Thursday, July 17, 2008

Distortion, AM versus SSB

A ham recently reported that his Flex, in transmit, sounded great in AM mode, but had distortion when he operated in SSB. When I heard this, I scratched my head and wondered how it could be possible? But...I ran an experiment, and I found it to be true.

My test was made using a second SDR-1000 in LSB mode as the receiver to copy my 5000's tramsit audio (when the 5000 was in AM mode, I listened to the lower sideband of the AM signal on my 1000 using LSB mode).

[I also performed the test with the 5K only in AM mode by varying the amount of carrier insertion (this is possible in the 5K setup menu). At 0 percent carrier insertion (i.e. DSB) the distortion is clearly audible, and it decreases as carrier insertion is increased. Again, the receiver I used for monitoring my audio was an SDR-1K in LSB mode.]

As to the reason why...I can't explain it myself, by somewhere there's sure to be some sort of mathematical analysis or intuitive explanation. I just don't know what or where they are.

By the way, if you'd like to hear the LSB vs. AM test, I can send you an mp3 file. Send an email to me at jca1955 "at" and request the "Flex AM vs LSB test. " The distortion in SSB is subtle, but you should be able to hear a difference between AM and SSB.


brianlloyd42 said...

Sure. When you generate AM the frequency, phase, and amplitude relationships between the carrier and sidebands is maintained because all that information is transmitted with with the information (sideband).

When you reinsert the carrier using an LO that relationship is NOT preserved. Most likely you have at least a few Hz of error (I can hear a BFO injection frequency error of about 3 Hz on a voice signal) and if you DO get the frequency spot on, there will be a phase error. Try to get the frequency within a fraction of a Hz and then listen as the phase of the reinjected carrier rotates with the frequency error. You will hear that subtle distortion come and go.

Brian, WB6RQN

Jeff said...

Thanks for the comment, Brian. The distortion I hear isn't a phase or frequency error in the signal. If it were, then one wouldn't hear a change in the distortion (with a corresponding correlation in measured THD) as the xmit signal level is varied.

Felipe Ceglia - PY1NB said...

Jeff, sorry to post this question here as a comment, but I could not find your email address elsewhere:

I have a FT1000D which has the same agc distortion as the one you
described in this old post:

Did you fix it? Did you keep notes on how you fixed it?


Felipe - PY1NB -