1138 
MONITORING 
grams (EMG), require relatively wide band- 
widths of several kilohertz. The signal's ampli- 
tude may vary from 50 microvolts to several 
millivolts, and the transmission range is usually 
from a few meters up to 30 meters at a power 
consumption of milliwatts. 
Figure 3 shows the two, single channel, con- 
tinuous wave FM transmitters ^ developed at the 
Engineering Design Center of Case Western 
Reserve University. Table II lists the perform- 
ance of the two transmitters. They can be pow- 
ered by a #312 Mercury battery with an extern- 
ally controlled magnetic reed switch to turn on 
and off the supply, or powered by RF induction 
power across the skin. For a 30" x 24' 
Figure 2.— Four popular telemetry transmitter circuits. cage, the RF power source required 
X 14" 
is 150 
limitations may be removed. Figure 2 shows 
four popular transmitter circuits from the early 
literature. 
The circuit in Figure 2(a) uses a piezoelectric 
crystal, as the feedback element in an oscillator, 
to achieve good carrier-frequency stability,^" it 
is widely used for animal tracking and narrow- 
band signal transmission over a range from 30 
meters to several kilometers; both CW and 
pulsed operation can be obtained by varying the 
value of resistor Ri. 
The circuit in Figure 2(b) is a common- 
emitter Hartley-oscillator circuit the trans- 
mitter is modulated by varying the rate of 
pulses of radio frequency oscillations. It is pop- 
ularly used to measure temperature, by replace- 
ment of the resistor R with a thermistor ; pres- 
sure, by allowing pressure changes to move the 
core M; pH, by attaching suitable electrodes 
across the voltage input X ; and other variables, 
by means of suitable transducers. Continuous 
wave operation can be obtained by reducing the 
value of R. 
The remaining two circuits of Figure 2 are 
common-base oscillators; circuit 2(c) is a Col- 
pitts circuit ;^2 circuit 2(d) is a Hartley circuit.^^ 
Both use the voltage-sensitive emitter-to-base 
capacitance of the transistor to modulate the 
frequency of the carrier. These circuits, de- 
signed to transmit electrical signals such as 
electrocardiograms (ECG) and electromyo- 
watts ; for 18" x 14" x 14" cage, 30 watts is re- 
quired. The absorbed power density in the cage 
Rl T.D. 
1.5 V 
^ B.D. 
K-5 
ic-,05 
K - 6 
Figure 3. — Two single channel FM transmitters. 
