Hi Thomas,
>
> As almost everyone knows Inrad makes a roofing filter kit for the Omni family
> of radios. A friend of mine in California, who has 2 Omni VI+, swears the mod
> provides a substantial increase in dynamic range.
The Inrad filter may make a substantial improvement. To say that it
increases dynamic range may not be exactly correct. I would say that,
given the dynamic range the Omni VI IF circuitry has, by having a
narrower than stock first IF "roofing" filter, one can take better
advantage of the dynamic range the radio has. Limiting the bandwidth,
that the active circuits after the filter have to deal with, improves
the performance within that smaller bandwidth, and with the dynamic
range available.
> He says it is just great in crowded contests when copying weak signals
> amongst the very strong signals. Here's my question:
>
> I have the optional Ten Tec 250 and 500Hz first IF CW filters. The Inrad CW
> roofing filter mod is available with the #763 normal pitch filter (750Hz), or
> #765 low pitch filter (500Hz). I prefer listening to CW at 500Hz vs. 750Hz.
> Will the low pitch filter work in conjunction with the standard Ten Tec CW
> filters?
>
Both the Inrad modification filter, and the first IF filters in the Omni
VI are operating in the 9 MHz IF. There is no adjustable heterodyne
frequency conversion, such as at the input and output of the 6.3 MHz IF.
Therefore the Inrad modification filter and whatever later filter in the
9 MHz IF you are using must have coincident bandpasses. Anything that is
not inside the bandpasses of all the 9 MHz filters in use will not get
through (or will be attenuated to the point of being unusable). You say
you have the 250 and 500Hz first IF filters. I presume you are referring
to their bandwidths, and not to the frequency of the audio note that
signals passing through them produce. What you need is for the new Inrad
filter to have the same bandpass center frequency as the later 9 MHz IF
filter. The bandpass center frequencies may not need to be exactly
equal, though they do need to be close enough so that a large portion of
the bandpasses of the Inrad and Ten-Tec filters overlap each other.
This becomes more critical as you are using narrower filters. The
Ten-Tec 9 MHz IF filters with 250 Hz bandwidth, that I know of, are the
model 219 and model 221. They have bandpass center frequencies of
9.000750 MHz and 9.000500 MHz respectively, and since in CW mode the
Omni VI uses a BFO frequency of 9.000000 MHz, will yield an audio note
of 750 Hz and 500 Hz. (They are 250 Hz wide, so really that would be 625
to 875 Hz and 375 to 625 Hz) . You need to pick the Inrad filter based
on both center frequency and bandwidth. Or, you could chose NOT to use
the either of your narrow Ten-Tec 9 MHz IF filters at the same time you
are using the Inrad filter, in which case their bandpasses do not have
to be coincident. I believe that the Inrad mod allows various control
wiring options, so that you could do that.
DE N6KB
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