M. Greenspan and C. £. Tschiegg 95 
Fig. 5.7. Sound head of a velocimeter built by the U.S. Naval Ordnance 
Laboratory. 
impedance is matched to the cable by an emitter—follower amplifier. For ocea- 
nographic applications, the cable need only be an insulated single conductor; the 
sea acts as the return. 
At the output end of the cable appears an attenuated, nearly sinusoidal wave. 
This is amplified and doubled. In any case, then, we have to measure small 
variations in a frequency of about 7 kc. 
The prf of the system may be measured by any of the standard techniques 
depending on the particular requirements and the available equipment. In the 
laboratory an electronic counter is very convenient, but a stable radio receiver 
tuned to a high-order harmonic (say 200) of the prf gives equally good results. 
In many field applications automatic operation and/or recording is required. 
In such cases the multiplied frequency is converted to an audio-frequency which 
may be fed to an ordinary frequency meter having a dc output suitable for opera- 
tion of a recorder. The local oscillator is preferably crystal controlled. 
