[Elecraft] mixed roofing filters for diversity

[wayne burdick]

wayne burdick wrote:

> Doug Faunt N6TQS +1-510-655-8604 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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