>> Is it possible I would actually be happier with 500 Hz filters in the 6.3
>> and 9 mHz IFs, since one op said they work a little better with the passband
>> tuning on the Omni VI+ than the 250 Hz filters do?
>>
When using narrow filters in both the 9 MHz IF and the 6.3 MHz IF it is
necessary to have the PBT adjusted so that the bandpasses of both IFs
coincide. Otherwise you won't hear much at all. The adjustment can be a
bit critical if both filters selected are really narrow. I think there
may be some drift in the 15.3 MHz VCXO on the passband tuning board,
requiring the PBT knob to not always be in exactly the same position to
get the two IF bandpasses coincidental.
The Inrad roofing filter modification put the new roofing filter before
the 15 kHz two pole filter, Q1 and the noise blanker pulse detector,
and the noise blanking switch diodes. Putting a narrower filter before
all these stages, instead of where Ten-Tec put the narrow 9 MHz filters
can help you copy weak signals amongst strong close in (in frequency)
signals that would otherwise produce intermodulation distortion in those
stages which the filter precedes. This is likely to be an advantage in
contest situations more than any other.
There is a possible disadvantage to putting a narrow filter before the
noise blanker circuit. The kind of noise that noise blankers work best
on, is short, strong pulses. The strong pulses are detected by the noise
detector, which then switches off the IF signal path using the noise
switch diodes. With the pulses short, you barely notice that ALL signals
are blanked for a short period. The pulse noise is blanked and the
signals you want to hear are mostly still there. By putting a narrow
filter in front of this, the pulses can be reduced in amplitude and
stretched out in time duration. So the period of IF signal path shut
off can be longer, and perhaps more noticeable, possibly rendering the
noise blanker less effective. This does not make the Inrad roofing
filter modification undesirable, it just means there are some trade
offs. And you can switch it in or out. Some people claim that noise
blankers are completely useless anyway. I find that I do occasionally
have noise that the blanker works very well on.
>> 3) If I were able to find the special N4PY "lowered tone" CW filter, what
>> other set of CW filters should I buy that are compatible with it?
>>
The "low note CW filter" for the Omni VI is the Ten-Tec model 221. It is
specified as 250 Hz bandwidth, centered on 9.000500 MHz, which will
result in a 500 Hz tone when the BFO is set to 9.00000 MHz as it should
be in CW mode. Ten-Tec does not list these as an available item any
more. I got mine through this reflector.
>> 4) In addition to crystal filters, should I invest in the Timewave
>> DSP-599zx especially for SSB summertime noise reduction? (As you probably
>> know, for hearing disabled, getting rid of as much background noise as
>> possible is a priority for both CW and SSB.)
>
With the kind of noise you are describing, you may not want to
compromise the functionality of your Omni VI noise blanker with a narrow
roofing filter before the NB circuit. I would suggest narrow 9 MHz
filters in the locations that Ten-Tec provided in their design.
I have not yet tried an outboard DSP NR system with my Omni VI. I'd be
interested to hear other peoples experience with this. Remember in the
Omni VI, the DSP circuitry is ALWAYS in the audio path, whether your
have NR turned on or off. So with an outboard DSP NR, the audio would be
going through two A to D and D to A processes. I've thought about
putting my JPS NIR-12 in line INSTEAD of the built in DSP rather than in
additon to it. Haven't got around to doing it yet.
DE N6KB
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