| 摘要 |
<p>1290578 Muting radio receivers INTERNATIONAL STANDARD ELECTRIC CORP 26 Feb 1970 [28 Feb 1969] 9269/70 Addition to 1238608 Heading H3T The invention relates to a muting arrangement for a frequency modulation receiver, in which, as in the parent Specification, the overall response prior to a frequency discriminator is made unsymmetrical relative to the signal frequency: the frequency discriminator produces a different D.C. output according to whether a signal or only noise is being received, and these two different D.C. outputs are used to mute and unmute an audio stage of the receiver. An embodiment comprises R.F. tuned circuit 9, frequency changer 10 and local oscillator 11, I.F. amplifier 12, limiter 13 and discriminator 14 of the integrator type: the R.F. tuned circuit 9 is mis-tuned away from the signal frequency. so that discriminator 14 produces zero D.C. output on noise and a D.C. output voltage U D on signal. The following audio stage 15 to which discriminator 14 is direct-coupled is then biased to cut off in the absence of a signal, but the D.C. output voltage U D biases it conductive for a signal. According to the present invention, . R.F. circuit 9 is tuned by means of a varactor diode 6 to which the D.C. output of the discriminator 14 is applied and so biases the diode as to resonate the circuit 9 when a signal is received.</p> |
