What is DC coupling?
The fundamental purpose of telemetry is to replace a signal wire – and wires display DC response. In frequency modulation (FM) systems, the information or intelligence is transmitted by varying of the carrier frequency around the assigned center frequency of the transmitter. The information is then recovered by demodulating or detecting this frequency variation via a FM receiver. In a positive sense transmitter, as the input or modulating voltage is increased, the carrier frequency increases a corresponding amount. The amount of this frequency increase divided by the input voltage increase is termed deviation sensitivity. In a DC coupled transmitter the carrier frequency will remain at this new frequency until the input is again changed whereas in an AC coupled transmitter the carrier frequency is always attempting to return to center frequency. The rapidity at which the carrier returns to center frequency in AC coupled transmitters is determined by the lower frequency response of the phase locked loop in the transmitter. There is, however, a trade-off involved - the lower the frequency response the longer the transmitter takes to initialize. In a DC coupled transmitter this tradeoff is avoided. For further information on DC coupled phase locked loops, the reader is advised to review Emhiser patents # 5311152, 5097230, & 5091706.