Hi, Everybody,
I'm reporting back on the carrier suppression problem in an Omni VI with two
(2) Inrad 2.8 kHz crystal filters.
Good operating practice dictates that you check your radio for problems like
this from time to time. I encourage owners who have installed wider filters
to listen for any residual SSB carrier on a second receiver, or, if you have
access to test equipment, actually measure it. The (unmodified) Omni VI spec
is 60 dB of carrier suppression.
Forgive a little bit more commentary before I start the tech talk. It is
terrific to have access to someone at the factory who knows what the score
is. Thanks to George Cutsogeorge at Inrad. I hope I am as helpful to my
customers.
After trading e-mail with George, I was convinced that I needed to put my
effort into the balanced modulator at least as much as investigating the
filters themselves. Even I seemed to remember being able to get a really
deep carrier null, deeper than I was now able to achieve. I was after at
least 40 dB of carrier rejection in the balanced modulator.
I think what was frustrating me was that R3 and R4, the two 1-turn pots on
the TX Audio board that adjust the carrier balance, just weren't giving me
enough fine adjustment to get right into the null. I replaced them with
miniature PC mount 25 turn pots to give me better control. What a
difference. Not only did this give me a >40 dB null, it hardly drifted after
the adjustment as it was prone to do with the original pots. This, more than
anything else I did, put me back to where I wanted to be relative to the
spec. My carrier is barely audible on a second receiver through a 60 dB
coupler while PTT on the Omni. (My "60 dB coupler" is actually a mediocre
coax switch with 60 dB of isolation.)
I hadn't noticed this until the wider filters were in and suddenly the
carrier was right up against that filter skirt. According to George, you
cannot expect more than 12 dB of carrier suppression from one of the 2.8 kHz
Inrad filters. This is in the ballpark of the 8-10 dB I was seeing (see my
original post). Traditionally, in the SSB filter method the carrier is
placed at the -20 dB point. That's what all the texts say. If you're going
to mess with the filter bandwidths, you have to plan for the difference, and
that is, of course, why I was going through this exercise.
The measured bandwidth of my #754 9 MHz filter is closer to 2.9 kHz. Part of
that is just manufacturing tolerance. I had to set the carrier points very
carefully and will be monitoring them for drift from time to time. I like
they way these filters sound. Looks like I will be keeping them.
I hope this helps anybody else who runs into this situation in the future.
Thank you everyone for your public and private comments.
Regards,
Al W6LX
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