Ratio + Foster Seeley detector functional differences

Thread Starter


Joined Jan 7, 2015

Does anyone here understand and perhaps you might explain the functional differences between an FM ratio detector and a Foster Seeley detector?

I understand they are similar but from the attached photos, a Foster Seeley has a connection from primary to centre of secondary (via a capacitor) whilst a ratio has no connection, only using a tertiary winding.

Also a Foster has diodes pointing in the same direction whilst a ratio uses diodes pointing in opposite directions.

I also understand the concept of phasors adding up to produce an output. But now, this is where my understanding FALLS DOWN....

On a ratio detector, how is it that the loosely coupled secondary produces a frequency sensitive output to one diode over the other diode? Why is one preferred to another? In any case they point in opposite directions so how would that work?

On the FS detector, I can see that both diodes point in the same direction so I'm guessing one diode is preferred to the other as each half of the secondary is tuned slightly off centre from the IF frequency (and so depending on the frequency shift direction, one half of the secondary will have the greater output)?

In both types of detector, why do we need a 90° shift in phase? Can't we simply have both halves of the secondary tuned either side of the IF frequency?

Kind regards,


Ratio ♤

Foster Seeley ♤


Joined Jul 10, 2017
The Foster–Seeley discriminator uses a tuned RF transformer to convert frequency changes into amplitude changes. A transformer, tuned to the carrier frequency, is connected to two rectifier diodes. The circuit resembles a full-wave bridge rectifier. If the input equals the carrier frequency, the two halves of the tuned transformer circuit produce the same rectified voltage and the output is zero. As the frequency of the input changes, the balance between the two halves of the transformer secondary changes, and the result is a voltage proportional to the frequency deviation of the carrier.

The ratio detector is a variant of the Foster–Seeley discriminator .but one diode conducts in an opposite direction, and uses a tertiary winding in the preceding transformer. The output in this case is taken between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large value capacitor, forming a dynamic limiter. The ratio detector has the advantage over the Foster-Seeley discriminator that it does not respond to amplitude modulation (AM) signals, thus potentially saving a limiter stage; however, the output is only 50% of the output of a discriminator for the same input signal. The ratio detector has wider bandwidth, but more distortion than the Foster-Seeley discriminator.

Ratio detector circuit


Joined Mar 14, 2008
The ratio detector has the advantage over the Foster-Seeley discriminator that it does not respond to amplitude modulation (AM) signals
It's probably apparent, but it's the amplitude noise on the signal, such as from atmospheric lightning, that you want to ignore, not necessarily an intentionally AM modulated signal.