OSCILLATORS
I originally used crystal
oscillators to generate the transmit and receiver
local-oscillator frequencies. This was done to ensure a
stable frequency for systems like TAPS. As it became harder
and harder to obtain crystals at the frequencies I needed,
I discovered digital direct-synthesis oscillators. Oh, joy!
These are so easy to use and fit very well with the
controllers that I prefer. Why would you use anything else?
I haven't sorted my design data out yet and probably won't
for some time yet. But you can see some DDS designs in the
schematics for the CPU/IO on TAPS-6 NewGen. I will pull
together the test code and test circuitry I used with the
Analog Devices AD9835 DDS chip to show how easy it is to
use these gadgets in a small sonar. In the case of a system
like TAPS or WHAPS or SandScan, I need to load the DDS
registers with two frequencies: a mulitiple (2X or 4X,
depending on the transmitter logic) of the transmitter
frequency and the transmit frequency plus an offset to
drive the mixer in the receiver. The AD9835, conveniently,
allows just this. Then the transmit/receive code in the CPU
sets the DDS to output the transmit frequency, toggles a
bit line to gate the transmitter on and off, and then
switches another bit line to change the DDS frequency from
transmit to receive. Happens in a flash! So the receiver is
tuned to the transmit channel immediately at the end of the
transmit pulse. Slick.