2006-11-11
SSB Exciter
I recently ordered 200 units of 11.98135 MHz crystals from Hy-Q for use in crystal ladder filters. (They were $5/100 in their surplus stock sale section) With the construction of my RF sweeper, I can now make precision filters suitable for SSB and CW transceivers.
Four crystals were selected with a minimum frequency within 50 Hz. The 30p to ground-shorted shift was measured and capacitors calculated for a 2.5 kHz bandwidth. Some fiddling later and a filter with fairly good characteristics was produced. The termination Z was selected to be 150 Ohms and some tests showed it offered pretty good ripple with the particular crystals used.
A crystal oscillator with padding inductor was constructed to generate the carrier, and a diode DBM was made with ferrite beads and 1N4148 diodes.
A quick lash-up on the desk proved the carrier oscillator and DBM alone produced a nice DSB signal.
The crystal filter was added with a buffer amplifier after it to compensate for its insertion loss. Some teaking of the carrier frequency later and a very clean LSB signal was produced (LSB is easier because of the filter shape). I used an AM radio as a signal source.
Next step is to make an microphone amplifier with AGC and then start thinking about switching the modules around to enable RX as well (I've already purchased DPDT mini-relays to do this). Then it is a relatively simple matter of building transverter-style circuits for the bands of choice. I think I'll investigate switchable carrier oscillator frequencies to enable LSB/USB selection. This will also give me more choices of LO injection frequencies.
40 metres will probably be my first band, I have a dipole for it already, and it is fairly open at the moment despite the solar cycle being at rock-bottom.
Carrier suppression could be better, the crystal filter knocks it down 3-6 dB, and it is already about 16 dB down. I may replace the DBM with a single-balanced modulator having a carrier nulling pot so I can reduce the carrier power as much as possible.
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