Hi Chuck,
Based on your experience copying 5T0CW on 30 meters....
"I had an experience yesterday, I'd like to share. During a time when I had strong line noise, some of the locals were working 5T0CW on 30 meters, and I couldn't copy at all. The Omni VI noise blanker helped, but not enough. Then I turned my NAR switch off ( which disengages both my narrow IF filter-- INRAD 753-- and the 600 Hz roofing filter. Now I could copy the 5T station! The noise blanker was more effective in smoothing out the harsh hiss of the line noise in this case. When the line noise is not present, I always have better copy with the NAR switch on."
I would say that the answer to your question....
"I've been considering dedicating the Omni VI to CW only by wiring the
roofing filter so it is in line all the time (using the NAR switch in
its normal mode). That would provide me with options in terms of CW
selectivity (since I have the VI, not the VI+). Now I'm wondering if
this would compromise my noise blanker. "
....is, yes, it would compromise your noise blanker.
I wonder if the Inrad modification inserts the roofing filter in the
wrong place? I think their choice of insertion point for the filter
modification was made based on the ease of installation, and to make the
modification more easily reversible. Many hams (particularly Ten-Tec
cultists) do not want to make any irreversable modification to their
radio, and a modification that seems too "drastic" would not sell as
well as one that can be fairly easily plugged in, and unplugged later.
By putting a narrow filter before the noise detector and the noise
blanking switch, they have changed the functionality of the noise
blanker. This could be because the noise pulses that trigger the noise
blanker circuit are attenuated and stretched out in the time domain by
the narrow filter.
Looking at the schematic, I think that a better place to insert a
roofing filter modification would be between T5 and T6. C20 could be
removed, and the cables to and from the add on board could solder to
the PCB pads where C20 had been. I do not know what the impedance is at
this point. So I cannot say whether a filter could be inserted there
without some additional circuitry to get the impedance matching to the
filter right. Likely there would need to be some gain to compensate for
the filter loss, as is done in the Inrad modification (or so I have heard).
Putting the new roofing filter in place of C20 puts it behind only one
more active gain stage, Q1, than where Inrad puts it. And this would
also put it after the noise blanker, which is also where the FM detector
gets it's feed. Like the Inrad mod location this also keeps it out of
the TX signal path, so that it does not need to get switched in and out
between TX and RX mode when operating SSB. Y2 would remain the filter
that passes one sideband and rejects the other for SSB TX.
I think your idea of a roofing filter that is in line all the time
sounds like a good idea, if it is in the right place. You could use a CW
bandwidth filter and the radio would no longer be usable for SSB, or you
could use a SSB bandwidth filter and still have all modes, probably with
some improvement in close spaced dynamic range. Perhaps not as much
advantage as the Inrad modification since Q1 could be overdriven by
strong signals make it through Y1.
This is not intended to be a criticism of Inrad's roofing filter
modification. I'm glad they have made their product available. I may buy
one myself. I'm just interested in discussing other possibilities.
Another possibility, if you're willing to give up both SSB TX and RX
capability, is to replace Y2 with a narrower filter (1.8 kHz or 1 kHz
maybe), and put your narrowest filter (500 or 250 Hz) in the NAR option
filter position.
DE N6KB
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