Dear Alan,
interesting Your crystal mixer exciter, but depending on a special diferential
variable capacitor. But I would not recommend varactor tuning unless You manage
to keep the r.f. voltage across the varctors rather low.
In my crystal mixer exciter I have chosen standard microprocessor crystals 5200
kHz and 5068.8 kHz. The 5200 kHz CO is fixed, and the 5068.8 kHz is a VXO with a
coil (in reality a molded choke) of 100 microhenries in series with an old brass
Philips trimmer of about (as far as I can remember) 15 pF variable capacity,
shunted by a 10 pF ceramic disc. With this arrangement I can also cover the
whole band. A different variable capacitor may need a different series coil, of
course. However, I also have not made exhaustive stability tests so far.
Keying is accomplished in the emitter of a third transistor operating as a
mixer. This allows soft keying to be accomplished. The emitter is blocked for LF
only, and two resistors of 390 ohms in series are used (this sum giving optimum
mixer gain in my case). The connection between both resistors is blocked by a
6,8 to 10 MFD tantalum electrolytic. The LF in the collector circuit is passed
through a lowpass filter to a final emitter follower (all transistors BC548B in
my case).
DF3LP is employing basically the same circuit using 4 MHz crystals but is keying
one of the oscillators, as You do. He has already done much QRS operation and is
very pleased with the stability achieved.
HW?
73 Ha-Jo, DJ1ZB
Alan Melia schrieb:
On the crystal mixer front, I lashed up a source from crystal modules out of
an old Pye Westminster (the fundamental oscillators, usually the TX pcbs) I
found that by taking the fixed padding (2 by 18pF) off and leaving the 12pF
trimmer, I could connect a differential capacitor across the crystal-trimmer
caps swinging one up and the other went down. That way I could cover the
whole band with a pair of carefully chosen rocks. The result seemed quite
stable but I have not given it exhaustive tests. These units are channel
switched by grounding the emitter line so can be keyed directly. The
resultant waveform with just one oscillator keyed doesn't look too bad.
The most probable problem could be that the differential cap is a single
bearing job (A bit of good old Govt surplus) and liable to vibrate
mechanically. It might be possible to do the same thing with varactors and a
twin gang pot, except that the tuning law would perhaps be a bit odd.
73 de Alan G3NYK
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