FM Demodulation analog communication types of demodulation
- 1. ELECTRONICS AND COMMUNICATION ENGINEERING COURSE CODE: 50 EC 501 COURSE NAME: ANALOG COMMUNICATION MODULE NAME: FM DEMODULATION FACULTY NAME: R.SATHEESHKUMAR
- 2. Methods for Frequency Demodulation What are Frequency demodulators ? They produce output voltage whose instantaneous amplitude is directly proportional to the instantaneous frequency of the input FM wave. Methods: 1. Balanced Slope Detector 2. Foster Seely Descriminator 3. Phase locked loop demodulator. 4. Ratio-detector. (old)
- 3. Basic Idea where = demodulated output signal (Volts) = demodulator transfer function (Volts per Hertz) = difference between the input frequency and the center frequency of the demodulator (Hertz). ( ) out d V t k f ( ) out V t d k f
- 6. Balanced Slope Detector (i) fin = fc: When the input frequency is instantaneously equal to fc, the induced voltage in the T1 winding of secondary is exactly equal to that induced in the winding T2. Thus, the input voltages to both the diodes D1 and D2 will be the same. Therefore, their dc output voltages Vo1 and Vo2 will also be identical but they have opposite polarities. Hence, the net output voltage Vo = 0.
- 7. Balanced Slope Detector fc < fin < (fc + Δf) Now consider the instantaneous frequency to be equal to fc + δf. Since T’ is tuned to this frequency, the output of D1 will be quite large. the induced voltage in the winding T1 is higher than that induced in T2. (fc - Δf)< fin < fc On the other hand, the output of D2 will be very small, since the frequency fc + δf is quite a long way from fc – δf. Similarly, when the input frequency is instantaneously equal to fc – δf, the output of D2 will be a large negative voltage, and that of D1 a small positive voltage. Thus in the first case the overall output will be positive and maximum, and in the second it will be negative and maximum.
- 8. Advantages and Limitations Advantages: (i) This circuit is more efficient than simple slope detector. (ii) It has better linearity than the simple slope detector. Limitations: (i) Even though linearity is good, it is not good enough. (ii) This circuit is difficult to tune since the three tuned circuits are to be tuned at different frequencies, and (iii) Amplitude limiting is not provided. Rizvi college of Engineering
- 10. Foster-Seeley Discriminator o in f f o in f f o in f f
- 15. basic operation of the circuit can be explained by looking at the instances when the instantaneous input equals the carrier frequency, the two halves of the tuned transformer circuit produce the same rectified voltage and the output is zero. If the frequency of the input changes, the balance between the two halves of the transformer secondary changes, and the result is a voltage proportional to the frequency deviation of the carrier.
- 16. Foster-Seeley Discriminator an un-modulated carrier is applied at the centre frequency, both diodes conduct, to produce equal and opposite voltages across their respective load resistors. These voltages cancel each one another out at the output so that no voltage is present. As the carrier moves off to one side of the centre frequency the balance condition is destroyed, and one diode conducts more than the other. This results in the voltage across one of the resistors being larger than the other, and a resulting voltage at the output corresponding to the modulation on the incoming signal.
- 17. Foster-Seeley… Advantages: procedure is simpler than balanced slope detector, because it contains only two tuned circuits and both are tuned to the same frequency . Better linearity, because the operation of the circuit is dependent more on the primary to secondary phase relationship which is very much linear. Limitations: It does not provide amplitude limiting. So in the presence of noise or any other spurious amplitude variations, the demodulator output respond to them and produce errors.
- 18. Ratio Detector Similar to the Foster-Seeley discriminator . (i) The direction of diode is reversed. (ii) A large capacitance Cs is included in the circuit. (iii) The output is taken different locations. Advantages: Easy to align. Good linearity due to linear phase relationship between primary and secondary. Amplitude limiting is provided inherently. Hence additional limiter is not required.
