|Using ceramic resonators in
|Although the output is
flat with frequency, pre-emphasis is added to the mic
audio before the first mixer, since this seems to enhance
readability of my particular voice. Compression is about
10 dB with the values shown and the lapel mic that I use.
In the past, I have used back to back diodes to clip the IF signal, but using one half of the SA604 limiter sector worked first time and the clipping looks really predictable. There was only 2mV of signal coming out of the ceramic filter, so the higher gain (60dB) second limiter stage was used on this occasion. To be sure that harmonics due to limiting didn't also get demodulated in the second SA602 mixer, a LPF was added after the limiter, and I suspect that this is why the processor ouyput is so well defined. It really needs a video to show this at its best, but the following still will have to do:
|A single board version was
then built (to replace the processor in the Weaver
transceiver that has never sounded brilliant on transmit)
and the same ICs used - the SA605s that were found and
which would reduce the IC count were found to be in a very
compact package with horribly close pin spacings).
The rather over-etched pcb opposite was just useable. This unit used 456 khz resonators, with coupling and shunt capacitances very similar to the first version. Having ordered 200 455 khz resonators from China, it will be interesting to apply a bit more logic in their use, since currently, I do not select resonators but use them randomly. With luck, the really cheap Chinese resonators (£1.32 per 50) will have a much wider tolerance actual frequency, so that selection of frequency rather than the pulling down of frequency with capacitance can be tried. You can certainly see the Q fall as parallel capacitance is added - sometimes this requires that the higher frequency resonator (with no pulling capacitance) has to be damped down with shunt resistance to make it similar to the other two, in order to achieve a flat top.