[Collins] KWM2A receive alignment - USB/LSB

[Gerald]

On Thu, 2006-02-02 at 21:07 -0500, Steve Garwood wrote:
> I went back and swept the calibrator S-meter reading on 20 meters @ 14.200.
> 
> Looking at the S-meter reading and noting the dial frequency, here is what I
> came up with:
> 
> On LSB, from zero beat, the S meter rose rapidly to S-8 and held until +2.0
> KHz.  At this point there is a sharp linear drop ending on S-0 at +2.5 KHz.
> 
> On USB, from zero beat, the S meter rose rapidly to S-8 and held until a
> sharp drop began at -2.0 KHz.  The S-meter began dropping in level to S-6.2.
> At -2.5 KHz it rose back up to S-7. This was a sharp peak.  From -2.5 KHz,
> the meter took a sharp linear drop to S-0 at -2.9 KHz.

If the bfo crystals don't match the slope of the filter, there should be
a difference in signal level at zero beat, and the number of Hz between
zero beat and S7.5 (3 dB down) will be different. You may have to
interpolate the 1 KHz dial increments to get more detail. There normally
is some variation across the pass band, but it should only be a couple
dB at the most (6 dB per S-unit). Usually there are three peaks in the
pass band, one near each edge and one at the middle.

When you sweep on USB you get the same curve, but opposite direction on
LSB. So you should see the same out of passband peak beyond zero beat on
LSB.

That signal out beyond 2 KHz (should out beyond 2.4 KHz, e.g. the 3 dB
point should be at 2.4 KHz) looks like a filter problem, not a BFO
crystal problem. There is generally a notch and a little signal again
past the notch but not nearly as much as you see.

The filter should be very close to symmetrical in its response, yours
isn't.
> 
> This was with a 50 ohm dummy load connected to the antenna jack.  With the
> antenna connected or with the antenna disconnected, the only difference was
> the S-meter reading.  The break points remained the same dial frequency
> wise.

The antenna connection is a long ways from the filter, but when
expecting a single signal from the calibrator can be confused with an
antenna contributing noise and other signals.
> 
> The track of these readings is very stable.  I can rock the PTO back and
> forth at the break points to see this very clearly.
> 
> Question now is probably the same as far as what to do about it.  These
> readings help visualize what I'm hearing. It seems to me that there is
> something going on with USB.  I don't have anything to compare the readings
> to.
> 
> Steve, N0CZV

You could try aligning the IF transformers to minimize the HF parts
beyond that notch on USB, but you may introduce slope in the main pass
band that won't help.

You could check the filter terminating capacitors, but they won't
probably affect anything but overall gain. They might affect the ripple
a little.

So you can shop for a filter, or live with the difference. Is the filter
round or rectangular? metal cased or plastic?

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer