Abstract: The demodulation circuit includes a Schmitt trigger circuit, first and second monostable multivibrators, a quartz oscillator, first and second low-pass filters, and a comparator. The Schmitt trigger circuit converts a modulated signal modulated by data into a pulse signal. The first monostable multivibrator converts an output from the Schmitt trigger circuit into a PPM signal. The quartz oscillator oscillates at the same frequency as the center frequency of the modulated signal. The second monostable multivibrator is formed on the same chip as the first monostable multivibrator and responsive to an output signal from the quartz oscillator. The second monostable multivibrator generates a signal comprising a pulse train having the same pulse width as that of the PPM signal output form the first monostable multivibrator and a constant pulse separation. The first low-pass filter receives an output from the first monostable multivibrator.

Abstract: A circuit arrangement comprises a controller (20) for producing a control signal selectively indicative of normal and diagnosis modes. Responsive to the control signal, an IF stage (12) processes desired RF and diagnosis input signals into an IF signal of an IF level. Responsive to the IF signal and a reference signal of a reference level squelch signal producing circuit (15) produces a squelch signal when the IF level is lower than the reference level. First and second signal generators (26, 27) generate first and second threshold signals of first and second threshold levels, respectively. The second threshold level is lower than the first threshold level. Responsive to the control signal, a switch circuit (22) selects the first and the second threshold signals as a selected threshold signal. A line (29) supplies the selected threshold signal to the squelch signal producing circuit as the reference signal.