[ARC5] Lopsided modulation

[Kenneth G. Gordon]

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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