CIRCUITS
This is intended as a general
repository for circuit notes about transmitters, receivers,
and ancillary circuits that are useful in ultrasonic
systems such as TAPS. Maybe you don't know it but
retirement is a full-time job and I don't have near the
spare time I used to have when I was working!
Designing self-contained acoustic systems requires one to
deal with two major problem areas: power consumption and
noise.
Since a self-contained system must run on batteries,
extravagant use of battery capacity leads to short
deployment life. This constrains one to use switching-type
power amplifiers, which then require low-pass filters as
part of the matching network to the transducer. Class A
power amplifiers draw too much current when idle to even be
considered. See the page on TRANSMITTERS.
Echoes from zooplankton, in particular, are quite weak.
Echo voltages can be in the micro-volt range. This requires
considerable gain in the receiver and very careful
attention to sources of noise. If you are required to
combine the transmitter and receiver on a single card, the
various oscillators required can cause noise to travel on
the groundplane into sensitive parts of the receiver,
swamping the weakest echoes. Since I always needed to
measure the echo amplitudes precisely, it turned out to be
a better design to have both + and - supplies for the
receiver. Problem was, I had a single battery to supply
power. I have had some success with voltage inverters,
although these can be a vexatious source of noise
themselves. Careful layout of ground planes, some
shielding, and a few inductors in the power leads seem to
make things work pretty well. The best receiver I ever
built had a voltage inverter section in it.
I have been intrigued with the notion of a simple receiver
(bandpass filter, impedance matching, maybe a little gain)
driving a fast ADC into a DSP chip. All of the processing,
including some filtering, could be accomplished in the DSP.
These are devilishly hard to program and I haven't yet made
a serious effort to learn. Sigh ... someday.