[ARC5] Lopsided modulation

Tom Lee tomlee at ee.stanford.edu
Sun Feb 25 02:28:39 EST 2018 

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

--Cheers
Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)

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
>
>
>
> ------------------------------------------------------------------------
> *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 2:15 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
>
>
>
> ------------------------------------------------------------------------
> *From:* AKLDGUY . <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.
>
>
>
>
>     ------------------------------------------------------------------------
>     *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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