Hi, Everybody,
I have wide, 2.8 kHz Inrad filters in both IFs of my Omni VI now. My
transmitted SSB signal showed all the signs of an incorrect BFO insertion
frequency: poor carrier rejection and poor opposite sideband rejection.
A friend of mine across town told me he could hear me on the opposite
sideband and actually make out what I was saying from the very low frequency
audio getting through.
All this was confirmed by measurement. Even after re-nulling the balanced
modulator carefully, I measured the carrier rejection at only 28 dB. 20 dB
of that rejection comes from the balanced modulator, and only 8 dB from the
filter. Using an RF generator to inject a 9 MHz transmit signal into the 9
MHz IF board, I confirmed that the filter was passing with little
attenuation both above and below the 9.000 and 9.003 MHz LSB and USB carrier
frequencies, respectively. (Remember that the mixing scheme inverts the
sidebands in the Omni VI.)
On purpose, since I happened to be testing in LSB mode, I moved the BFO
frequency to (9.00000 MHz - 100 Hz = 8.9999 MHz), in other words, an
additional 100 Hz away from the lower skirt of the filter, and that really
cleaned up the signal. So I did the same thing to the USB BFO crystal. Same
good results. However, one undesirable side effect of doing this is that the
frequency readout is now off by 100 Hz. I guess I have no choice but to get
used to this.
The stock 2.4 kHz filters do not exhibit this behavior. I have measured them
and listened to them and they have sufficient rejection at the BFO
frequencies.
I know several folks on here have gone to these filters, so have you guys
really checked carefully the carrier suppression by listening on another
receiver or by measurement with test equipment? Has anyone found what I
found? Or do I just have a filter that's simply too wide to operate
correctly inside the Ten Tec 9.000 to 9.003 MHz IF window? I guess what I'm
asking is 1/ is the Inrad filter really exactly supposed to be 2.8 kHz wide
or 2/ in reality is it wider, giving rise to the problems I've described, or
3/ do I have a filter that's out-of-spec? Before you answer #3, I would
really appreciate hearing from you if you've actually measured this.
I want to be fair and clear that I haven't spoken to Inrad yet about this to
give them a chance to respond. I am looking for more of a sanity check right
now. Thanks.
Al W6LX
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