Comparison Between AM and FM Reception

21/06/20162 FM Receiver

21/06/20163 FM Receiver

21/06/20164 Frequency Demodulation Frequency modulation causes a carrier frequency to deviate from its rest value by an amount linearly proportional to the voltage of a baseband signal. That is The reverse process whereby the original baseband is recovered from the modulated carrier in a process called frequency demodulation or detection. The operation is to obtain a voltage proportional to the frequency deviation of a signal. That is

21/06/20165 Frequency Demodulation The general situation is as in Figure 1 below: Fm demodulator V fm inV m out Figure 1: General idea of FM demodulation The main requirement is that the output of the circuit is the voltage linearly proportional to the deviation.

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7 Frequency Demodulation In general an FM demodulator should meet three performance requirements: Linearity – V o is linearly proportional to  over the whole modulation range. This is expressed as a constant, the demodulation sensitivity (k), where Range – adequate linearity must extend over the whole deviation range of the input signal as stated above. Sensitivity – the demodulator must produce a large enough output voltage to reproduce adequately the dynamic range of the original baseband.

21/06/20168 Frequency Demodulation The detection of the FM signal typically done in two stages: Converting FM to AM Envelope detecting AM signal The FM to Am stage is given the general name of discriminator. The discriminator typically has a frequency response curve of S- shaped as shown in Figure 2.

21/06/20169 Frequency Demodulation VoVo  Range At least  Figure 2: Discriminator S-characteristic Slope k

21/06/ Frequency Demodulation Simple Demodulators: Differentiating Circuit Tuned Circuit Differentiating Circuits Based on the properties of inductance and capacitance whereby V and I are related by a factor  resulting from the differentiation of one or the other of them. If one quantity is kept constant, the other varies proportionally to  as shown in Figure 3. These properties can easily be used to produce fm demodulation using a simple filter such as the CR.

21/06/ Frequency Demodulation Figure 3: Differentiation by L and C

21/06/ Frequency Demodulation These discriminator are linear with a wide range but are not very sensitive. Mathematically:

21/06/ Frequency Demodulation DiscriminatorAM detector

21/06/ Frequency Demodulation Tuned Circuit Discriminator or Slope Detector The tuned circuit has a useful property that their response changes rapidly with the frequency either side of the resonance peak and is linear over a small range. This property can be used as a simple sensitive discriminator. The drawback of this simple method is that its range is limited because of the limited extent of the linear slopes. This can be overcome by using two tuned circuit back-to- back at slightly different resonant frequencies spaced equally either side of the IF frequency. This produce a much longer linear region.

21/06/ Frequency Demodulation Figure 4: Action of tuned circuit discriminator

The FM signal is applied to transformer T 1, made up of L 1 and L 2. L 2 and C 1 form a series resonant circuit. The response curve of this tuned circuit is shown in (b). At the resonant frequency, f r, the voltage across C 1 peaks. At lower or higher frequencies, the voltage falls off. To use the circuit to detect or recover fM, the circuit is tuned so that the center or carrier frequency of the FM signals is approximately centered on the leading edge of the response curve. As the carrier frequency varies above and below its center frequency, the tuned circuit responds as shown in the figure. 16

If the frequency goes lower than the carrier frequency, the output voltage across C 1 decreases. If the frequency goes higher than the carrier frequency, the output voltage across C 1 goes higher. Thus, the ac voltage across C 1 is proportional to the frequency of the FM signal. The voltage across C 1 is rectified into dc pulses that appear across the load R 1. These are filtered into a varying dc signal that is an exact reproduction of the original modulating signal. The main difficulty with slope detectors lies in tuning them so that the FM signal is correctly centered on the leading edge of the tuned circuit. In addition, the tuned circuit does not have a perfectly linear response. It is approximately linear over a narrow range, but for wide deviations, amplitude distortion occurs because of nonlinearity. 17

The slope detector is never used in practice, but it does show the principle of FM demodulation, i.e. converting a frequency variation to a voltage variation. Numerous practical designs based upon these principles have been developed. These include the Foster-Seeley discriminator and the ratio detector. 18

Pulse-Averaging Discriminators –A pulse-averaging discriminator uses a zero crossing detector, a one shot multivibrator and a low-pass filter in order to recover the original modulating signal. –The pulse-averaging discriminator is a very high- quality frequency demodulator. –Originally this discriminator was limited to expensive telemetry and industrial control applications. –With availability of low-cost ICs, this discriminator is used in many electronic products.

The FM signal is applied to a zero-crossing detector or a clipper/limiter which generates a binary voltage-level change each time the FM signal varies from minus to plus or from plus to minus. The result is a rectangular wave containing all the frequency variations of the original signal but without amplitude variations.

The FM square wave is then applied to a one-shot (monostable) multivibrator which generates a fixed-amplitude, fixed-width dc pulse on the leading edge of each FM cycle. The duration of the one shot is set so it is less than one-half the period of the highest frequency expected during maximum deviation. The one-shot output pulses are then fed to a simple RC LPF which averages the dc pulses to recover the original modulating signal. 21

At low frequencies, the one-shot pulses are widely spaced; at higher frequencies, they occur very close together. When these pulses are applied to the averaging filter, a dc output voltage is developed, the amplitude of which is directly proportional to the frequency deviation. When a one-shot pulse occurs, the capacitor in the filter charges to the amplitude of the pulse. When the pulse turns off, the capacitor discharges into the load. If the RC time constant is high, the charge on the capacitor does not decrease much. 22

When the time interval between pulses is long, the capacitor loses some of its charge into the load so the average dc output is low. When the pulses occur rapidly, the capacitor has little time between pulses to discharge; the average voltage across it therefore remains higher. Hence, the filter output voltage varies in amplitude with the frequency deviation. The original modulating signal is developed across the filter output. 23

The filter components are carefully selected to minimize the ripple caused by the charging and discharging of the capacitor while at the same time providing the high frequency response for the original modulating signal. Some pulse-averaging discriminators generate a pulse every half- cycle or at every zero crossing instead of every one cycle of the input. With a great number of pulses to average, the output signal is easier to filter and contains less ripple. 24

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