FM Ratio Detector Scheme

I understand how a Foster Seeley discriminator works, but in this Ratio detector, which is a modified version of the former, I don't understand the purpose of having three transformer windings, or having a diode in reverse (why would you want a diode reversed if it can't pass any signal?) or how the signal flows through it.

I also don't understand the purpose of C5, but I understand there's a dynamic filtering function going on (how?)

And lastly, since I don't understand the purpose of three windings, I don't understand the purpose of R3, C6, R5, C7 and C8.

Can someone explain the whole circuit?

  • 1
    \$\begingroup\$ Some of these circuits use vector-combining to create the demodulated output. Are all these circuits at the same resonant frequency? \$\endgroup\$ Jul 7, 2019 at 16:37

2 Answers 2


The diagram in your question comes from a NEETS (US Navy Electricity & Electronics Training Series) module, to be more specific it comes from module nº12 . The documentation for these modules seems to be freely available in the internet from several sources. Here as a PDF, or here as a webpage.

You can find a detailed discussion of circuit operation in pages 3-15 to 3-17. The discussion is focused on how the circuit operates above, below and at resonance. You should read the full discussion, but I'll give some short answers here to your questions:

  1. The RATIO DETECTOR uses a double-tuned transformer to convert the instantaneous frequency variations of the fm input signal to instantaneous amplitude variations. These amplitude variations are then rectified to provide a dc output voltage which varies in amplitude and polarity with the input signal frequency. This detector demodulates fm signals and suppresses amplitude noise without the need of limiter stages.
  2. Diodes CR1 and CR2 work as rectifiers. They block the negative part of the FM signal cycle.
  3. The third transformer winding L3 provides additional inductive coupling which reduces the loading effect of the secondary on the primary circuit. You get an in-phase sample of the input voltage without having to load the primary.
  4. C5 is a smoothing capacitor. It keeps the voltage approx constant when CR1 and CR2 are reverse biased during the negative part of the FM signal cycle. This can be achieved by letting the time constant of C5, R1 and R2 be much greater that the FM signal period.
  5. Resistor R3 limits the peak diode current and furnishes a dc return path for the rectified signal.
  6. R5, C6, and C7 form a low-pass filter to the output.
  7. C8 is a coupling capacitor. It AC couples the demodulated signal to the next stage.
  • \$\begingroup\$ Thank you, great answer. Although, why is there a shortcut to output just before the signal passes CR2? You said the path was the resistance next to the third winding. Will it take both? And also, I thought low pass filters were resistance-capacitor but this is c-r-c, does it act as a high and low pass? \$\endgroup\$
    – The AFOH
    Nov 15, 2019 at 2:17
  • \$\begingroup\$ @TheAFOH if you're talking about the "short" between node A & D (just after CR2), that is not a short since a connection dot isn't drawn there. \$\endgroup\$
    – qrk
    Jun 12, 2021 at 2:24

the real secret to the ratio detector is eh phase relationship between the 3rd winding and the tuned center-tapped secondary.

When the frequency goes up beyond the tuned center frequency, the phase shifts towards the positive rectifier, and when frequency drops below the center frequency, the phase shifts towards the negative rectifier.

Hence the name "ratio detector", it is the ratio between the positive and negative sides of the rectified output based on the phase shifting caused by a tuned circuit above and below its center frequency.


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