Frequency demodulator designed as phase control circuit - has mixer with VCO with current impression transistors connected to variable reactance

摘要

The demodulator has a phase locked loop (PLL) with a VCO controlled by a mixer voltage. The mixer is balanced, has current impression transistors and delivers through a low-pass filter the frequency demodulated output voltage. The current impression transistors (Q7, Q8) are connected as a part of the oscillator to a variable reactance (D1, D2) controlled directly or indirectly by the mixer (Q3, -Q6). The variable reactance is provided by the combination of a diode and a capacitor in series with it.