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Old 21st Mar 2020, 4:40 pm   #1
Vintage_RC
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Default Dymar 1525 sig gen AM modulation

Recently I needed a decent signal generator for a project I was working on. I got my Dymar 1525 out of the back of the equipment cupboard, this is a 1970's bit of equipment with a useful range of features but despite being all transistor is quite bulky and heavy (see photo). Since I hadn't used for some considerable time I decided to run some tests on it to make sure it was performing acceptably. One thing I noticed (and something I remember from last time I used it) was the long period before the slow frequency drift stabilised. I can live with that, I just leave it on all the time while it might be required. Everything looked OK and I decided to check the AM modulation, as you can see the envelope at 50% mod looks to be as you would expect. Out of interest I took a look at the spectrum with and without AM modulation (see photos). I was surprised to see quite noticable asymmetry with AM modulation (in FM mode it is symmetrical), I assume this is due to some unwanted FM component. I don't think this will cause any problems for my application but I wondered if this is typical of a generator such as my Dymar.
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Old 21st Mar 2020, 4:51 pm   #2
Jon_G4MDC
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Default Re: Dymar 1525 sig gen AM modulation

Sweeping too fast? Will your Rigol allow 10sec or even longer sweeps?

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Old 21st Mar 2020, 6:37 pm   #3
Vintage_RC
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Default Re: Dymar 1525 sig gen AM modulation

Thanks for your reply, I've viewed the spectrum with narrower RBW's and longer sweep times but the asymmetry is always the same. Looking in my copy of the HP App Note 150-1 "Spectrum Analysis AM and FM Modulation" it comments that when AM and FM modulation are present together this results in sideband asymmetry (hence my conclusion). I was wondering if this is typical if sig gens such as the Dymar.
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Old 21st Mar 2020, 6:39 pm   #4
Radio Wrangler
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Default Re: Dymar 1525 sig gen AM modulation

The asymmetry of the sidebands is a sreong pointer to the presence of both FM/PM with AM.

The 1MHz wide phase noise pedestal on the wide spectrum picture makes it look like noise is getting into the oscillator, possibly via the FM system.

The modulated waveform on the scope looks good, but doesn't show FM/PM effects, of course. The narrow spectrum is terrible. Surely they can't normally be that bad?

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