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  FM Detector

Introduction to FM Detector

Fm detector is termed as discriminator. Fm detector is a balanced double diode demodulator. Fm detector splits audio signal from frequency modulated IF signal.

 The FM detector circuits are categorized like this:

TYPES OF FM DETECTOR

1. Travis discriminator

2. Foster-Seeley discriminator

3. Radio detector.

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It is a 3 stages tuned (or) stagger tuned discriminator. In this type, three tuned circuits are employed that are tuned to three dissimilar frequencies. In the final IF transformer the first tuned circuit that is tuned to the intermediate frequency. It contains two secondary coils.

Two Signal diodes are employed. They are associated to the secondary coils. Two filter networks are connected in the output section. These filter networks are R.C filters. Output audio signal is taken beyond the filter condensers.

While the input signal rises from the center frequency first diode conducts and second it doesn't conduct. Therefore half cycle of audio signal is charged in output. At the output, Half cycle the two half cycles are joint and taken like audio signal. The R.F signals are bypassed through the condensers. This audio signal is applied on to the audio amplifier for amplification.

FOSTER SEELEY DISCRIMINATOR

Foster Seeley discriminator is also termed as phase shift discriminator or center tuned discriminator. Only two tuned circuits are employed that are tuned to center frequency in this circuit. From the final IF amplifier (limiter) input signal is applied to it. The primary and secondary coils have been got by the final IFT.

Two signal diodes are used. Among the primary and center tap of the secondary a coupling capacitor is connected. Thus by this coupling condenser and mutual induction the input signal is applied to the secondary.

The diodes are balanced while equal signal voltage is applied. Therefore the output is similar and net audio signal output is 0. At another levels (apart from centre frequency) the output of one diode is larger than another. The output is positive, if the total output is larger than centre frequency, and on the other hand while output is smaller, the output is negative. Carrier signals are filtered and AF signal is split and provided to the output stage.

 

RATIO DETECTOR

This circuit looks like the discriminator circuit. The major variation is that the signal diodes are connected in reverse direction. It is not essential to make use of a limiter stage. The two tunrf circuits are contained by final IF transformer. By coupling capacitor and output section, the input signal is applied two filter condensers are connected and one load resistor is employed. While, the unmodulated signal is applied, the output signal voltage is similar. And while the input signal changes the output voltage also changes. While input signal frequency is raised, more output voltage is generated. If input signal frequency is get decreased, less output voltage is generated. If input signal is raised more than centre frequency, the output of first tuned circuit is gets high, and while the output of second tuned circuit is low. Therefore the output is +ive. If input signal is low, the output becomes -ive. So the two signal voltages are combined and so audio signal is acquired.

DOUBLE CONVERSION

If two different IF's are employed in a receiver, it is termed as Double conversion. It is employed in communication receivers.

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