A technique for the design of pulse time multichannel radio systems

Abstract

The description of a technique for the design of pulse time multichannel radio systems, the application of the design to an automatic 24-channel system, the description of a first 18-channel experimental system and its performance in radio links are given.The fundamental parts in a pulse multichannel system are: the distributor (transmitter) and the synchronized distributor (receiver) producing the correct time distribution between channels, the modulator (transmitter), the demodulator (receiver) and the mixing and radio frequency transmitting and receiving circuits.Simple conceptions are introduced in the design of each of these parts.The distributor uses a delay line, in a special feedback circuit, and a square wave. Each channel is selected by a correct tapping on the delay line and the channel boundaries are accurately timed by the square wave. The whole scheme is entirely automatic and within large limits it is unaffected by the pulse shape distortions appearing at the tappings along the line. In the distributor, and generally in the system, a special three-valve multivibrator is used as pulse generator. It has the property of producing pulses of great peak power in low impedances.The modulation is obtained by means of trapezoidal pulses triggering the short pulse multivibrator generators. When a signal is added to the trapezoidal pulses, the instant, of triggering is effectively time modulated. The advantage of using trapezoidal pulses is that the pulse time modulation never crosses the boundaries of the channel.A special type of short pulse multivibrator has been designed. It uses an inductance or, more exactly, a shock-excited tuned circuit, to define the length of the pulse. A three-valve multivibrator of this type has the advantage of producing pulses of great peak power. This type is called in the paper a “power modulator.”The demodulation process consists of transforming the time modulation into variable length pulse modulation and then to filter the signal with a low-pass filter.To change phase modulation into length modulation a special type of multivibrator is used. The circuit is so adjusted that the multivibrator becomes sensitive only during the time allocated to the corresponding channel and it triggers only when the corresponding channel pulse appears. This continues until the time allocated to the channel comes to an end. Here also “power multivibrators” are used to give a considerable gain in power.An important feature of the modulators and demodulators is the use of one circuit for many channels. This appreciably reduces the number of valves required. Such circuits are called in the paper “multiplex modulators” and “multiplex demodulators.” It is shown that if the power multivibrator is used, the cross-talk in these circuits is negligible.The mixer circuit comprises cathode followers with cathodes connected together. When multiplex modulators or demodulators are used, about three valves are required per two channels including mixing valves.This technique is particularly valuable for systems with a large number of channels. The optimum seems to be between 20 and 40 channels. The maximum number appears to be one hundred.Block and simplified schematic diagrams are given for a 24-channel system, as an example.Finally, a very early experimental 20-channel system, built in 1943, is described and its performance is discussed. This system was satisfactorily tested up to distances of about 36 miles in mobile vans.