 730, 740, 840 series, 940
 
455 kHz 1000 series, EB35 Mk III, 1570 series, 1590 series
 
465 kHz 700, 820, 870 series, 960, EB35, EB37, EC10
 
As random as it appears at first glance, one may actually impute some rationale to this pattern.
 
All of those receivers with the 450 kHz IF tuned down to 480 kHz on their MF band.  Presumably 450 kHz was the comfortable upper limit for the IF in that case.
 
The 870 tuned down to 540 kHz.  So it looks as if Eddystone saw the “standard” 465 kHz number as being an appropriate choice.
 
Evidently the same applied to the 820 FM-AM tuner, although by the time it was released, 470 kHz had become the UK norm for MW reception.  But the 820 looks very much as if it were developed by dipping into the existing parts bin.
 
In the case of the 700, the 465 kHz IF was used inter alia for the 585 to 1550 kHz and 95 to 250 kHz ranges, with 110 kHz used inter alia for 240 to 600 kHz.  So there was no need to depart from the 465 kHz standard number, it being well clear of the respective band edges.
 
The 960, effectively a transistor derivative of the 960, tuned down to 500 kHz, so again it looks as if 465 kHz was seen as an appropriate IF choice – no need to go down to 450 kHz.
 
Then that precedent was followed for the EB35, EB37 and EC10, all of which had 550 kHz as the lower end of the MF tuning range.
 
The 1000 series of 1972 introduced the 455 kHz IF to Eddystone practice.  By then, 465 kHz had faded away, and 455 kHz was generally recognized as the normal “low” IF for HF receivers.
 
Then 455 kHz was applied to the EB35 Mk III (a major rework of the original) and following that the 1570/1590 series.
 
 
Returning to late 1940s HF receivers, GEC used 455 kHz for the BRT400, apparently following American practice, although it used 465 kHz for domestic MF-HF receivers such as the BC5082/L.
 
The Copenhagen Plan resulted in 470 kHz being a better choice for UK MF reception than 465 kHz, and there was a general shift with domestic receivers.  But there was no compelling need to make the change for export-oriented receivers.  For example, Murphy made this comment in respect of its TA160 bandspread export receiver of late 1951:  “The early receivers will have their i.f. circuits adjusted to 465 Kc/s, but later models will have an i.f. of 470 Kc/s. This change has been made to standardize production procedure and, in general, no advantage can be claimed for one i.f. over the other; the i.f. circuits need not be re-adjusted for this reason alone.”
 
“Standardization of production procedure” probably meant that it had changed to 470 kHz for its domestic receivers, and it was just easier to do the same for its export models.
 
By way of comparison with similar receivers, Ekco used an IF of 460 kHz for its A182, and Pye used 470 kHz for its PE80.
 
Armstrong had used 465 kHz for its late 1949 revamped range of radiogram chassis, mostly export models, but some domestically oriented.  In 1954, it changed to 470 kHz for the FC48, essentially a domestic chassis, and for the EXP125C, primarily as export chassis but also promoted domestically, but stayed with 465 kHz for the other export models, such as the RF41, EXP119 and BS125.  Then quite oddly, after using 470 kHz for the AM side of its early AM-FM chassis, it jumped to 430 kHz c.1958.
 
 
Cheers,
 
Steve
 
Posted : 14/08/2022 11:26 pm
Synchrodyne
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The recently mentioned 700 kHz IF used by Marconi for its mid-1950s Atalanta marine receiver appeared again in the Marconi Monitor marine reserve receiver of the later 1960s.  This was a valved unit, intended to work from 24-volt DC supplies, and thus using some of the automotive radio low-HT valves (ECH83, EBF83, etc.)
 
Whereas the Atalanta had continuous coverage from 15 kHz to 28 MHz, the Monitor had broken coverage.  It also used a mix of single- and double-conversion, as well as being switched to TRF mode for the spot 500 kHz frequency.
 
In this case, 700 kHz was the only IF for frequencies below 3.8 MHz, and it was the second IF for frequencies above 3.8 MHz, for which the first IF was 2700 MHz.
 
Coverage was as follows:
 
165 to 235 kHz single-conversion
405 to 535 kHz single-conversion
500 kHz TRF
1.6 to 2.5 MHz single-conversion
2.5 to 3.8 MHz single-conversion
4.063 to 4.438 MHz double-conversion
6.205 to 6.525 MHz double-conversion
8.195 to 8.815 MHz double-conversion
12.33 to 13.20 MHz double-conversion
16.47 to 17.36 MHz double-conversion
22.00 to 22.72 MHz double-conversion
 
Insofar as the 700 kHz IF had worked for the Atalanta, and in particular as it lay in a gap of the Monitor coverage range, it was probably seen as a logical choice for the lower frequencies.  It may also be inferred that it was seen that adequate selectivity could be obtained at this frequency, without there being any need for another conversion to a low frequency such as 85 kHz.
 
The Monitor had a single-stage RF amplifier, as compared with two stages in the Atalanta, and that might have indicated the need for a higher first IF for the higher frequencies in order to obtain adequate image rejection.  Perhaps something like 1.6 MHz, not uncommon in HF practice, might have done the job.  Against that, the chosen 2.7 MHz looks to have been on the high side.  But its use ensured that none of the first IF harmonics fell into any of the reception bands for which it was used.  That would not have been true of 1.6 MHz.  Also, assuming that infradyne conversion from 2.7 MHz to 700 kHz was used, it would have required an oscillator frequency of 2 MHz, for which none of the oscillator harmonics fell into those bands, although one, the 11th at 22 MHz, was right on the lower edge of the 22.00 to 22.72 MHz band.  The 2.7 MHz IF, 2.0 MHz oscillator combination might have been the lowest frequency pair available that met the no in-band conflicts criterion, assuming that it applied.  (Supradyne conversion would have required an oscillator frequency of 3.4 MHz, which had an in-band harmonic of 17 MHz.)
 
 
Cheers,
 
Steve
 
 
Posted : 16/08/2022 2:51 am
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