[ARC5] Lopsided modulation

[AKLDGUY .]

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> 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.
>
>
>
> ------------------------------
> *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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