92 Lecture 5 
silver solder. The two transducers in each set are oppositely poled for proper 
operation with the circuit to be described. Ifanother inverting stage of amplifica- 
tion were added, then the transducers would have to be similarly poled. 
The transducer mounts are better seen in Fig.5.4. They are hollow and con- 
tain only the electrical connections and silicone grease (DC-4). The transducers 
are held in place by the spring retainers shown, but are not cemented or otherwise 
fastened. The grease is centrifuged into the mount to eliminate air bubbles and 
the final seal is made using a thin neoprene gasket under the perforated top cap. 
The ceramic disc is thus under substantially hydrostatic pressure so that there 
is no tendency for water to leak in at the edges of the disc or to percolate 
through it. We have never been able to fabricate a rigidly backed transducer, 
even using heavily plated material especially processed for low porosity, which 
is completely impervious at pressures above a few hundred atmospheres. The 
hollow mounts have been repeatedly tested to 1500 atm without evidence of leakage. 
5.3.2. The Electronics 
As may be seen from the schematic diagram, Fig. 5.5, the circuit employs 
8 microalloy transistors, all of the same type. The power required is 8 ma at 
6.5 v and may be obtained from any sufficiently stable supply. For instruments 
that are to be lowered to great depths, the most practical supply is a pair of 
mercury batteries, visible at the top of Fig. 5.6. 
The blocking oscillator (Fig. 5.5) utilizes a miniature, ferrite-core pulse 
transformer of one-to-one turns ratio. It is adjusted to run free at about 6 kc, 
somewhat lower than any synchronized prf which will occur in practice. The 
blocking oscillator applies to the sender a positive pulse of amplitude about 6 v 
and width somewhat less than 1ysec. The sendor oscillates briefly at its natural 
frequency of 3.6 Mc and a corresponding pressure variation is transmitted 
through the water to the receiver. It is only the initial transient, confined to less 
than the first quarter-cycle of this disturbance, which is of interest here. Sup- 
pose the rising part of the positive BO pulse expands the sender, then a pressure 
wave travels through the water and compresses the receiver. It follows from the 
piezoelectric equations that the receiver would put out a positive-going voltage 
if the transducers were poledalike; inourcase, they are oppositely poled and the 
input to the base of common emitter stage Q1 is negative-going. 
The amplifier section consists of common emitter Ql, common collector 
Q2, and common emitter Q3. Stage Q3 is heavily saturated with negative-going 
output. Detector Q4 rectifies and amplifies; the output is about 3.6v rf positive- 
going. It also removes mostof the base line noise. The first four stages Q1-4 are 
intended as a rise-time amplifier; for example, corresponding to an input slope 
of 5 mv/psec is an output slope of 75 v/usec. The relevant gain in the linear 
range is thus about 15,000. In operation, however, stage Q3 is heavily saturated, 
and the output of Q4 is at its limiting value, 150 v/ysec. Note that it is possible 
to apply the output of Q4 directly to the sender, thus obviating the necessity for 
stages Q5-8. This arrangement has been tried out in the laboratory. It has the 
disadvantage that it is not self-starting, and also that one or more independent 
series of pulses may arise adventitiously. 
The remainder of the circuit removes these inconveniences. Schmitt trigger 
