| 摘要 |
<p>A frequency converter for use in a receive-transmit ground station in a satellite communication system, having improved frequency control characteristics and reduced cost. The converter utilizes three local oscillators, one of which is a voltage controlled oscillator. In the transmit channel, an output signal for transmission is obtained which is derived from the sum signals from each of three serially connected mixers, each receiving one of the local oscillator signals. In the receive channel, the receive signal is subtracted from the third local oscillator signal, the second local oscillator signal is subtracted from the result, and the final local oscillator signal is added to the result. A pilot tone is extracted from the resulting baseband signal, which is used to control the voltage controlled oscillator. The voltage controlled oscillator can be used either as the first or third local oscillator. The result is a converter in which drift caused by the third oscillator (closest to the antenna) is cancelled. Thus a much less expensive oscillator which need not be a precision oscillator can be used.</p> |
