Re: Wide filter on QCX?


Thanks Johan.  I was thinking it was not as simple as just changing the menu frequencies and readjusting.  I have 2 QCX's I've widened the filter on and while they do appear to
have a flatter response, imaging on strong signals is apparent.  I did look at the ARRL article and that is the way to do it.  Much more complicated though.  
Another thing I've noticed is both widened QCX's exhibit a spur at 1400 Hz that was not there  with the stock QCX filter.  It appears to be audio not rf as it will not move with tuning.
It's not avbig issue but I'd like to clean it up.

I also have  a QRPLabs RX with Polyphase network.  I'll take a look at that.  It may be a better solution.



On Tue, Aug 31, 2021 at 5:13 PM Johan Bodin <jbodin@...> wrote:
Hi Bob

There is no problem with widening the filter built around IC8/IC9 (on
QCX mini schematic) but the precision of the 90 degree I/Q phase shifter
(IC6/IC7) is too narrow to give a good opposite sideband suppression
over the entire SSB bandwidth. Widening the bandpass filter will work,
but, expect bad opposite sideband rejection outside the CW tone
passband. It will behave just like a simple direct conversion receiver
with (almost) equally strong response either side of zero beat except
around the CW center frequency.

At least 3+3 stages of all-pass filters are needed for good opposite
sideband suppression over a voice bandwidth. See for example

Jim Tonne (forgot his callsign...) has written a nice program for
designing 90 degree I/Q phase shifters. Look for the program "QuadNet"


WA1EDJ wrote:
> After reading a few threads on widening the CW filter on the QCX, I've
> come up with some R and C values to try for an SSB bandwidth.
> What I'm not sure of is what frequencies the alignment menu items:  IQ
> Balance Freq, Phase Adjust LO and Phase Adjust HI should be set to for
> SSB bandwidth?
> I know IQ Balance Freq was 700 Hz for the original CW QCX.  So for my
> wider filter, I'd like center to be 1500 Hz, so set IQ Bal Freq
> accordingly?
> Original Phase Adj LO was 600 Hz.  What would be appropriate for the
> wider SSB filter?  Likewise for the Phase Adj HI which was 800 Hz.
> I realize the QCX was not intended to be for SSB reception.  I'm just
> trying to tune the best I can for a wider filter.
> TNX!
> Bob

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