- 19. Ratio Detector
- 20. The operation of the ratio detector centres around a frequency sensitive phase shift network with a transformer and the diodes that are effectively in series with one another. When a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and the capacitor C3 charges up as a result. The transformer enables the circuit to detect changes in the frequency of the incoming signal. It has three windings. The primary and secondary act in the normal way to produce a signal at the output. The third winding is un-tuned and the coupling between the primary and the third winding is very tight, and this means that the phasing between signals in these two windings is
- 21. PLL FM Demodulator (Phase Locked Loop) A Phase-Locked Loop (PLL) is basically a negative feedback system. It consists of three major components such as re multiplier, a loop filter and a voltage controlled oscillator (VCO) connected together in the form of a feedback loop. A VCO is a sine wave generator whose frequency is determined by the voltage applied to it from an external source. It means that any frequency modulator can work as a VCO.
- 22. PLL FM Demodulator (Phase Locked Loop) PLL is also useful for synchronous demodulation of AM-SC (i.e., Amplitude Modulation with Suppressed carrier) signals or signals with few cycles of pilot carrier. Further, PLL is also useful for demodulating FM signals in presence of large noise and low signal power. This means that, PLL is most suitable for use in space vehicle-to-earth data links or where the loss along the transmission line or path is quite large. Recently, it has found application in commercial FM receivers.
- 24. Working Operation The operation of a PLL is similar to any other feedback system where the feedback signal tends to follow the input signal. If the signal fed back is not equal to the input signal, the error signal will change the value of the fed back signal until it is equal to the input signal. The difference signal between s(t) and b(t) is called an error signal.
- 25. A PLL operates on a similar principle except for the fact that the quantity feedback is not the amplitude, but a generalized phase Φ(t). The error signal or difference signal e(t) is utilized to adjust the VCO frequency in such a way that the instantaneous phase angle comes close to the angle of the incoming signal s(t). At this point, the two signals s(t) and b(t) are synchronized and the PLL is locked to the incoming signal s(t).
- 26. Phase-locked Loops • PLLs when fed with an FM signal directly produce an output signal that is proportional to the message signal. • PLL has low cost and superior performance even at low SNR (signal-to-noise ratio) • Where do we take the output? Compare with the case of carrier acquisition.
- 27. A PLL is a device whose output is a periodic signal synchronized in phase with an input reference (periodic or almost periodic) signal. Phase Lock Loop (PLL)
- 28. 2 VCO: oscillator that produces a period waveform with a frequency that may be varied around free running frequency, . VCO output frequency = when ( ) 0. Phase Detector (PD): o o o f f v t utput is a function of the phase difference between incoming signal ( ) and ( ). PLL has two modes: - Narrowband mode: tracks average frequency of ( ). - Wideband mode: t o in in v t v t v t racks instantaneous frequency of ( ). Lock: When the PLL tracks the (average or instantaneous) frequency of ( ). Hold-in range: When the PLL is in lock, the range of frequency of ( ) to remain in in in v t v t v t in lock. (Also called the lock range.) Pull-in range: When the PLL is not in lock, the range of frequency of ( ) to capture a lock. (Also called the ca in v t pture range.) Maximum locked sweep range: When the PLL is in lock, the maximum rate of change of the frequency of ( ) to remain in lock. A PLL can be made in analog (APLL) or d in v t igital (DPLL) circuits.
- 29. Different Phase Detector Characteristics
- 30. Performance Comparison of FM Demodulators S.No. Parameter of Comparison Balanced Slope detector Foster-Seeley (Phase) discriminator Ratio Detector (i) Alignment/tuning Critical as three circuits are to be tuned at different frequencies Not Critical Not Critical (ii) Output characteristics depends on Primary and secondary frequency relationship Primary and secondary phase relation. Primary and secondary phase relation. (iii) Linearity of output characteristics Poor Very good Good (iv ) Amplitude limiting Not providing inherently Not Provided inherently Provided by the ratio detector. (v) Amplifications Not used in practice FM radio, satellite station receiver etc. TV receiver sound section , narrow band FM receivers.
- 32. Pre-Emphasis And De-Emphasis Pre-Emphasis Pre-emphasis refers to boosting the relative amplitudes of the modulating voltage for higher audio frequencies from 2 to approximately 15 KHz. De-Emphasis De-emphasis means attenuating those frequencies by the amount by which they are boosted.
