Telephone Hybrid Circuit
for Radio Transceivers
Not all radio transceivers have capability to connect to a telephone line and transmit and receive information simultaneously. The document on the right is a circuit that I designed and was patented by Motorola for the AN/MARC-66 radio communications. It is comparatively simple, and it outperformed a large AT&T passive hybrid circuit in telephone central stations. The patent period has expired, and this information is not open to the public. The circuit diagram of Fig.1 illlustrates a typical connection betwee two transceivers (mounted in a moving vehicle) through a telephone switchbaord. Fig. 2 illustrates an block diagram of the entire telecommunications system, although six or more radios can receive and transmit simultaneously. Fig. 3 pictures the electronic hybrid circuit. Only one vacuum tube was required in the circuit. Modern FET transistors can as well be used with proper parts value adjustments. The hybrid produces comparatively low level "talkback" levels, power gain and good transfer efficiency in all pathways. The grid circuit provides phase shift to compensate for line impedance variations with proper setting of the balance potentiometer and parts 46 and 47, which are adjusted for minimum unwanted signal feedthrough from receiver to transmitter. If the line impedance can vary, the cathode resistor can be added so that talkback does not become a problem.
Noise Blanker for Radio Receivers
Noise blankers provided noise reduction for impulse noise. Impulse noise can be generated by spark ignition noise, power lines and electrical storms. There are various circuits, but many of them are not very effective. Th radio receiver circuit noise blanking circuit on the right is an expiered patent, of a circuit that produced significant noise reduction. A demonstration was conducted in the near vicinity of a powerline transformer, and the noise completely overwhelmed the signal with the noise blanked turned off. Upon turning it on, the noise could barely be heard. Fpr effectove noise blanking, the noise signal should be located where the bandwidth of the receiver is the highest. This would be at the antenna terminals, but the signal level is generally too low to produce good blankiing pulses, so the output from the first mixer of the receiver was used. A two-stage amplifier boosts the noise pulses to a level suitable for signal processing. The most important section of the circuiit is the pulse processor, which is clearly nonlinear, delaying and shaping the blanking pulse. After more amplification of the noise pulses, they are fed to the grid of the first IF amplifier. The noise pulses and the pulse noise pulses are in phase when the phase shifts of the amplifer and noise blanking circuits are properly adjusted for the proper delay and pulse stretch.
See next page (link below) for the measured results!
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