Pulse Position Modulation PPM using 555 timers

 Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2^M possible time-shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second. It is primarily useful for optical communications systems, where there tends to be little or no multipath interference

          Pulse position modulation (PPM) use pulses that are of uniform height and width but displaced in time from some base position according to the amplitude of the signal at the instant of sampling. The position of each pulse, in relation to the position of a recurrent reference pulse, is varied by each instantaneous sampled value of the modulating wave. Pulse position modulation is also sometimes known as pulse-phase modulation

ADVANTAGE OF PPM:

Pulse position modulation has the advantage over pulse amplitude modulation (PAM) and pulse duration modulation (PDM) in that it has a higher noise immunity since all the receiver needs to do is detect the presence of a pulse at the correct time; the duration and amplitude of the pulse are not important.

Sensitivity to multipath interference:

Aside from the issues regarding receiver synchronization, the key disadvantage of PPM is that it is inherently sensitive to multipath interference that arises in channels with frequency-selective fading, whereby the receiver's signal contains one or more echoes of each transmitted pulse. Since  the information is encoded in the time of arrival (either differentially, or relative to a common clock), the presence of one or more echoes can make it extremely difficult, if not impossible, to accurately determine the correct pulse position corresponding to the transmitted pulse.

Non-coherent detection

One of the principal advantages of PPM is that it is an M-ary modulation technique that can be implemented non-coherently, such that the receiver does not need to use a phase-locked loop (PLL) to track the phase of the carrier. This makes it a suitable candidate for optical communications systems, where coherent phase modulation and detection are difficult and extremely expensive. The only other common M-ary non-coherent modulation technique is M-ary Frequency Shift Keying (M-FSK), which is the frequency-domain dual to PPM.

 

Variation In Output:

In  pulse width modulator modulate the active factor (the duty cycle), t(on)/(t(on)+t(off)), without changing the total period of  t(on)+t(off), while pulse position modulator modulate the t(on) and keep t(off) fixed. In the pulse-position-modulation, the frequency of the pulse is modulated as well as the duty cycle.

Circuit Diagram:

In the circuit there are two stages of 555 timers. One is acting as MONO-STABLE MULTI-VIBRATOR while the second one is acting as an ASTABLE MULTI-VIBRATOR.  The input signal is applied to a-stable stage and Its out-put is connected to the input of mono-stable stage which at its output gives us, pulse position modulated signal. The input signal is applied to the control terminal of the a-stable multi-vibrator when the input signal amplitude varies the control terminal voltage changes and the switching voltage of the a-stable multi-vibrator changes accordingly and the width of the output pulse of the a-stable multi-vibrator changes and the time when the mono-stable is triggered varies 

with the width of the output of the a-stable multi-vibrator and the position of the output pulses

  In A-stable configuration the timer continuously changes its output high to low and vice vrsa, which is achieved by charging and discharging of the capacitor connected to pin THR and TRIG of the timer. When output is high the capacitor is charging because the transistor through which discharging has to occur is off, when the capacitor is charged to Vcc (2/3) the output of the comparator connected to reset goes high and  the output of 555 timer goes low and the transistor is turned on ,due to which the capacitor starts discharging and when Vcc goes below Vcc(1/3) the output again goes high and the capacitor again stars charging  along with it. This process continues and we a square wave at the output of the timer.

Mono-stable configuration:

This circuit diagram shows how a 555 timer IC is configured to function as a basic mono-stable multi-vibrator. A mono-stable multi-vibrator is a timing circuit that changes state once triggered, but returns to its original state after a certain time delay.  It  got its name from the fact that only one of its output states is stable.  It is also known as a 'one-shot'.

   

In this circuit, a negative pulse applied at pin 2 triggers an internal flip-flop that turns off pin 7's discharge transistor, allowing C1 to charge up through R1. At the same time, the flip-flop brings the output (pin 3) level to 'high'.  When capacitor C1 as charged up to about 2/3 Vcc, the flip-flop is triggered once again, this time making the pin 3 output 'low' and turning on pin 7's discharge transistor, which discharges C1 to ground. This circuit, in effect, produces a pulse at pin 3 whose width t is just the product of R1 and C1, i.e., t=R1C1

Output Waveform: