| 摘要 |
A multiple frequency RF modem includes a frequency synthesizer for providing local oscillator signals to double balanced mixer circuits in both the transmitter and receiver sections. To test the frequency synthesizer, the synthesizer is set to produce a local oscillator signal having a nominal frequency of 3 IF. The transmitter RF filter is tuned to suppress all harmonics and sidebands other than the 4 IF sideband. A ganged set of switches is employed to connect the transmitter output directly to the receiver input in the modem. Another switch in the set isolates the receiver mixer circuit from the frequency synthesizer during testing and connects it instead directly to the output of an IF oscillator in the transmitter section. Due to its non-linear operating characteristics, the receiver mixer circuit generates a strong third harmonic of its local oscillator input. During operation in the testing mode, this third harmonic has a frequency of 3 IF. The mixer circuit combines the 3 IF harmonic with the nominal 4 IF signal received through the wrap line. If the received signal has an actual frequency of 4 IF, a strong signal will appear at the output of the IF filter in the receiver. Since the wrapped signal has an actual frequency of 4 IF only if the frequency synthesizer output during testing is actually 3 IF, a strong output signal indicates that the frequency synthesizer is operating properly.
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