70 cm in front of the transducer-pair, in the 
clear water where the flow pattern is unaffected 
by the presence of the meter. 
Since the current meter has no moving parts 
it can respond almost instantaneously to flow 
changes, being limited by the transit time of 
an acoustic wave from the point of measurement 
to the receiver (less than 1 ms). Spectrum 
broadening and rapid shifts of frequency of the 
returned signal can be expected in turbulent 
flows and should provide a reliable index of 
turbulence in the measured current. 
DESCRIPTION OF THE CURRENT METER 
The current meter consists of an oscillator 
driving the transmitting crystal, an amplifier 
tuned to the oscillator frequency (10 mc) and 
driven by the receiving crystal, a detector, 
and an audio frequency stage with emitter fol- 
lower output. These elements are enclosed in 
a pressure-tight case. The instrument operates 
on 1.5 v and, except for the battery, is com- 
pletely enclosed in a cylindrical package about 
12 cm long and 3.2 cm in diameter. The crystal 
transducers are mounted on one end of the cyl- 
inder, while the electrical connectors exit 
from the other. The complete current meter is 
shown in Fig. l. 
Complete isolation of the oscillator and 
amplifier stages is provided by the internal 
shields shown in Figs. 2 and 3. The outer 
case provides structural strength to withstand 
the water pressure and is sealed at both ends 
with O-rings. The two electrical conductors, 
one for -1.5 v and the other for the audio 
Signal output, pass through small O-ring stuff- 
ing boxes to prevent leakage around them. 
The transmitting and receiving crystals 
are standard stock barium titanate discs 6.5 
mm in diameter and about .25 mm thick. The 
thickness-resonant frequency is about 10 mc. 
The theoretical half=-beamwidth of these crys- 
tals at 10 mec is 1.7° and the actual measured 
half-beamwidth is about 2.5°, as shown in 
Fig. 4. The crystals are mounted 6 mm on 
centers and backed with .8 mm of epoxy circuit 
boarde 
The oscillator is of unique design, employ- 
ing a single tunnel diode as the active element. 
The tunnel diode is superior to conventional 
transistors in this application because of its 
small size and excellent current-handling 
ability at high frequencies. Although the im- 
197 
pedance of the crystal is low at 10 mc, it is 
possible to apply 2.0 v peak-to-peak across the 
crystal with the tunnel diode. 
The tuned amplifier stages employ MADT, 
high-frequency transistors operating well below 
their rated voltage. There are three stages 
with a gain of about 20 per stage. There is 
enough acoustic coupling between the transmit- 
ting and receiving crystals to provide the 
reference frequency; hence, no electronic mixing 
is required. The Doppler frequency appears as 
amplitude modulation on the 10-me carrier. 
The Doppler frequency is extracted from the 
10-me carrier with a highly sensitive backward 
diode detector, then amplified. The Doppler 
frequency shift for a flow range of 1 mm/sec to 
10 m/sec is 13 to 130,000 cps. In this form 
the data can be processed for recording on tape 
recorders or for telemetering to data-gathering 
statims. 
The Doppler-shift flowmeter requires very 
little power and operates on 1.5 v, drawing only 
50 mae It could be operated continuously on a 
single rechargeable flashlight cell for about 
8 hours. 
The Doppler-shift ocean-current meter has 
been tested in the laboratory in a flow tube. 
Results indicated that, if sufficient scatterers 
are present, the instrument will function well, 
although it was necessary to add scatterers to 
Palo Alto tap water to get satisfactory returns. 
The observed Doppler frequency checked closely 
with the measured velocity in the tube. In 
order to try the meter in "ocean" water, it was 
taken to Palo Alto Yacht Harbor on San Francisco 
Bay for additional tests. Sufficient scatterers 
were present in the yacht harbor water to get 
return, and the meter performed as anticipated. 
The signal-to-noise ratio was low, but work is 
underway to decrease the noise level. 
CONCLUSIONS 
This paper describes an ocean-current meter 
which uses the Doppler shift in continuous-tone 
volume reverberation to measure flow rates. 
Operation of the instrument depends on suf= 
ficient scatterers being present in the water 
that is to be measured. It is assumed that, at 
an operating frequency of 10 mc, there are 
