mixed roofing filters for diversity

[Doug Faunt N6TQS [phone]]

The question that came into my head late last night:
Can I mix 8-pole filters for diversity reception?

If I have a 250 Hz filter and a 400 Hz in my subRX can I do diversity
reception using that combination?  What about a 2.1kHz and a 1.8kHz?

73, doug
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[wayne burdick]

On Jan 23, 2009, at 2:53 PM, Doug Faunt N6TQS [phone] wrote:

> The question that came into my head late last night:
> Can I mix 8-pole filters for diversity reception?

Yes. They all have equal offsets (0.00), so even though a different
filter may be selected on the two receivers at different bandwidths,
there will be no long-term phase drift.


>
> If I have a 250 Hz filter and a 400 Hz in my subRX can I do diversity
> reception using that combination?  What about a 2.1kHz and a 1.8kHz?

Yes and yes.

73,
Wayne
N6KR

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[wayne burdick]

wayne burdick wrote:

> Doug Faunt N6TQS [phone] wrote:
>
>> The question that came into my head late last night:
>> Can I mix 8-pole filters for diversity reception?
>
> Yes. They all have equal offsets (0.00), so even though a different
> filter may be selected on the two receivers at different bandwidths,
> there will be no long-term phase drift.

Doug,

My apologies, but this actually will *not* work in all cases. In some
modes, if the automatically-selected crystal filter is wider than will
fit above the carrier (Fc), then the I.F., 1st LO, and BFO may all end
up shifted to compensate. Thus the two sythesizers will not be on
exactly the same frequency, and a phase offset is possible.

Using your 400 and 250 Hz filters as an example: If your sidetone pitch
were say 500 or 600 Hz and you had no passband SHIFT in effect, both
synths would be set at the same frequency. But if you used a much lower
sidetone pitch (say 300 Hz), the nominal position of the 400 Hz filter
(but not the 250 Hz filter) might be shifted slightly upward, moving
the synths apart, even though the perceived receive pitch is identical.
This could produce a phase offset.

So allow me to humbly retract my claim and suggest that all filters
involved in diversity RX should, ideally, have matched bandwidths. In
addition to eliminating phase offset in situations like I described, it
will result in nearly identical close-in dynamic range performance for
both receivers. Having a wide filter on just one of the receivers might
leave it open to adjacent signals, detracting from the 'pure'
experience of diversity mode that I'd like you to have  ;)

Thanks, Lyle (KK7P) for catching this.

73,
Wayne


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[Doug Faunt N6TQS [phone]]

Thanks.  That's almost as useful as if it would work.

73, doug

I want a lime-green (or black) Elecraft hoody.
And a circular Elecraft slide rule.


   From: wayne burdick <[hidden email]>
   Date: Fri, 23 Jan 2009 16:01:09 -0800

   wayne burdick wrote:

   > Doug Faunt N6TQS [phone] wrote:
   >
   >> The question that came into my head late last night:
   >> Can I mix 8-pole filters for diversity reception?
   >
   > Yes. They all have equal offsets (0.00), so even though a different
   > filter may be selected on the two receivers at different bandwidths,
   > there will be no long-term phase drift.

   Doug,

   My apologies, but this actually will *not* work in all cases. In some
   modes, if the automatically-selected crystal filter is wider than will
   fit above the carrier (Fc), then the I.F., 1st LO, and BFO may all end
   up shifted to compensate. Thus the two sythesizers will not be on
   exactly the same frequency, and a phase offset is possible.

   Using your 400 and 250 Hz filters as an example: If your sidetone pitch
   were say 500 or 600 Hz and you had no passband SHIFT in effect, both
   synths would be set at the same frequency. But if you used a much lower
   sidetone pitch (say 300 Hz), the nominal position of the 400 Hz filter
   (but not the 250 Hz filter) might be shifted slightly upward, moving
   the synths apart, even though the perceived receive pitch is identical.
   This could produce a phase offset.

   So allow me to humbly retract my claim and suggest that all filters
   involved in diversity RX should, ideally, have matched bandwidths. In
   addition to eliminating phase offset in situations like I described, it
   will result in nearly identical close-in dynamic range performance for
   both receivers. Having a wide filter on just one of the receivers might
   leave it open to adjacent signals, detracting from the 'pure'
   experience of diversity mode that I'd like you to have  ;)

   Thanks, Lyle (KK7P) for catching this.

   73,
   Wayne


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