FM (frequency modulation), PAM (pulse amplitude modulation), PWM or PDM (pulse width or 
duration modulation), and PCM (pulse code modulation). The PCM systems were considered 
highly desirable for this application because of their high accuracy, but the equipment was 
rather large and complex for use as control equipment. The other types of pulse systems had 
little to offer in accuracy over the FM systems, and had the disadvantage that such systems 
are somewhat subject to loss of synchronization if the radio signal is momentarily lost. 
Investigation of the FM systems disclosed that system accuracies of better than 
1 percent should be possible if components were carefully selected and used under limited 
environmental conditions. This type of system could also be expanded very easily if addi- 
tional data channels were required at a later date. 
Subcarrier oscillators and discriminators were commercially available with stability 
and accuracy within + 1/4 percent or better. Subcarrier oscillators and phase-modulated 
transmitters were generally available in subminiature form; however, at the time there had 
not been a large demand for subminiaturized discriminators and receivers since this is normal- 
ly ground-based equipment, and for this reason, only a few types were available. Transistor- 
ized subcarrier equipment was becoming available, but the stability and accuracy were not 
comparable to the best subminiature vacuum tube gear. 
The final decision was to use vacuum tube FM-PM telemetering gear for both the 
control and readout portions of the system. It was realized that this equipment would be too 
large for use in 10- to 15-foot models. However, since a single system could not possibly 
meet the conflicting weight requirements for small models and the servo and propulsion power 
requirements for large models, it was considered desirable to have a system suitable for use 
in the models of the sizes most frequently used. Although commercial subcarrier circuitry 
could also have been used for control of on-off functions, such as fail-safe and propulsion 
motor reversing, it was felt that low-frequency tone circuitry would be more suitable for these 
purposes. This would permit a great saving of weight. By using high-Q circuits it would be 
possible to handle several on-off channels in the frequency band occupied by one standard 
FM subcarrier channel. Further reductions in the weight of this equipment could be gained 
by the use of electromechanical oscillator controls and frequency detectors which eliminated 
the need for large high-Q inductors in the frequency-determining circuitry. 
In order to maintain as high a degree of stability and accuracy as possible, the use of 
regulated power supplies was considered mandatory. Vacuum tube regulators could be used 
for the console circuitry, but to keep the power drain in the model equipment low, transistor- 
ized regulators were required. These regulators have an efficiency of about 80 percent as 
compared to the 25-percent efficiency, and greater size and weight of vacuum tube 
supplies. 
There were no servo actuators commercially available meeting the variable-speed and 
torque requirements for the rudder servo. A unit of special design was required to meet all 
of the system requirements including compatibility with existing dynamometers. To meet the 
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