FRANKLIN, VATNER, HIGGINS, PATRICK, KEMPER AND VAN CITTERS 
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The signal is radiated from the transmitter on 
the animal to a remote receiver vi^here the 
original signals are separated by frequency fil- 
tering. Each of the separate frequency mod- 
ulated signals are restored to their original 
form using frequency discrimination. The out- 
put of the discriminators are voltages propor- 
tional to the measured variables and are re- 
corded on magnetic tape and a strip chart 
recorder. 
The basic VHF transmitter employed is a 50 
MHz oscillator v^ith a varactor diode in the fre- 
quency determining circuitry of the oscillator. 
The modulating signal is coupled to the oscil- 
lator to modify the capacitance of the varactor 
and thus modulate the frequency of the 50 MHz 
oscillator. The 50 MHz signal is coupled to a 
second amplifier v^^hich serves as a frequency 
doubler and power amplifier-buffer. The 100 
MHz output of this amplifier is matched into a 
stub antenna for short range telemetry, i.e., a 
few hundred feet. Where longer telemetry 
ranges are needed, the 100 MHz signal is multi- 
plied to 400 MHz which allows use of a full 
resonant transmitting antenna with consequent 
increase in radiation eflflciency and range (line 
of sight). 
As was indicated above, each of the variables 
to be transmitted must be in the form of a rela- 
tively low frequency, frequency modulated sig- 
nal. The Doppler flowmeter is unique in this 
manner in that the output of the Doppler flow- 
meter is inherently a frequency modulated, 
audio frequency signal. Thus, no further signal 
conditioning must be provided for FM/FM 
radiotelemetry, i.e., the output of the Doppler 
flowmeter can be coupled directly to the input 
of the frequency modulated radiotelemetry 
transmitter for transmission. This approach re- 
tains the inherent zero stability of the Doppler 
flowmeter and the signal need not be demod- 
ulated within the instrumentation carried by 
the animal. A separate subcarrier oscillator is 
provided for each of the other variables, i.e., 
pressure and dimension. Typically, the pressure 
signal is coupled to a voltage controlled oscil- 
lator operating on IRIG band #15 to produce a 
signal varying in frequency from 28 kHz to 
32 kHz as pressure varies from 0 to 300 mm Hg. 
This 30 kHz FM signal is summed with the 0-15 
kHz signal from the Doppler flowmeter and the 
composite signal is coupled to the 50 MHz oscil- 
lator within the telemetry transmitter. Alterna- 
tively, the pressure signal is coupled to a chan- 
nel 15 voltage controlled oscillator and the 
diameter signal is coupled to a channel 13 volt- 
age controlled oscillator and these two signals 
are summed to drive the telemetry transmitter. 
Power and Remote Control 
Power for the components carried on the ani- 
mal is provided by 2000 ma hr mercury bat- 
teries (Mallory TR 236-R). Although the 
discharge voltage from mercury batteries is rel- 
atively constant, voltage regulators are included 
to provide power to the voltage sensitive com- 
ponents, primarily the voltage controlled oscil- 
lator and pressure gauge. Power consumption 
is typically 400 mw for each of the measuring 
instruments and 150 mw for the telemetry 
transmitter, so that total continuous operating 
time of the instruments on one set of batteries 
is typically 24 hours. 
There are instances when continuous opera- 
tion is not appropriate for an experimental sit- 
uation. For example, where ^ baboon is to be 
observed intermittently for a number of days, 
continuous operation of the measuring instru- 
ments would require the baboon to carry an 
inordinate number of batteries. To overcome 
this difl^culty, a remote control device for turn- 
ing the power on and off at will is built into 
the system. The animal carries a miniature, low 
power, 72 MHz AM radio receiver which is con- 
tinuously powered. The output of this radio 
receiver is coupled to 4, low frequency, narrow 
band filters. When a signal passes through one 
of the filters a latching relay is thrown. In this 
way one can switch a latching relay by trans- 
mitting a 72 MHz signal modulated by a fre- 
quency corresponding to one of the filters. The 
latching relay is thrown to its original position 
by transmitting a tone corresponding to one of 
the other filters connected to switch the relay. 
In other words, one can control 4 functions by 
transmitting the 4 tones corresponding to the 
frequency of the 4 filters. This remote control 
capability has advantages beyond simple con- 
trol of power. The animals are typically instru- 
mented with 3 flow transducers at three differ- 
