[ARC5] Lopsided modulation

[K5MYJ]

Applying AM to a LC oscillator create FM.
Applying AM to a crystal oscillator creates PM.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Tuesday, February 27, 2018 12:02 PM
Subject: Re: [ARC5] Lopsided modulation


> Although I am probably the least qualified, in an engineering sense, to 
> offer much to this
> discussion, I do feel I have the right to, at least, express some 
> carefully considered
> opinions.
>
> Since we know that "best practices" dictates that the final tank circuit 
> of any transmitter
> "should" have an unloaded "Q" of somewhere between 10 and 12, let's take 
> 12 for the
> purposes of discussion, it is quite plain to anyone who has looked at even 
> the ARRL
> Handbook on this issue to realize that that value of "Q" is complete 
> insufficient to cut a
> sideband, no matter how far "off tune" that circuit might be.
>
> The final tank circuit is, essentially, a "single" tuned circuit and it is 
> simply impossible for
> any NORMAL such circuit to be sharp enough to cut sidebands, while leaving 
> transmitter
> output power at normal levels. Period.
>
> Now, since Mac insists that he has experienced sideband asymetry in these 
> transmitters in
> actual practice, and has, by experience, determined that this problem was 
> related to the
> final tank circuit being "off tune", from the above it seems clear to me 
> that since circuit "Q"
> cannot possibly be the cause, there must be some OTHER cause for what Mac 
> has
> observed, which is related to the tuning issue, but has absolutely zero to 
> do with circuit "Q".
>
> Others have discussed at some length the effect on AM of an incidental 
> FM... or PM, and
> from what I have read here, that seems to me to be, most probably, the 
> true reason for this
> effect.
>
> I am wondering if having the final tank circuit off-tune at some critical, 
> but minor, point
> causes PM (not FM) in the VFO or in the input signal to the final tank 
> circuit?
>
> One reason I suggest PM not FM is that Neil has not observed any FM of the 
> VFO's
> frequency. However, I have never been really certain of how the generation 
> of FM differs
> from the generation of PM either.
>
> I wonder that if one were to drive the finals with an extremely stable 
> buffered external
> source, then check the symetry of the plate-modulated transmitter's 
> sidebands, whether
> doing this could at least eliminate any incidental FM... or PM.
>
> Or, contrary to that, purposely introducing some PM into the input to the 
> final amp and
> observing the effect on sideband symetry.
>
> I don't presently have a working "ARC-5" transmitter or I would try this 
> sort of thing myself.
>
> Thoughts?
>
> Ken W7EKB
>
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