OT: sideband filtering (USB v. LSB)

[John Shadle]

Hi all,
Please forgive me for this non-Elecraft intrusion. I'm hoping that
someone can help me out with a question about modifying rigs to
operate on the opposite sideband. Please continue reading below for my
reason why...

I'm trying to modify Dave Benson's PSK-20 rig to operate on USB (it
currently operates on LSB, even though it is a 20m rig for PSK31). I
want to replace the 5.07MHz LO with a DDS module to allow for tuning
across the entire 20m ham band.

I emailed Dave about this and he responded that "I suspect your
talking about going anywhere in the band - a DDS-based signal in place
of that oscillator would let you go anywhere in the band." That's
exactly what I'm hoping for. He gave me another hint in the following
sentence:

"...you need to pull the 9MHz LO to the other side of the filter
passband so that the rig is operating on the (standard) upper sideband
on 20m."

The theory of operation and the schematic may be found on pages 6-9 here:

http://smallwonderlabs.com/PSK20_Manual.pdf

I'm no RF engineer, but just a kit builder who is trying to understand
how his radios work. ;-)

The manual states (on page 7) that "Single-sideband filtering is
performed by a second filter comprising Y7-Y10 and related capacitors.
After passing through the transmit IF filter comprising Y7-&10 and
related components, the signal has been reduced to a 9 MHz SSB signal.
It's output drives 2nd mixer U7. This mixer is also driven by a signal
from the 5.07 MHz LO, Colpitts oscillator Q9 and associated
components."

>From this, I am guessing that:
- Y7-Y10 and related capacitors reject the USB signal
- Something must change (the value of the capacitors C19-C23?) to
allow for the LSB signal to be rejected and the USB signal to pass

I'm not sure where to figure out how to pass the USB signal, though.
That's where I'm getting stumped.

Also, I'm not sure exactly where to place the outputs of the DDS
signal generator. This will replace the 5.07MHz LO (page 9, middle of
page, right-hand side). Should I just not install the crystal (Y6) and
put the output of the DDS in its place, or should I remove all
components related to the 5.07MHz LO and have the input of the DDS at
R48/R49?

If there are other places I might be passing my questions to, please
let me know. Sorry again for the intrusion.

73
-john W4PAH
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