[ARC5] Lopsided modulation

Sun Feb 25 15:15:15 EST 2018

  Lopsided sidebands are not unusual in older AM test equipment. A classic is the GR-1001a signal generator. My first good spectrum analyzer ( around 1970) was a SB-12a with a 5MHz IF. I did not have a local oscillator at the time and wanted to test it. I had just modified my old SB-8 spectrum analyzer power supply to work with the SB-12.  I took to the EE lab I worked in at Tennessee Tech and used the GR-1001 there. Set it on the 1.5-5 MHz band and tuned it into the bandpass of the SB-12. When I switched to high resolution I noticed a carrier with only one sideband.  I thought something with wrong with the spectrum analyzer but using a my SB301 receiver I verified it was SSB with on sideband.

  At the time I was taking a class in information and communication theory and I realized it must be incidental FM doing this. The signal generator had a triode oscillator driving a amplitude modulated 6L6 buffer.

  Then, needless to say, the last thing I looked at was the specs for the signal generator. There it was, I actually stated the maximum incidental FM modulation. And it was max due to the fact I was at the highest frequency for that range, minimum tuning capacitance. On the next range that started at 5 MHz both sidebands were there but still not equal. The GR-1001 was considered the state of the art signal generator at when it came out.

73

Bill wa4lav

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From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Fuqua, Bill L <wlfuqu00 at uky.edu>
Sent: Sunday, February 25, 2018 2:42 AM
To: Tom Lee; arc5 at mailman.qth.net
Subject: Re: [ARC5] Lopsided modulation

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Yep, except you are left with the carrier.

Bill

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From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Tom Lee <tomlee at ee.stanford.edu>
Sent: Sunday, February 25, 2018 2:28 AM
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] Lopsided modulation

Combining AM and FM is in fact one way to produce SSB. Flip the polarity of one to select the other sideband.

--Cheers
Tom

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On 2/24/2018 11:21 PM, Fuqua, Bill L wrote:

   You can use spectrum lab to experiment with simultaneous AM-FM moduation. Low modulation levels you can completely cancel out one sideband and double the amplitude of the other. It is sort of fun thing to play with.

  73

Bill wa4lav

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From: arc5-bounces at mailman.qth.net<mailto:arc5-bounces at mailman.qth.net> <arc5-bounces at mailman.qth.net><mailto:arc5-bounces at mailman.qth.net> on behalf of Fuqua, Bill L <wlfuqu00 at uky.edu><mailto:wlfuqu00 at uky.edu>
Sent: Sunday, February 25, 2018 2:15 AM
To: AKLDGUY .; ARC-5 List
Subject: Re: [ARC5] Lopsided modulation

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FM does not manifest itself on a carrier but as sidebands. Provided that bandwith limitations don't exist, the only way a plate amplitude modulated signal can produce unequal sidebands is by simultaneous Frequency Modulation.

73

Bill wa4lav

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From: AKLDGUY . <neilb0627 at gmail.com><mailto:neilb0627 at gmail.com>
Sent: Sunday, February 25, 2018 12:29 AM
To: Fuqua, Bill L; ARC-5 List
Subject: Re: Lopsided modulation

Interesting analysis, thank you.

Two things:
1. The Command transmitters do have neutralization
2. I appreciate that FM would not be discernable in a monitoring receiver equipped with BFO, but I'm pretty sure that it would show in the SDR's waterfall display, which is, in effect, a spectrum analyzer. No such FM of the carrier appears in either my signal or my friend's on the waterfall.

Neil ZL1ANM

On Sunday, February 25, 2018, Fuqua, Bill L <wlfuqu00 at uky.edu<mailto:wlfuqu00 at uky.edu>> wrote:

Narrow Band Frequency modulation produces sidebands just like Amplitude Modulation.

A 20% AM signal and a FM signal with .2 Modulation index produce sidebands of equal amplitude.

The only difference is that the AM sidebands are in phase with each other and the FM sidebands are out of phase with each other.

Being out of phase the FM modulation does not have a modulated envelope.

When you have both simultaneous FM and AM modulation one set of sidebands subtract and the other adds. You don’t lose any sideband energy and would only notice it if you either used a spectrum analyzer or a very narrow band receiver and tuned the sidebands individually.

  Since the final amplifiers of the ARC transmitters are directly coupled to the oscillator and that the finals are not neutralized the final acts as a reactactance tube modulator.  The effective input additional capacitance of the tetrode (Miller Capacitance) is equal to the internal feedback capacitance times (the gain of the output tube +1)  Cm=Cgp(Voltage Gain+1).  At positive peaks modulation the effective gain of the tube is greatest and at the negative peaks the gain (at 100% modulation) is zero.  This varying capacitance is directly across the tank circuit of the oscillator thus frequency modulating the signal.

I think even if the final amplifier were neutralized the audio rate changing of the load on the oscillator could cause it to FM. I suspect that during modulation the grid current of the final fluctuates.

You will not hear this FM using a BFO because it is at a audio rate.

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From: arc5-bounces at mailman.qth.net<mailto:arc5-bounces at mailman.qth.net> <arc5-bounces at mailman.qth.net<mailto:arc5-bounces at mailman.qth.net>> on behalf of Richard Knoppow <1oldlens1 at ix.netcom.com<mailto:1oldlens1 at ix.netcom.com>>
Sent: Saturday, February 24, 2018 8:02 PM
To: AKLDGUY .; ARC-5 List
Subject: Re: [ARC5] Lopsided modulation

     It depends on whether its FM-or PM- ing the oscillator. If
it is its causing the asymmetrical sidebands. You should hear it
on the oscillator signal. Try receiving it as a PM or NBFM
signal, that is, slope detect it.
     Its not your antenna.

On 2/24/2018 4:09 PM, AKLDGUY . wrote:
> You may be onto something with the B+ kicking downward on
> modulation. I notice that the dynamotor voltage does kick down
> significantly. This drop may be causing the plate voltage to
> bottom out, or even go negative!, on modulation peaks.
>
> The MD7 modulator schematic doesn't show significant capacitance
> decoupling the bottom of the mod tranny feed to the 1625 final
> stage - about 1.2 uF IIRC, and I followed that when building my
> own modulator. I suspect it may not be sufficient decoupling.
>
> But even if the final plate voltage is bottoming out or going
> negative, where is the literature that says this causes lopsided
> modulation?
>
> Neil ZL1ANM

--
Richard Knoppow
1oldlens1 at ix.netcom.com<mailto:1oldlens1 at ix.netcom.com>
WB6KBL
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