Carrier recovery network for QPSK modems employing synchronized oscillators

DETAILED DESCRIPTION The present invention is directed to a carrier recovery network of greatly simplified construction which is able to track both high and low frequency carriers without the inherent disadvantages of the prior art carrier recovery network.
The present invention is also directed to the particular and specific apparatus which enables the above-mentioned simplified carrier recovery network in accordance with the present invention to operate over a relatively wide range of conditions.
More specifically, the apparatus enabling the carrier recovery network in accordance with the present invention to operate in such a manner comprises a synchronous oscillator.
The synchronous oscillator performs a number of functions which readily lend themselves to operation in a carrier recovery network.
Specifically, the synchronous oscillator in accordance with the present invention provides synchronization capability so that carrier drift may be tracked by the carrier recovery network.
The synchronous oscillator also provides frequency division and multiplication capabilities as well as a bandpass filtering capability, the Q of the bandpass filter being adjustable.
Furthermore, the filtering characteristics of the synchronous oscillator allow a substantially constant signal-to-noise ratio level over a wide variety of operating conditions since the bandwidth of the filter is reduced as the signal input level is reduced.
Furthermore, since amplitude variations only effect the synchronous oscillator filter bandwidth, the output of the synchronous oscillator is substantially of constant amplitude, thereby obviating the need for automatic gain control circuitry.
The synchronous oscillator may be used in one of several carrier recovery networks in accordance with the present invention.
The prior art carrier recovery network may be simply modified by replacing the multipliers and dividers by synchronous oscillators to provide the various frequency multiplication and division functions.
The prior art carrier recovery network may be further simplified by providing a synchronous oscillator to perform the carrier frequency tracking functions as well as any necessary multiplication and division functions