unit-6 pulse-de-modulation
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The document describes how pulse position modulation (PPM) signals can be converted to pulse width modulation (PWM) signals. A flip-flop is used to generate a train of pulses with widths determined by the delay of incoming PPM pulses. This results in a PWM output proportional to the amplitude of the original signal. PWM signals can then be demodulated using time averaging by a low-pass filter.
unit-6 pulse-de-modulation
- 1. Conversion of PPM to PWM First the PPM signal is converted to PWM using a simple technique. The SR edge triggered flip-flop is set by +ve edge of the clock. It remains set so that output Q is high, till a +ve edge from PPM resets it. The more the delay in arrival, the longer the duration Q remains high. It is again set in the next clock period by the rising edge of the clock pulse.
- 2. Conversion of PPM to PWM Thus the output of the flip-flop is a train of pulses, the width of which is decided by how late PPM pulses arrives. Thus we get a PWM output at the flip-flop output, the width of which in each cycle is proportional to the amplitude of the message signal. PWM demodulation can be achieved by simple time averaging of PWM pulses by an averaging low pass filter.
- 3. PPM to PWMPPM to PWM
- 4. For PWM demodulation a ramp signal is started at the the +ve edge and stop it when the negative edge comes. Since the widths are different these ramps will reach different heights in each cycle. These ramps are directly proportional to the pulse width and inturn the amplitude of the message signal. This when passes through a LPF will follow the envelope, and demodulation is done. PWM DemodulationPWM Demodulation
- 5. PPM Demodulation In PPM demodulation the ramp starts at one +ve edge of the pulse and stops at +ve edge of the next pulse. Thus the delay between the pulses decides the height of the ramp generated and in turn closely follows the message signal. A LPF after that filters out the envelop information as demodulated signal. Transistor and RC combinations can be used for ramp generation and filtering to implement a demodulator circuit.
- 6. Time Division Multiplexing Block diagram of TDM system
- 7. Conceptual diagram of multiplexing- demultiplexing.
- 8. Time Division Multiplexing Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing. Two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel.
- 9. Time Division Multiplexing Each input signal is restricted in bandwidth by a low- pass anti-aliasing filter. The LPF outputs are then applied to a commutator, which is usually implemented using electronic switching circuitry. The functions of the commutator are (sampling & Multiplexing) To take a narrow sample of each of the N input messages at a rate fs, that is slightly higher than 2W.(W- cut off frequency of LPF)(The commutator must operate at a rate that satisfies the sampling theorem for each channel) To sequentially interleave these N samples inside the sampling interval Ts.
- 11. Time Division Multiplexing The multiplexed signal is applied to a pulse modulator , to transform the multiplexed signal into a suitable form for transmission over the channel (PAM) At the receiving end, the signal is applied to a pulse demodulator which performs the reverse operation of pulse modulator. The demodulated output is distributed to the appropriate Low pass reconstruction filters by means of a decommutator. The decommutator operates in synchronism with the commutator. Unlike FDM, TDM is immune to nonlinearities in the channel.
- 12. Channel Bandwidth for PAM Suppose that we have N independent baseband signals m1(t),m2(t), etc., each of which is band limited to fm. The communication channel will allow all N signals to be transmitted simultaneously using PAM time-division multiplexing. The Bandwidth required will be need not larger than Nfm. Multiplexing a number of signals by PAM TDM requires less bandwidth than required to multiplex using FDM using SSB transmission.
- 13. Frequency Division Multiplexing Assignment of non-overlapping frequency ranges to each “user” or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies. A multiplexer accepts inputs and assigns frequencies to each device. The multiplexer is attached to a high-speed communications line. A corresponding demultiplexer, is on the end of the high-speed line and separates the multiplexed signals.
- 15. Frequency Division Multiplexing Analog signalling is used to transmits the signals. Broadcast radio, television and cable television use frequency division multiplexing. This technique is the oldest multiplexing technique. Since it involves analog signalling, it is more susceptible to noise.