- 33. Characteristics Of Pre-Emphasis Pre-Emphasis: Applies a high-pass filter to the signal before transmission Boosts or amplifies the high-frequency components Commonly boosts frequencies above 2-3 kHz Provides up to 10 dB of gain to high frequencies Done prior to transmission or recording Takes advantage of high-frequency noise immunity
- 34. De-Emphasis: Applies a low-pass filter to the received signal Attenuates or reduces the boosted high frequencies Rolls-off highs above 2-3 kHz The reverse process of pre-emphasis Provides gain reduction equal to the pre- emphasis Restores original frequency spectrum Reduces noise and distortion picked up during transmission Characteristics Of De-Emphasis
- 36. Pre-Emphasis Circuit At the transmitter, the modulating signal is passed through a simple network that amplifies the high frequency, components more than the low- frequency components. The simplest form of such a circuit is a simple high-pass filter of the type shown in Fig (a). Any combination of resistor and capacitor (or resistor and inductor) giving this time constant will be satisfactory. Such a circuit has a cutoff frequency fco of 2122 Hz. This means that frequencies higher than 2122 Hz will be linearly enhanced.
- 37. Pre-Emphasis Circuit The output amplitude increases with frequency at a rate of 6 dB per octave. The pre-emphasis curve is shown in Fig (b). This pre-emphasis circuit increases the energy content of the higher-frequency signals so that they will tend to become stronger than the high- frequency noise components. This improves the signal-to-noise ratio and increases intelligibility and fidelity.
- 39. To return the frequency response to its average level, a de-emphasis circuit is used at the receiver. This is a simple low-pass filter with a constant of 75 πs. See Figure (c). It features a cutoff of 2122 Hz and causes signals above this frequency to be attenuated at the rate of 6bB per octave.
- 40. The combined effect of pre-emphasis and de- emphasis is to increase the high-frequency components during transmission so that they will be stronger and not masked by noise.
- 42. Differences Between Pre-Emphasis And De-Emphasis In Audio Signals: Pre-Emphasis Applies a high-pass filter to boost high frequencies prior to transmission or recording Boosts treble range signals that may get attenuated during transmission Improves signal-to-noise ratio for high frequencies Commonly used time constants are 50μs and 75μs Can introduce distortion if the boost is too aggressive Found in FM radio, cassette recording, NTSC video
- 43. De-Emphasis Applies a high-cut filter to attenuate boosted high frequencies Reverses the frequency contour applied by pre- emphasis Reduces noise in high frequencies that got boosted Must use same time constant as pre-emphasis stage Restores flat frequency response of the original signal Found in FM receivers, tape playback, NTSC video
- 44. Key Differences: Pre-emphasis boosts treble, de-emphasis cuts treble Pre-emphasis occurs before transmission/storage De-emphasis happens during reception/playback Pre-emphasis improves SNR, de-emphasis reduces noise Must use matched time constants to reverse effect
- 45. Pre-Emphasis: Advantages: Improves signal-to-noise ratio - Boosts high frequencies before transmission which are more susceptible to noise Increases transmission range - Pre-emphasis provides 6-10 dB gain allowing greater transmission distance Compatible with transmitter characteristics - Takes advantage of nonlinear compression in transmitters Reduces channel crosstalk - Attenuates lower frequencies to prevent crosstalk between channels
- 46. Disadvantages: Increases transmit power - Requires more transmitter power to accommodate pre-emphasis Susceptible to non-linearity - Can introduce distortion if pre-emphasis is applied incorrectly
- 47. De-Emphasis: Advantages: Restores original frequency response - Rolls- off highs to counteract effects of pre-emphasis Reduces noise - Attenuates amplified high frequencies and the noise introduced during transmission Improves overall SNR - Combined pre-emphasis and de-emphasis give better SNR Prevents adjacent channel interference - Reduces high frequencies that may cause interference
- 48. Disadvantages: Requires complex circuitry - Needs tuned circuits or active filters to provide specific de- emphasis Risk of incorrect de-emphasis - Can further distort signal if de-emphasis doesn't match pre- emphasis