Proportional control or readout data are applied to potentiometer-controlled subcarrier 
oscillators. Variation of the potentiometer slider causes frequency modulation of the sub- 
carrier oscillator. For model equipment simplicity and increased readout accuracy, reactance- 
controlled subcarrier oscillators are used in the rpm readout circuits (Figure 3). In this 
application, soft iron slugs are used to vary the inductance of a tank circuit which in turn 
controls the frequency of the subcarrier oscillator. Separate subcarrier oscillators and controls 
are used for each channel of information. Standard IRIG* subcarrier frequencies are used with 
band selection based on use of preferred bands and type of data to be transmitted. The fre- 
quency range of 200 to 500 cycles per second is used for the on-off control tones. This re- 
quires the elimination of IRIG subcarrier band No. 1 which has a center frequency of 400 
cycles per second. 
The control tones are generated by a group of resonant-reed oscillator-control units. 
The resonant reed has a very high Q and good frequency stability which permits the use of 
many channels within a small range of frequencies. Feedback power for driving the reed is 
provided by a single-stage vacuum-tube amplifier. Because of their high-Q and long buildup 
time, the oscillators are allowed to run continuously and their output is switched on or off 
as required. 
The outputs of the resonant-reed circuits are mixed in a resistance network and then 
passed through a low-pass filter to remove harmonics. The filtered tones are then mixed 
with the outputs of the subcarrier oscillators. In the control link (Figure 1), the mixing is 
performed in an operational amplifier circuit which is capable of driving the transmitter 
circuitry via the interconnecting cable in the console. In the model readout link (Figure 3), 
the operational amplifier is replaced by a simple resistive mixing network which drives the 
transmitter through a very short cable. 
The transmitters are crystal-controlled, phase-modulated units and have a maximum 
power output of 3.5 to 4.5 watts. Since full power is not required for this application, the 
transmitters are operated at reduced plate voltage to extend tube life. Power output under 
these conditions is about 2 watts. 
Ground plane antennas are used for all transmitting and receiving circuits. These 
antennas are small, lightweight, and omnidirectional, and provide proper impedance match 
for the transmitters and receivers. 
Miniature crystal-controlled FM receivers are used in both the control and readout 
circuits. The receivers contain a plug-in RF section for channel selection, but have no 
controls to be adjusted during operation. The output of the receivers is sufficient to drive 
all subcarrier and tone-channel circuitry. The receivers have an automatic gain-control 
circuit for operation over a wide range of signal levels. The AGC circuit output is also 
used for control of external circuitry. 
*Inter-Range Instrumentation Group. 
II-2 
