The AM Forum

Ken, W7MZG , has a Harvey Wells T-90 Bandmaster that puts out carrier + one sideband.   Really.   I've talked to Ken a couple of times, watched his signal using a Flex radio, switched to USB and to LSB.   There is no LSB.

As I understand it, this thing is a conventional plate modulated 1955 transmitter -- a 6146 modulated by a pair of sweep tubes, 6AQ5 as a clamper.  How can a conventional plate modulated AM transmitter put out carrier + one sideband?   Where should Ken look first to try to fix it?

Anyone seen anything like this?

Jon

I've "seen" this effect on the air before... using my spectrum scope.... and I've also seen it making measurements of some of my boat anchor equipment (see the last sentence below).

I believe that the effect is caused by simultaneous amplitude (as desired) and phase (spurious) modulation of a carrier. Phase modulation is, of course, a variation of frequency modulation. It would occur if the VFO did not have a well-regulated power supply, or for a variety of other reasons.

It is a little complicated mathematically (depending on how comfortable you are with equations) but one can show that phase modulation of the carrier produces sidebands that are just like the sidebands produced by AM, except: one of the two sidebands is in-phase with the sideband that is produced by AM, and the other of the two sidebands is 180 degrees out-of-phase with the sideband that is produced by AM.

If you have simultaneous AM (desired) and phase (spurious) modulation, then the contributions to one of the two sidebands will add (since they are in-phase), and the contributions to the other of the two sidebands will subtract (since they are 180 degrees out-of-phase). 

This does not necessarily result in perfect cancellation of one of the sidebands... but it does result in a signal that has more of one sideband than the other.

Under the right conditions (the right mix of amplitude and phase modulation), you can get enough cancellation of one of the sidebands to make the signal look like it has only one sideband (even though the cancellation is not anywhere near as good as it would be in a real SSB transmitter).

As an aside, this is a good way to check to see if your transmitter has any phase/frequency modulation when you AM-modulate it. I.e., modulate with a sine wave (e.g., 1 kHz)... and see if the two sidebands are equal in strength.

Best regards
Stu 

Stu.   That makes a lot of sense.   I remarked to Ken that he was "FM-ing" so the possibility that he was phase modulating as well as amplitude modulating makes a lot of sense.

The most likely culprit is lousy regulation of the VFO.   I'll ask him to try plugging in a rock.

Jon