[ARC5] Lopsided modulation

[Fuqua, Bill L]

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.



________________________________
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Richard Knoppow <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
WB6KBL
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