Add a signal-strength display to an FM-receiver IC

You can obtain the information related to the intensity of the received signal at the output of the IF filter (IC1, Pin 12). You can easily process this voltage with common op amps, because the IF signal is centered on 70 kHz. The voltage at Pin 12 is dc-coupled to an amplifier, IC2. Next, an envelope detector, IC3, yields a dc voltage proportional to the received-signal strength. The Siemens (www.siemens.com) TCA965 window discriminator, IC4, compares this envelope voltage with a voltage derived from R1, R2, and R3 for the window's center (and R4 and R5 for the window's half-width). Three LEDs show the result of the comparison (Low, OK, Good), but the display is valid only if the tuning is correct. If it's correct, the voltage at IC1, Pin 1 reaches its maximum value, and the LM311 comparator, IC5, enables the TCA965.