Chapter 4 Pulse Width Demodulator

Transcription

1 Chapter 4 Pulse Width Demodulator

2 Pulse Width Demodulator 4-1: Curriculum Objectives 1. T o underst and the o perat io n t heo r y o f pu ls e w idt 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 chapter 3, we use square wave oscillator and monostable 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 V b 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 V th. 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.

3 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 V a, and send the signal V a to the low-pass filter to obtain the demodulated PWM signal. Figure 4-4 is the internal circuit diagram of MC1496. The transistor Q 5 and Q 6 is the differentiator. They are used to activate the double differential amplifiers, which are formed by Q 1,Q 2, Q 3 and Q 4. The Q 5 and Q 6 can also be used to control the current capacity of double differential amplifiers. The transistors Q 7 and Q 8 are the current source. They supply a constant current to Q 5 and Q 6. We can connect a resistor between pin 2 and pin 3 to control the gain of the whole circuit.

4 (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.

5 (a) Circuit diagram of PWM to PPM. (b) Waveform of PWM to PPM Figure 4-2 Circuit diagram and waveform of PWM to PPM

6 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).

7 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 U 1 and U 2 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 pp to 1400 mv pp.if the PWM signal is higher than 300 mv pp,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. VR 1 and VR 2 are used to adjust the gain of U 1 and U 2. VR 3 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 U 4, C 11, C 12, R 11,R 12 and R 13 which is the 2 nd order low-pass filter to obtain the PWM demodulation signal.

8 Figure 4-5 is the circuit diagram of product detected PWM demodulator 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.

9 4-3: Experiment Items Experiment 1: PWM demodulator 1. Refer to figure 4-5 or figure DCT4-1 on GOTT DCT 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 VR 1 to minimize the output signal distortion of U1 (µa741). 6. Adjust VR 2 and VR 3 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 filter (test point TP6) and demodulation PWM signal (Audio O/P). Finally record the measured results in table 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.

10 4-4: Measured Results Table 4-1 Measured results of PWM demodulator. (V m =500mV amplitude, f m = 70OHz) Test Points Output Signal Waveforms Carrier Signal Input Terminal Audio O/P TP1

11 Table 4-1 Measured results of PWM demodulator. (Continue) (V m = 500mV Amplitude, f m = 700 Hz) Test Points Output Signal Waveforms TP2 TP3 TP4

12 Table 4-1 Measured results of PWM demodulator. (Continue) (V m =500mV Amplitude, f m = 700 Hz ) Test Points Output Signal Waveforms TP5 TP6 Audio O/P

13 Table 4-2 Measured results of PWM demodulator. (V m = 500mV Amplitude, f m = 500 Hz ) Test Points Output Signal Waveforms Carrier Signal Input Terminal Audio O/P TP1

14 Table 4-2 Measured results of PWM demodulator. (Continue) ( V m = 500mV amplitude, f m = 500 Hz ) Test Points Output Signal Waveforms TP2 TP3 TP4

15 Table 4-2 Measured results of PWM demodulator. (Continue) (V m = 500mV amplitude, f m = 500 Hz) Test Points Output Signal Waveforms TP5 TP6 Audio O/P

16 4-5: Problems Discussion 1. What are the functions of VR 1 and VR 2 in figure 4-5? 2. What are the functions of VR 3 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?