Sunday, October 3, 2010

Keying by Counter

Here's another trick I used in one of my rigs, chirp-free keying by means of a counter.
In previous entries, it was indicated that counters could be used to reach a certain frequency, in particular in the LF and MF bands.
Here's an example, well, this example actually reflects the design of my 600m keyed exciter.
To reach 500kHz the signal of a 4MHz ceramic resonator VFO (74HCT04 in Pierce configuration, with some gates used as buffers) is divided by 8. This involves preferably a ripple-counter. My exciter design involves a 7493 which has got an "enable" circuit. And here is where keying is done. The oscillator, since it is using a different IC, is not affected by the keying and therefore does does not create any chirp.
This concept does not provide "click prevention", however, I believe clicking is more acceptable than chirp.