Thursday, April 21, 2011

FM demodulators

There are a number of circuits that can be used to demodulate FM. Each type has its own advantages and disadvantages, some being used when receivers used discrete components, and others now that ICs are widely used.Below is a list of some of the main types of FM demodulator or FM detector.
  • Slope FM detector
  • Balanced Slope FM detector
  • Foster-Seeley FM detector
  • Ratio detector
  • PLL: Phase locked loop FM demodulator
  • Quadrature FM demodulator / Coincidence FM demodulator
Slope detection
The very simplest form of FM demodulation is known as slope detection or demodulation. It simply uses a tuned circuit that is tuned to a frequency slightly offset from the carrier of the signal. As the frequency of the signal varies up and down in frequency according to its modulation, so the signal moves up and down the slope of the tuned circuit. This causes the amplitude of the signal to vary in line with the frequency variations. In fact at this point the signal has both frequency and amplitude variations. The final stage in the process is to demodulate the amplitude modulation and this can be achieved using a simple diode circuit. One of the most obvious disadvantages of this simple approach is the fact that both amplitude and frequency variations in the incoming signal appear at the output. However the amplitude variations can be removed by placing a limiter before the detector. Additionally the circuit is not particularly efficient as it operates down the slope of the tuned circuit. It is also unlikely to be particularly linear, especially if it is operated close to the resonant point to minimize the signal loss.
Limitations of balanced slope detector
*Linearity is not defined
*It does not take care about the noise and spurious signals therefore the output of balanced slope detectors may contain some noise

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.
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.
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:
  1. Free running
  2. Capture
  3. Tracking  
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 VCOAs 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.
    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.