FM detectors convert the frequency variations of the carrier back into a replica of the original modulating signal. There are 5 basic types of FM detectors:
1. SLOPE DETECTOR
The slope detector is the simplest type of FM detector. A schematic diagram of a slope detector appears below:
The operation of the slope detector is very simple. The output network of an
amplifier is tuned to a frequency that is slightly more than the carrier
frequency + peak deviation. As the input signal varies in frequency, the output
signal across the LC network will vary in amplitude because of the band pass
properties of the tank circuit. The output of this amplifier is AM, which can be
detected using a diode detector.
The circuit shown in the diagram above looks very similar to the last IF
amplifier and detector of an AM receiver, and it is possible to receive NBFM on
an AM receiver by detuning the last IF transformer.
If this transformer is tuned to a frequency of approximately 1 KHz above
the IF frequency, the last IF amplifier will convert NBFM to AM.
In spite of its simplicity, the slope detector is rarely used because it has poor linearity. To see why this is so, it is necessary to look at the expression for the voltage across the primary of the tuned transformer in the sloped detector:
The voltage across the transformer's primary winding is related to the
square of the frequency. Since the frequency deviation of the FM signal is
directly proportional to the modulating signal's amplitude, the output of the
slope detector will be distorted. If the bandwidth of the FM signal is small, it
is possible to approximate the response of the slope detector by a linear
function, and a slope detector could be used to demodulate an NBFM signal
2.
The Foster-Seely Discriminator is a widely used FM detector. The detector consists of a special center-tapped IF transformer feeding two diodes. The schematic looks very much like a full wave DC rectifier circuit. Because the input transformer is tuned to the IF frequency, the output of the discriminator is zero when there is no deviation of the carrier; both halves of the center tapped transformer are balanced. As the FM signal swings in frequency above and below the carrier frequency, the balance between the two halves of the center-tapped secondary are destroyed and there is an output voltage proportional to the frequency deviation.
The discriminator has excellent linearity and is a good detector for WFM and NBFM signals. Its major drawback is that it also responds to AM signals. A good limiter must precede a discriminator to prevent AM noise from appearing in the output.
3. RATIO DETECTOR
The ratio detector is a variant of the discriminator. The circuit is similar to the discriminator, but in a ratio detector, the diodes conduct in opposite directions. Also, the output is not taken across the diodes, but between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large capacitor, which eliminates AM noise in the ratio detector output. The operation of the ratio detector is very similar to the discriminator, but the output is only 50% of the output of a discriminator for the same input signal.
4. QUADRATURE DETECTOR
The Quadrature Detector does not require a center-tapped RF transformer; therefore it can be integrated onto a single chip, unlike the other detector circuits. The IF signal and a square wave at the carrier frequency are combined in an XOR gate. The square wave is phase shifted 90 degrees with respect to the IF carrier.
The time interval between a zero crossing of the square wave and the IF signal will depend on the instantaneous frequency, which makes the gate output a pulse whose width depends on the time interval. In essence, the quadrature detector converts an FM signal to a PWM (pulse width modulated) signal, as indicated in the figure above. The original audio signal is recovered by passing the PWM signal through a low-pass filter.
5. PHASE-LOCKED LOOP (PLL) DETECTOR
The PLL detector uses PLL technology to demodulate FM signals. The diagram below shows a simple PLL detector. The phase detector compares the phase of the FM input and the VCO output. Frequency deviation of the carrier results in a phase difference between the two and the phase detector sends an error voltage to the low pass filter. The filtered error signal is used to change the VCO output frequency in order to reduce the phase error. The output of the low pass filter has an amplitude that is proportional to the deviation of the FM input, so it is actually a replica of the original modulating signal. FM is converted directly to audio.
A PLL can operate in three different modes:
In the free running mode, the input frequency is not close enough to the VCO frequency and the PLL runs at the free running frequency determined by the timing circuits of the VCO. The error voltage is outside the range of the VCO
As the input frequency gets closer to the VCO frequency, the error voltage reaches a value at which it can begin to change the VCO frequency. This is the capture mode. The error voltage will continue to decrease as the VCO frequency gets closer to the input frequency.
Finally, when the VCO is operating at the same frequency as the input, the PLL is in the tracking mode. The VCO will track changes in the input frequency as long as the input frequency remains in a range of frequencies known as the hold-in range.
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