JAMES U. MEINDL 
1175 
+ 1.3V 
Q 
6 MHz AM RECEIVER 
Figure 6. — Schematic diagram of low noise AM receiver circuit. 
transistor power switch which provides a con- 
nection between the flowmeter electronics and 
its battery power source only during those in- 
tervals when a command signal is present. Fig- 
ure 11 illustrates a photomicrograph of the 
command receiver chip. 
A PULSED VELOCITY PROFILE OR 
VOLUME FLOWMETER 
A simplified block diagram of a chronically 
implantable pulsed Doppler ultrasonic flow- 
meter which measures blood velocity profile and 
lumen diameter and hence instantaneous volume 
of blood flow is illustrated in Figure 12.21.22,23 
The portion of the system within the upper 
dashed rectangle is implanted; the lower rec- 
tangle contains a block diagram of one channel 
of an eight channel external system. As with 
the CW flowmeter, the pulsed system is de- 
signed to capitalize fully on the size, power 
drain and reliability advantages offered by 
monolithic integrated circuits. Because of the 
complexity of this system, it is virtually a neces- 
sity to implement the implantable portion with 
integrated circuits specifically designed for the 
purpose. 
As diagrammed in Figure 12, a novel two- 
oscillator scheme is used in this particular 
Doppler system. The 6 MHz ultrasonic oscil- 
lator which excites the transducer is synchro- 
nized to the 20 kHz oscillator immediately be- 
fore each transmitted burst of ultrasonic waves. 
This ensures the necessary phase coherence of 
