Demodulation/ Detection Chapter 4

FM Detection/Demodulation FM demodulation is a process of getting back or regenerate the original modulating signal from the modulated FM signal. It can be achieved by converting the frequency deviation of FM signal to the variation of equivalent voltage. The demodulator will produce an output where its instantaneous amplitude is proportional to the instantaneous frequency of the input FM signal.

FM detection (cont’d) To detect an FM signal, it is necessary to have a circuit whose output voltage varies linearly with the frequency of the input signal. The most commonly used demodulator is the PLL demodulator.

PLL Phase Locked Loop
PLL: a circuit that locks on a signal and changes with it slightly as it changes.
Invented in 1932

Has many applications in communications: FM stereo decoders, FM Demodulation networks for FM operation. Frequency synthesis that provides multiple of a reference signal frequency. Used in motorspeed controls, tracking filters. Used in frequency shift keying (FSK) decodes for demodulation carrier frequencies.

PLL Demodulator

Phase detector (Multiplier)
Loop filter: low pass filter Three main components of PLL
Phase detector (Multiplier)
Loop filter: low pass filter
Voltage controlled oscillator (VCO): a FM system

Phase detector (Multiplier) A phase detector basically a comparator which compares the input frequency with the feedback frequency and generates a voltage according to the phase difference between the two signals. The low pass loop filter removes the high-frequency component of the multiplier output.

Loop filter: low pass filter This filter is used to filter the output from the phase comparator in the PLL The loop filter is a low pass linear filter. The bandwidth of the loop filter is roughly the same as the bandwidth of the message m(t), so output is also low frequency signal

Voltage controlled oscillator (VCO) The output frequency of the VCO is directly proportional to the input signal. Both the input frequency and output frequency are compared and adjusted through feedback loops until the output frequency equals the input frequency.

VCO Classification VCO can be broadly classified into linear voltage controlled oscillators and relaxation type voltage controlled oscillators. Linear voltage controlled oscillators are generally used to produce a sine wave Relaxation voltage controlled oscillators are used to produce a sawtooth or triangular waveform.

Question ……Problem In this problem we study the idea of mixing in a super-heterodyne receiver. To be specific, consider the block diagram of the mixer shown below that consists of a product modulator with a local oscillator of variable frequency f, followed by a band-pass filter. The input signal is an AM wave of bandwidth 10 kHz and a carrier frequency that may lie anywhere in the range 0.535-1.605 MHz; these parameters are typical of AM radio broadcasting. It is required to translate this signal to a frequency band centered at a fixed intermediate frequency (IF) of 0.455 MHz. Determine the range of tuning that must be provided in the local oscillator in order to achieve this requirement.  

Solution ……Problem