JAMES D. MEINDL 
1171 
ers must use relatively sizeable battery packs 
and cannot be implanted unless provision is 
made for battery recharging through the skin. 
The electromagnetic flowmeter cuff must fit 
rather tightly around a blood vessel in order 
to provide good electrical contacts betv^een the 
vessel wall and the three signal carrying elec- 
trodes of the cuff. A looser fit is permitted for 
ultrasonic flowmeter cuffs since direct contact 
with the vessel walls is unnecessary; however, 
proper alignment of the transducer (s) must be 
assured. Because it requires only a single ultra- 
sonic element, which can be mounted in a silastic 
holder, the pulsed Doppler transducer is per- 
haps the least traumatic of the various flow 
transducers. 
One of the significant advantages which a 
telemetry system offers in the collection of 
physiological data is the opportunity to use 
unanesthetized, unrestrained, undisturbed ani- 
mals. By providing the telemetry system with 
an RF controlled switch or command receiver 
which connects or disconnects the system (flow- 
meter) electronics to its power supply upon 
command from a remote control transmitter, 
automated data collection can take place at all 
hours of the day while still maintaining a small 
average power drain. Ultrasonic flowmeters are 
well suited to this mode of operation. 
A CONTINUOUS WAVE VELOCITY FLOWMETER 
A block diagram of a chronically implantable 
CW Doppler ultrasonic flowmeter which meas- 
ures blood velocity at the center of the lumen 
is illustrated in Figure S.^^^o -phe portion of the 
system within the dashed rectangle is im- 
planted ; the remainder is external. In order to 
reduce its size and power drain as well as im- 
prove its reliability, the implantable portion of 
this system has been designed to use a unique 
family of low voltage micropower silicon mono- 
lithic integrated circuits. This family consists of 
four chips : an excitor oscillator circuit for en- 
ergizing the transmitting transducer; an AM 
receiver circuit for amplifying and mixing the 
signal from the receiving transducer; an audio 
amplifier-FM transmitter circuit for transmit- 
ting the telemetry signal ; and an RF switch or 
command receiver for disconnecting the flow- 
meter electronics from its power source during 
periods when data is not required. The signifi- 
cance of this family of circuits is that it repre- 
sents the initial application of a custom family 
of monolithic integrated circuits to a substantial 
biomedical instrumentation problem. For this 
reason some of the unique details of the circuit 
design are discussed in the following para- 
graphs. 
An additional point of interest regarding the 
flowmeter illustrated in Figure 3 is that it meas- 
ures only the absolute magnitude of local flow 
velocity and is not sensitive to direction of flow. 
A CW system capable of sensing both magni- 
tude and direction of flow velocity is under de- 
velopment in the Integrated Circuits Laboratory 
of Stanford University at this time. 
Exciter Oscillator Circuit 
The schematic diagram of the exciter-oscil- 
lator circuit is illustrated in Figure 4. This cir- 
cuit consists of a basic Colpitts oscillator, a 
buffer-driver amplifier and a class C output 
stage. The design is optimized in several key 
respects for both monolithic construction and 
operation from a single Hg cell power source. 
For example, the collector current of the oscil- 
lator transistor Qg is set (equal to that of Qi) 
by exploiting the virtually perfect match be- 
tween two adjacent monolithic transistors Qi 
and Q2 ; operating transistors Q3 and Q4 at OV 
collector-base voltage eliminates two discrete 
(i.e., external to the integrated circuit) by- 
pass capacitors and conserves battery volt- 
age; using direct coupling between Qe, Q7 and 
Qs precludes the need for several discrete cou- 
pling capacitors ; and operating Qg with a tuned 
load permits a collector voltage swing which 
ideally is twice the supply voltage. 
This circuit operates at room temperature 
over a supply voltage range of 1.0 to 1.35 v with 
a frequency stability better than 1 % ; its cur- 
rent drain is 5 ma; it drives the transmitting 
transducer with a 2 v p-p 6 MHz output voltage 
at an overall circuit efficiency of 30% ; and it 
uses six discrete chip components — Li, Ci, C2, 
C4, C5 and L2. Figure 5 is a photomicrograph 
of the monolithic circuit fabricated in a 70 x 90 
mil silicon die. 
