1176 
MONITORING 
Figure 7. — Photomicrograph of AM receiver chip. 
the bursts and it is simpler and less power- 
consuming than the normal technique of deriv- 
ing the pulse repetition frequency from the 
ultrasonic frequency by a series of binary di- 
viders. The transducer emits a 1 fisec burst 
of sound, then acts as a receiver for the follow- 
ing 49 /x,sec. The received signal is amplified 
and phase-detected against the 6 MHz oscillator, 
with the resulting video signal of 0.5 MHz 
bandwidth telemetered along with a timing 
pulse. In the external electronics, multiple range 
gates with different range settings are used 
simultaneously to give the instantaneous veloc- 
ity profile across the vessel. 
In the integrated circuit design for this sys- 
tem, emphasis is on low voltage operation and 
low current drain. To minimize the number of 
discrete components, two Hg cells are used to 
give ±: 1.3 V supply voltages and thus provide 
three ac ground points in the circuits. The nec- 
essary acoustic power is minimized (about 25 
mW peak, 0.5 mW average) by transducer and 
RF amplifier design; the use of a command 
receiver enables the flowmeter electronics to be 
switched on only when data is being collected. 
Due to the need to measure flow velocity at the 
near wall of the vessel, transducer ringing time 
following a transmit pulse and receiver overload 
recovery time are minimized. 
An additional point of interest regarding the 
flowmeter illustrated in Figure 12 is that it 
measures the absolute magnitude of the flow 
velocity profile and is not sensitive to direction 
of flow. A pulsed system capable of sensing both 
magnitude and direction of flow profile is now 
under development in the Integrated Circuits 
Laboratory of Stanford University. 
