Chapter 4
Pulse Width
Demodulator
Pulse Width Demodulator
4-1: Curriculum Objectives
1 . T o u nd e r s t a nd t he o p e r at io n t he o r y o f p u ls e w id t h demodulator.
2 . To understand the operation theory of PWM to PAM pulse width demodulator.
3 . To understand the operation theory of PWM to PPM pulse width demodulator.
4. To design and implement the product detector pulse width demodulator.
4-2: Curriculum Theory
In chapt er 3, we use square wave oscillat or and monost able multi-vibrator to produce
the PWM signal. This modulated signal is used to transmit digital and analog data. The amplitude of
PWM signal maintains constant, but the pulse width will be varied by the amplitude of input audio
signal. This chapter is mainly introducing the demodulation method of PWM signal and the
produced PWM signal is according to the monostable multi-vibrator in chapter 3.
There are many types of pulse width demodulator, we will introduce common PWM demodulators
in this chapter. The first type is let the signal convert to PAM (pulse amplitude modulation) signal,
then via a low-pass filter and demodulation. If there are three consecutive PWM pulses close to
each others as shown in figure 4-1(a), then input these pulses into the integrator and voltage
threshold circuit. When the pulse appears, then it will start to integrate and produce ramp until the
last pulse appears. So, the height of the ramp is proportional to the width of the pulse, the ramp
is maintained at the maximum peak time, after the fixed time, the voltage will return to zero
voltage. Then it will produce a signal V b as shown in figure 4-1(b). The demodulator
needs to produce a fixed amplitude and a same signal pulse width, then sum this pulse signal
with signal Vb of figure 4-1(b), finally we get the signal V C as shown in figure 4-1(c). Let the
signal V C passes through the limiter circuit if the signal higher than the reference voltage Vth.
The result is shown in figure 4-1(d), which is the PAM signal. Finally let the PAM signal passes
through the low-pass filter to obtain the PWM demodulation signal.
The second type is let the PWM signal convert to PPM (pulse position modulation) signal, as shown
in figure 4-2. In figure 4-2, we can obtain the PPM signal by differentiating the PWM signal. After
that, it will produce a negative edge PPM pulse at the positive edge of the PWM pulse until the
end of the negative edge PWM pulse. At the same time, reset a positive edge pulse until the period
of the pulse terminated, that is the PPM(1) signal in figure 4-2. Next we utilize diode to eliminate
the negative edge pulse of the PPM(1) signal, then we obtain PPM(2) signal. If we let this signal
pass through a monostable multi-vibrator, which can provide a fixed bandwidth with square wave
signal, then we can obtain the typical PPM signal as shown in PPM(3) in figure 4-2. Finally, let
the PPM signal pass through a narrow timer and R-S flip-flop with edge trigger effect, then at the
output port, we can obtain the demodulated PWM signal.
Another type of PWM demodulator is the product detector. The block diagram and signal waveform
of product detector PWM demodulator are shown in figure 4-3. If we let the PWM signal and carrier
signal input to the product detector, at the meanwhile, let the PWM signal and carrier signal to be
positive or negative simultaneously, then the output terminal of the product detector will
receive a pulse signal Va, and send the signal Va to the low-pass filter to obtain the demodulated
PWM signal.
Figure 4-4 is the internal circuit diagram of MC1496. The transistor Q5and Q6 is the differentiator.
They are used to activate the double differential amplifiers, which are formed by Q 1 ,Q2 , Q3 and
Q4. The Q5 and Q6 can also be used to control the current capacity of double differential amplifiers.
The transistors Q7 and Q8 are the current source. They supply a constant current to Q5 and Q6.
We can connect a resistor between pin 2 and pin 3 to control the gain of the whole circuit.
(a) PWM signal waveform
(b) The waveform after processed by the integrator and voltage threshold circuit
(c) The output signal of Vb with pulse signal
(d) PAM signal waveform
Figure 4-1 Signal diagram of PWM to PAM.
(a) Circuit diagram of PWM to PPM.
(b) Waveform of PWM to PPM
Figure 4-2 Circuit diagram and waveform of PWM to PPM
Figure 4-3 Block diagram of signal waveform of product detector PWM demodulator.
The PWM modulated signal is inputted through pin 1 and pin 4. The carrier signal is inputted
through pin 8 and pin 10. The bias current of amplifier is determined by the external resistor,
which is normally connects to the bases of Q 7 and Q 8 at is pin 5. This is because the detector
has two output terminals (pin 6 and pin 12), so we can let one of the output terminals to be the
output of the detector. Then another output can be used as the automatic gain control (AGC).
Figure 4-4 Internal circuit diagram of MC1496.
