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Video Sweep Generator: Any good for CTV Alignment?
Topic Rating: +1 Topic Rating: +1 (1 votes) 
April 18, 2017
3:01 pm
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Cathovisor
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To do IF alignment you need a Philips PM5334 or similar. An interesting fact I learned about these is that they are also sold under the Nordmende brand in Germany as the SW3330. Also, they have a small 0 - 30V PSU in them to simulate AGC voltages.

You could go mad and buy a Knott or an R&S SWOB5...

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Chris
To know yet to think that one does not know is best; not to know yet to think that one knows will lead to difficulty. Lao Tzu
April 18, 2017
6:56 pm
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Chris said
Definitely PAL

Nothing on a TV when fed into a modulator.

Occiput beat me to it, we're both on the same page in reviewing your PIX and 'scope display.

I didn't see a 75 ohm termination between your generator and your scope, so amplitudes will be off, probably twice the expected 1.0V pk-pk. Some generators have source terminated outputs, meaning the generator has an internal 75 ohm source impedance. Others mix components of the final signal at the output connector and when not terminated in 75 ohms produce very distorted results. A piece of kit you should have looks something like this:

BNC_Termination.jpgImage Enlarger

If one is not to hand rig a 75 ohm resistor (or two 150 ohms in parallel) across the scope end of the cable. Purists (like myself) might cringe, as this should be a 0.1% (yes, that accurate) tolerance and constructed for RF signals. Without that 75ohm load your measurements will be in error or make little sense.

I also noticed that your scope readout thinks the signal is more than twenty volts amplitude. Impossible! This has to be a measurement error, and perhaps nothing more than your scope 'thinks' you are using a 10X probe and the readout is scaled up. Without the 75 ohm termination we would expect a 2.0V pk-pk signal, or possibly a 3.0V signal is the generator conforms to the much older standard of 1.0V video and 0.5V sync amplitude.

Heres the results on the scope and some internals.

This little photo tour is like 'Catnip' for me. The layout is clean, logical, and a lot of engineering love was baked in to this product. As the saying goes "They don't make them like this anymore". Compare and contrast to a high volume telly of the same age...! BTW, did you happen to spot any date codes? Caps and ICs will reveal when they were made, and give an approximate date of manufacture for this kit.

If I'm reading the setting correctly in your PIX the generator output is composite sync (CCIR I, 625/50 as Occiput pointed out) and a line-time sinewave at 7.62MHz. No typical base-band video system will pass this signal, so the Monitor/TV display should a plain grey (50% amplitude) raster.

A TV fed from an RF modulator should pass, say, a 1MHz sinewave, so perhaps fiddle with the front panel controls and look for that on the scope and TV/Monitor.

The one place where this type of generator would be indispensable doesn't come up very often; the design and testing of long base-band video cables. Typically found in broadcast studio cameras and related kit where analogue video is (was?) send down long camera to CCU (Camera Control Unit) cables that require compensation for high frequency roll-off.

One of the widely used camera cable types has 101 conductors inside!

  Here's a typical CCU control panel, note the cable length compensation adjustment. The circuit adds HF boost to offset the natural losses as the cable length increases. Swapping out different cable lengths or brands would require adjustment using (you guessed it) a sinewave sweep generator. The generator would also produce correct timing and syncs as the signal likely passes through processing amps and clamps.

wv-rc35-front-panel.jpegImage Enlarger

Enjoy!

 
 

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April 18, 2017
7:49 pm
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Chris
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Hi Peter,

I don't have a 75Ω terminator never had the need for until now. Had a rummage in the draws and found a 50Ω

term.jpgImage Enlarger

It was a direct bnc to bnc connect no 75R termination. The scope was set to x10 as I only use x10 probes so its always set for that. When I was starting out with scopes on the TV's a few engineers used to give me all sorts of grief for using x1, so now I don't.

I'll did look for a date on what was easy to view but nothing obvious , nothing on the big caps either. Bit wary of removing the daughter boards. I'll have another look with the mag lamp, I may have missed a date code somewhere.

Thanks to both yourself and Occi for some excellent input thumb_gif

April 18, 2017
9:51 pm
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If you are going to get the amplitudes right you need to use the correct value of termination. 50R is an r.f. communications termination: for video work you need 75R: ideally with 1% tolerance or better.

They are easy enough to make out of a BNC plug and a close-tolerance 75 ohm resistor.  They are also available on ebay at about GBP4 a pop - for which price, I'd regard it as not really worth turning the soldering iron on.

When I posted earlier, I was assuming that "30V pk-pk" on the display of your 'scope meant what it said.  This seemed possible in view of the wide range of attenuation apparently available on the generator.  I would politely suggest that you need to be clear what gain settings you have on your 'scope and how these are reflected in the reading shown on the screen.

709379

April 18, 2017
10:03 pm
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occiput said
If you are going to get the amplitudes right you need to use the correct value of termination.

occiput said
available on ebay at about GBP4 a pop - for which price, I'd regard it as not really worth turning the soldering iron on.

occiput said
I would politely suggest that you need to be clear what gain settings you have on your 'scope and how these are reflected in the reading shown on the screen.  

Agreed and noted on all points

April 18, 2017
11:22 pm
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Chris said
I don't have a 75Ω terminator never had the need for until now. Had a rummage in the draws and found a 50Ω

term.jpgImage Enlarger

Great! That's better than no termination, but any amplitude measurement will be off (by 50/75 or 2/3 of spec)

I can't speak for Occiput... Video Engineers (and perhaps RF Engineers) will be quite prickly about these terminator things.

In any analogue broadcast plant many signals have to go to many places without harm. A common piece of kit is a Distribution Amplifier, which might have a specialty; such as Video, Audio, or Pulse. These are building blocks with one input and one or more outputs. They buffer the signal and drive multiple loads. Inside they have an amplifier with a gain of exactly 2.00, non-inverting.

The VDA input may be internally terminated in 75 ohms (for video and pulses, and perhaps 600 ohms for audio), or have two parallel connected input connectors. The second one, called a "loop-through", may feed another VDA (Video Distribution Amplifier) down the chain, or have the kind of termination seen above. Either way, the sending signal is happy, it sees only one 75 ohm load.

On each VDA output there's a 75 ohm "build out" or "back terminated" resistor to match the cable's impedance, and when terminated at the other end of that cable divides the signal by exactly one half. The original 1V signal became 2V, then was restored to 1V at the destination. Everyone is happy!

What happens if the VDA has several outputs and the build out resistors are not exactly 75 ohms? There will be a slight change in amplitude. If one is 1% high the destination will be 75/75.075 or 1% too much, while another might be 1% too little. For a broadcast transmitter a 1% error means a lot of the station's signal power is lost.

BTW, without the correct impedance at each end a coaxial cable behaves as a filter. It can greatly distort the waveform, for video this is typically ringing on fast edges, or affects some frequencies differently to others. 

A little story, if I may. Enter ex-Pat Peter Hughes, while working for GVG (Grass Valley Group) he saw this as a major issue, mostly because GVG made and sold a lot of VDAs, so he started his own company HEDCO (Hughes Electronic Development Company) in Grass Valley and made very nice VDAs and other things. HEDCO sold literally tens of thousands of high precision 75 ohms 0.1% BNC terminations over the years. Back at GVG we were a top customer of HEDCO, our engineering labs and production test floor was always knee deep in those little rascals!

Sadly, last time I visited Peter he was fighting cancer, RIP. He sold HEDCO about 1995 or so, but kept the terminator side-business for his widow to continue.

Mods: Perhaps move some of this material to a new home? "Video Engineering 101"?

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