unit-6 pulse-de-modulation

unit-6 pulse-de-modulation

• 1.
  Conversion of PPMto 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 PPMto 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 PWMPPMto PWM
• 4.
   For PWMdemodulation 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  InPPM 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 Blockdiagram of TDM system
• 7.
  Conceptual diagram ofmultiplexing- 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.
• 10.
  TDM-Three information sources.
• 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 forPAM  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.
• 14.
  Frequency Division Multiplexing
• 15.
  Frequency Division Multiplexing Analogsignalling 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.