Figure 4-4 is the circuit diagram of PWM demodulator by using MC1496 product detector,
which was previously introduced in Chapter 3. There are two op-amps U1 and U2 in the circuit, the
objective is to adjust the voltage level of the PWM signal and carrier signal so that the MC1496 can
operate properly. The actual input signal is normally between 300 mV ppto 1400 mVpp.If the PWM
signal is higher than 300 mVpp,then the product detector operates in the linear region. R 7 is used
to control the gain of MC1496. Capacitors C 1 , C 2 , C 4 , C 5 and C 9 are the coupling
capacitors, which are used to block the DC signal and let the AC signal pass through. VR1 and
VR2 are used to adjust the gain of U1 and U2. VR3 is used to changethe input signal range of PWM.
In this experiment, the output signal of MC1496 is the pulse signal, let this pulse signal passes
through U4, C11, C12, R11,R 1 2 and R 1 3 which is the 2 nd order low-pass filter to obtain the
PWM demodulation signal.
Figur e 4-5 is t he circuit diagram o f product det ect ed PWM demodu lat or or
synchronous PWM demodulator. The implementation is very simple, which only need a
multiplier IC and a few of op-amps. Practically, it is difficult to obtain the synchronous carrier
signal; therefore, we need some extra synchronous or phase-locked circuit. Figure 4-1 shows the
method of PWM to PAM demodulation or called asynchronous PWM .Modulator. Since this circuit
needs not the synchronous signal, therefore, we can save the phase-locked circuit, this is another
demodulation technique.
Figure 4-4 Circuit diagram of PWM demodulator by using MC1496.
4-3: Experiment Items
Experiment 1: PWM demodulator
1. Refer to figure 4-5 or figure DCT4-1 on GOTT DCT-6000-02 module.
2. In this experiment, the modulated PWM signal is produced by the PWM modulator in
figure 3-8. At this moment, the audio signal input terminal amplitude is 500mV amplitude and
input frequency is 700 Hz.
3. In figure 3-8, the astable multi-vibrator output terminal (i.e., pin 3 of U1 LM555, test point TP3)
connects to the carrier signal input terminal (Carrier I/P).
4. In figure 3-8, the mono-stable multi-vibrator output terminal (i.e., pin 3 of U2 LM555)
connects to PWM signal input terminal (PWM I/P).
5. Adjust VR1 to minimize the output signal distortion of U1 (µA741).
6. Adjust VR2 and VR3 until we obtain the demodulated signal correctly.
7. By using oscilloscope, observe on the PWM input signal, carrier signal, U 1 output signal (test
point TP1), U 2 output signal (test point TP2), output signal of MC1496 pin 10 (test point
TP3), output signal of MC1496 pin 1 (test point TP4), output signal of MC1496 pin 12 (TP5
test point), input signal of low-pass filt er (test point TP6) and demodulation PWM
signal (Audio O/P). Finally record the measured results in table 4-1.
8. Refer to figure 3-8, change the input audio signal frequency to 500 Hz and the others remain the
same.
9. Repeat step 5 to step 7 and record the measured results in table 4-2.
4-4: Measured Results
Table 4-1 Measured results of PWM demodulator. (Vm =500mV amplitude, fm = 70OHz)
Test Points
Carrier Signal
Input Terminal
Audio O/P
TP1
Output Signal Waveforms
Table 4-1 Measured results of PWM demodulator. (Continue)
(Vm= 500mV Amplitude, fm = 700 Hz)
Test Points
TP2
TP3
TP4
Output Signal Waveforms
Table 4-1 Measured results of PWM demodulator. (Continue)
(Vm =500mV Amplitude, fm = 700 Hz )
Test Points
TP5
TP6
Audio O/P
Output Signal Waveforms
Table 4-2 Measured results of PWM demodulator. (Vm = 500mV Amplitude, fm = 500 Hz )
Test Points
Carrier Signal
Input Terminal
Audio O/P
TP1
Output Signal Waveforms
Table 4-2 Measured results of PWM demodulator. (Continue)
( V m = 500mV amplitude, fm = 500 Hz )
Test Points
TP2
TP3
TP4
Output Signal Waveforms
Table 4-2 Measured results of PWM demodulator. (Continue)
(V m = 500mV amplitude, fm = 500 Hz)
Test Points
TP5
TP6
Audio O/P
Output Signal Waveforms
4-5: Problems Discussion
1. What are the functions of VR1 and VR2 in figure 4-5?
2. What are the functions of VR3 in figure 4-5?
3. Refer to figure 3-8, if we adjust the audio signal frequency to a minimum value, then,
what the values of the components should be changed in figure 4-5?