94 Lecture 5 
Fig. 5.6. Velocimeter with case removed. 
Q5-6 further cleans up the base lineand removes most of the rf. The output here 
is a rectangular positive pulse of about 3 v amplitude and 8 psec width. Stage Q7 
is an inverting, low-impedance-output power amplifier which produces the 
negative-going pulse necessary to synchronize the blocking oscillator. It is 
adjusted to provide a pulse large enough to trigger the blocking oscillator reli- 
ably, but not so large that there is any possibility of doubling. 
5.3.3. Frequency Measurement 
The input to the frequency measurement equipment istaken from the emitter 
of the blocking oscillator where a narrow rectangular pulse of amplitude -1 v is 
available. The prf, which is inthe neighborhood of 7 kc, may be measured direct- 
ly, but in the case of a very long cable connecting the sing-around circuit to the 
measuring equipment, as in deep-sea work, the signal is first converted to a 
square wave (about 3.5 kc) by means of an Eccles— Jordan type of divider. The 
