Frequency Modulation Tutorial Includes:
Frequency modulation, FM Modulation index & deviation ratio FM sidebands, bandwidth FM demodulation FM slope detector FM ratio detector Foster Seeley detector PLL FM demodulator Quadrature demodulator MSK GMSK
Modulation formats: Modulation types & techniques Amplitude modulation Phase modulation Quadrature amplitude modulation
The FM quadrature demodulator and its relation the coincidence detector or demodulator provide another form of FM detector circuit that is used in many areas, being easily incorporated into integrated circuits.
The quadrature detector required the use of very few components, but does require a coil. However as the inductor is only a coil and not a transformer the cost impact can be accommodated for many radio receiver designs.
In terms of its performance, the quadrature detector and its relation the coincidence detector offer a high level of performance with excellent linearity.
FM quadrature demodulator basics
The circuit for the quadrature demodulator is quite straightforward consisting of a mixer, a resistor, two capacitors and an inductor. There is also a low pass filter to follow the mixer, but this also can be quite straightforward.
The incoming signal is split into two components. The first enters the mixer directly, but the second is phase shifted. The phase shift is 90° for the carrier, but the deviation on the carrier will cause the phase shift to change slightly. Dependent upon the amount of deviation.
The original signal and the phase shifted signal are then passed into a multiplier or mixer.
The mixer output is dependent upon the phase difference between the two signals, i.e. it acts as a phase detector and produces a voltage output that is proportional to the phase difference and hence to the level of deviation on the signal.
If the operation of the system is designed to ensure that the deviation remains well away from the ±90° points, then the linearity remains very good.
In terms of performance, the quadrature detector is able to operate with relatively low input levels, typically down to levels of around 100 microvolts and it is very easy to set up requiring only the phase shift network to be tuned to the centre frequency of the expected signal. It also provides good linearity and this results in low levels of distortion.
Coincidence FM demodulator
Often the analogue multiplier is replaced by a logic AND gate and the input signal is hard limited to produce a variable frequency pulse waveform.
The operation of the circuit is fundamentally the same, but it is known as a coincidence detector. Also the output of the AND gate has an integrator to 'average' the output waveform to provide the required audio output, otherwise it would consist of a series of square wave pulses.
Quadrature detector advantages & disadvantages
The quadrature detector offers significant advantages for many circuits, but as with any decision, a number of different advantages and disadvantages have to be considered when selecting a given circuit for FM demodulation.
Advantages of FM quadrature demodulator
- Offers good level of performance and including linearity.
- Can be incorporated into an integrated circuit.
- Very simple circuit - easy to ensure it operates correctly.
Disadvantages of FM quadrature demodulator
- Requires the use of a coil - this is not a major issue as a simple coil is used and an RF transformer is not needed as in the case of the ratio and Foster Seeley circuits.
- Some designs may require setting during manufacture.
The quadrature detector is ideal for many radio receiver applications. Despite the fact that it needs a coil to provide the required phase shift, it is often used because it lends itself to easy incorporation into an integrated circuit and its is very simple to implement. The facts that the quadrature detector and coincidence detectors require a coil is not a major disadvantage because a simple coil can be used.
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