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Old 4th Jan 2020, 10:09 pm   #1
Boulevardier
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Default FM generator vs sweep generator question

As I've said elsewhere, I have done very little RF work before, and I'm feeling my way slowly through the definitions. Simple question, and hopefully simple answer. When it comes to aligning/testing VHF FM radios, is there any great difference between the functionality and usefulness of an "FM generator" as against a "sweep generator"?

Mike
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Old 4th Jan 2020, 11:57 pm   #2
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Default Re: FM generator vs sweep generator question

Hi!

Yes!

An F.M. generator frequency modulates a carrier wave over a specified frequency range (typically 75 kc/s or 100kc/s for an f.m. broadcast receiver, at a rate depending on the frequency of the audio modulation, whereas a sweep generator sweeps the frequency of a modulated or unmodulated carrier over a range determined by (usually) the "Sweep Range" control, and the sweeping is usually at a very low frequency compared with the instantaneous frequency changes of an f.m. generator.

A sweep generator can also sweep over a much larger range of frequencies to produce a "response curve", that can be traced on an oscilloscope directly!

For aligning conventional f.m. broadcast receivers, an f.m. generator (more cred orrectly, an f.m. modulated signal generator), is very much more convenient for f.m. i.f. alignment as the receiver's a.f. output can be used, monitored by ear or with an a.c. voltmeter, whereas a sweep generator's output needs to be monitored with an oscilloscope!

Chris Williams
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Old 5th Jan 2020, 12:42 am   #3
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Default Re: FM generator vs sweep generator question

Thanks Chris. A Sylvania book on their Type 131 and 132 scopes (at a guess, 1930s or 1940s) that Ive been looking at indicates that an FM generator can also provide a IF reponse curve that can be displayed on an oscilloscope by using its sync output to trigger the scope. Have I got that right?
Thanks for your help.
Mike
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Old 5th Jan 2020, 1:02 am   #4
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Default Re: FM generator vs sweep generator question

The FM generator will need to operate over a frequency band that is a bit wider than the pass-band of the filter. With the aid of a 'scope, the pass-band can then be seen dynamically, so as adjustments are made to that filter, the resultant changes are immediately apparent. Therefore, the FM generator will operate over a relatively narrow range of frequencies. But if the pass-band was seriously wide - say several 100's of kHz, a 'sweep generator' will be required.

Al.
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Old 5th Jan 2020, 1:26 am   #5
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Default Re: FM generator vs sweep generator question

Thanks Al. I was hesitantly reaching that conclusion, but typical service manuals only ever seem to specify sweep generators. FM generators are very rarely mentioned for VHF FM alignment procedures. Good to have it confirmed that one can be used to adjust symmetry of response.
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Old 5th Jan 2020, 10:31 am   #6
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Default Re: FM generator vs sweep generator question

For an FM system, to simulate a signal, your FM sig gen doesn't need to swing outside the centre of the passband of the selectivity of your receiver. To sweep the filter and see its shape, you need a generator that does swing outside the passband, so you can see its shape, -3dB points and at least part of the way down to the stopband. Ideally you want to go far enough out to see that the adjacent channel rejection part of the skirts is correct.

Some generators can do both FM and wide sweeps.... the Marconi TF2008 sitting on my shelf can, and also the HP8601A is a classic. Modern synth sig gens can do FM and they can be set to step their centre frequency through a sweep range. You need a storage scope of some sort in this latter case.

You often want slower sweeps than the FM modulation bandwidth of a sig gen allows so that the filter being swept has time to settle, else you get a distorted picture.

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Old 5th Jan 2020, 2:54 pm   #7
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Default Re: FM generator vs sweep generator question

Thanks for very clear and detailed response David. I can now see the answer to my question clearly. But it does leave me wondering when an FM generator is adequate for VHF FM alignment (which is presumably at least a part of what they were designed for), and when you need to use a full sweep generator.

Is the answer to that question that you need a sweep generator when you are designing and developing a new or improved circuit for the job, but when you are simply checking or realigning an already well-designed circuit, then an FM generator is adequate?

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Old 5th Jan 2020, 6:11 pm   #8
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Default Re: FM generator vs sweep generator question

Working at HP, I was always a bit spoiled for test equipment ;-)

In setting up an FM receiver, you have two tasks needing a swept measurement. Firstly, adjusting the IF bandwidth. This isn't needed as such in radios with the usual pizo-acoustic ceramic filters, there are no adjusters and they dominate the selectivity, but there may be a stage or two of LC resonators which could spoil the shape so you want to at least check.Also some ceramic filters have been found to be a bit 'off' and people have changed them and not as matched pairs. Tuners with L-C bandpass filters do need careful alignment. Some high-end jobs use L-C filters so they have more control over shape, group delay etc and hit tighter specs than the ceramic jobs were made to. I have a Revox B261 which is all L-C filtered and avoids the usual IF and decoder chips. To align one of these high performance IFs, you do need a wider sweep and you may want to see the group delay/phase characteristic. With an FM signal non-linear phase versus frequency creates distortion in the received audio. But this is getting into vector network analyser territory.

Secondly, there is the linearity of the discriminator to set up. This definitely needs some sort of swept display... an FM generator ought to be fine. Use the FM modulating signal as the X input to your scope.

So this is one of those how deep do you want to go areas. Most people are comfortably familiar with passband amplitude shapes. Some people know of the issues with linear phase response (== flat group delay). Relatively few people have been involved in the effects of AM to PM conversion, a problem common in limiting amplifiers, like FM radio IFs.

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Old 6th Jan 2020, 1:00 am   #9
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Arrow Re: FM generator vs sweep generator question

Quote:
Originally Posted by Boulevardier View Post
But it does leave me wondering when an FM generator is adequate for VHF FM alignment (which is presumably at least a part of what they were designed for), and when you need to use a full sweep generator.
The essential choice is simply based on just how much information you want to see and what test equipment (and its spec.) is available. In essence, for narrow bandwidths: FM generator; for wide bandwidths: sweep generator. It's all a question of choosing the right tool for the job - such is the case with many things, not necessarily 'electrical'. The knowledge of 'which tool' comes down to experience (if manufacturer's service data is not available) - like so many things in Life.

Al.
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