On Sun, 2009-03-29 at 16:36 -0400, John Graves wrote:
> If I may jump in with a question. I have a jupiter but my Drake 2B rcvr
> might need this, after I recap it.
>
> I have a MFJ 259B antenna tester, which will work as a frequency counter
> that does resolve to 1 Hz.
Resolution is distinct from accuracy. What is the precision of the
counter's clock? It can be as bad as 100 ppm which is a 100 Hz error at
1 MHz or 900 Hz at 9 MHz which is disastrous when aligning a BFO to a
filter.
> Do I need to build a probe with a clip on
> the outboard end, or is placing my antenna close to the circuit
> sufficient?
You might need a close connection but you have to watch that the
connection doesn't change the oscillator frequency. Try it without (if
the accuracy is adequate) and if you don't get a reading, then you need
a direct connection.
> There is a max of 5V Peak-to-peak on the input of the counter
>
> John Graves
> WA1JG
> jh.graves@verizon.net
An alternative way of setting the BFO is how we did it before crystal
BFOs. Using a stable frequency and amplitude signal, like the crystal
calibrator, tune across the pass band and then down the LF slope of the
filter (LF as at audio on the desired sideband) 20 dB as closely as
possible. Tune the BFO for zero beat. Repeat for the other side band.
Then start over until they don't need adjusting anymore. In Omni V and
VI they interact because its the same crystal being shifted by switching
in more capacitance to lower the BFO for USB injection AT THE IF
FREQUENCY. Or rather than 20 dB down you could look for the 3 dB down
point and go out another 250 to 300 Hz which ought to be close to 20 dB
down for typical filter skirts. This technique takes practically no test
equipment if the receiver has a crystal calibrator, but it takes more
time than having a quality counter.
For a signal source, tune by 14.318 MHz. That's four times the NTSC
color burst frequency and most PCs have an oscillator at that frequency
for (unused) video. Or with an analog color TV on, tune to 3579 KHz. Or
a local MW AM broadcast signal. A shortwave broadcast signal may not
have good enough amplitude stability.
Of course, if the receiver has 10 Hz resolution on the dial, once you
have found the two 20 dB down points you can make note of their
frequencies and return by the dial, not by measuring 20 dB down again.
73, Jerry, K0CQ
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