| 摘要 |
A threshold generator for an FSK demodulator compensates for selective fading of the FSK signal. The peak positive and negative amplitude excursions in a composite envelope signal, representative of detected mark and space tone signals, are sampled and respectively stored in two sample and hold circuits. The average value of the amplitude values stored in the sample and hold circuits is computed to establish a threshold signal. When no transitions occur in a demodulated data signal for a predetermined period of time due to the absence of incoming information, the composite envelope signal is conducted to ground, thereby forcing the output signals of the sample and hold circuits to go to zero volts. The sample and hold circuits continuously sample the grounded signal to maintain the threshold level at zero volts as long as no transitions occur in the demodulated data signal.
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