# How does this simple FM slope detector work?

I understand what happens from the diode onwards, it's a rectifying amp and a low pass filter that filters any unwanted frequencies from the carrier, but what is going on in terms of voltage between the first capacitor and the primary coil of the transformer, and between the secondary coil of the transformer and the second capacitor, that allows the FM signal to vary in amplitude according to the frequency input?

The FM slope detector relies on the signal entering the input terminals (on the left) to be current in nature. This can be achieved by placing it in the collector of a common-emitter stage but can also work when feeding it from a voltage source via a high value resistor. If the transformer has fairly low coupling between primary and secondary this can also work with a voltage drive at the input.

The peak parallel tuning of the circuit is offset from the FM carrier frequency (usually an I.F. of 10.7 MHz) by several hundred kHz hence, the circuit output amplitude rises as the modulated carrier gets closer to peak tuning and falls as the modulated carrier gets further away from the peak tuning point: -

Picture from here.

This means that the carrier is regarded as being placed on the slope of the parallel resonant tuning and hence, is called a slope detector. It converts a flat-amplitude and modulated FM carrier to a modulated carrier where the frequency modulation effects are transferred to the amplitude and are resolved with a conventional diode detector (as shown).

• 10.7MHz is standard FM IF; otherwise another excellent answer. Feb 27, 2019 at 13:04
• @analogsystemsrf oops my mistake - fixing.... Feb 27, 2019 at 13:07

The input goes through a tuned transformer with a bandpass response. This should be tuned so that the frequencies of the input signal is on either side of the slopes of the filter response, like in this examples where the input is at the right side of the peak:

The output will then be a varying amplitude depending on the frequency.

Afterwards, yes, then a slope detector is used to filter out the high frequency content like in an simple AM demodulator.