III.
THEORY OF OPERATION
TRANSCEIVER CIRCUITRY
1.
General Description
The transceiver uses a single tuning section which is
time-shared between the transmitting and receiving functions.
Three co-ordinatinq signals, generated in conjunction with the
repetition rate signal,
control the transceiver switching it
between transmitting and receiving modes.
2.
As a Receiver
Echo signals,
generated when an echo strikes the face of
the transducer,
are presented to the receiver input via R1.
Diodes D1 and D2 protect the receiver input from the out-going
transmit pulse by limiting signals at the receiver input to
plus or minus one diode drop.
Q1 amplifies the echo signal and presents it to the base
of 03 via C3.
(Q2 is associated with the transmit function
and will be discussed later) Q3 provides two receiver functions.
Firstly,
it is a tuned amplifier operating in conjunction with
the tank circuit formed by the inductance of the primary winding
of Tl and C5.
Secondly, Q3 provides time varying gain to over-
come transducer ringing which occurs immediately after trans-
mittinq.
Just after transmitting,
when ringing is most severe, Q3
is driven by very strong signals. These signals cause a buildup
of voltage at the emitter of Q3. R9 and C6 have been chosen so
that the excess voltage at the emitter of Q3 decays away at a
rate which matches the decay of ringing within the transducer.
During the first 1.5 milliseconds after the transmit pulse is
sent,
the receiver is rendered in-operative by a blanking signal.
By the time the blanking signal is removed the decay process is
well under way.
Q3 operates as an amplifier whose gain increases
with time in a way that ensures it is always insensitive to
transducer ringing but still sensitive enough to respond to any
strong echo which may be returned from a nearby target.
3-l
