PHASE DEMODULATION

In phase modulation (pm) the intelligence is contained in the amount and rate of phase shift in a carrier wave. You should recall from your study of pm that there is an incidental shift in frequency as the phase of the carrier is shifted. Because of this incidental frequency shift, fm demodulators, such as the Foster-Seeley discriminator and the ratio detector, can also be used to demodulate phase-shift signals.

Another circuit that may be used is the gated-beam (quadrature) detector. Remember that the fm phase detector output was determined by the phase of the signals present at the grids. A QUADRATURE DETECTOR FOR PHASE DEMODULATION works in the same manner.

A basic schematic is shown in figure 3-19. The quadrature-grid signal is excited by a reference from the transmitter. This may be a sample of the unmodulated master oscillator providing a phase reference for the detector.

Figure 3-19. - Phase detector.

The modulated waveform is applied to the limiter grid. Gating action in the tube will occur as the phase shifts between the input waveform and the reference. The combined output current from the gated-beam tube will be a series of current pulses. These pulses will vary in width as shown in figure 3-20. The width of these pulses will vary in accordance with the phase difference between the carrier and the modulated wave.

Figure 3-20. - Phase-detector waveforms.

Q.31 Where is the intelligence contained in a phase-modulated signal?
Q.32 Why can phase-modulated signals be detected by fm detectors?
Q.33 How is a quadrature detector changed when used for phase demodulation?

PULSE DEMODULATION

Pulse modulation is used in radar circuits as well as communications circuits, as discussed in chapter 2. A pulse-modulated signal in radar may be detected by a simple circuit that detects the presence of rf energy. Circuits that are capable of this were covered in this chapter in the cw detection discussion; therefore, the information will not be repeated here. A RADAR DETECTOR, in its simplest form, must be capable of producing an output when rf energy (reflected from a target) is present at its input.

In COMMUNICATIONS PULSE DETECTORS the modulated waveform must be restored to its original form. In this chapter you will study three basic methods of pulse demodulation: PEAK, LOW-PASS FILTER, and CONVERSION.

PEAK DETECTION

Peak detection uses the amplitude of a pulse-amplitude modulated (pam) signal or the duration of a pulse-duration modulated (pdm) signal to charge a holding capacitor and restore the original waveform. This demodulated waveform will contain some distortion because the output wave is not a pure sine wave. However, this distortion is not serious enough to prevent the use of peak detection.

Pulse-Amplitude Demodulation

Peak detection is used to detect pam. Figure 3-21 includes a simplified circuit [view (A)] for this demodulator and its waveforms [views (B) and (C)]. CR1 is the input diode which allows capacitor C1 to charge to the peak value of the pam input pulse. Pam input pulses are shown in view (B). CR1 is reverse biased between input pulses to isolate the detector circuit from the input. CR2 and CR3 are biased so that they are normally nonconducting. The discharge path for the capacitor is through the resistor (R1). These components are chosen so that their time constant is at least 10 times the interpulse period (time between pulses). This maintains the charge on C1 between pulses by allowing only a small discharge before the next pulse is applied. The capacitor is discharged just prior to each input pulse to allow the output voltage to follow the peak value of the input pulses. This discharge is through CR2 and CR3. These diodes are turned on by a negative pulse from a source that is time-synchronous with the timing-pulse train at the transmitter. Diode CR3 ensures that the output voltage is near 0 during this discharge period. View (C) shows the output wave shape from this circuit. The peaks of the output signal follow very closely the original modulating wave, as shown by the dotted line. With additional filtering this stepped waveform closely approximates its original shape.

Figure 3-21A. - Peak detector. CIRCUIT OF PEAK DETECTOR

Figure 3-21B. - Peak detector. AMPLITUDE MODULATED PULSES

Figure 3-21C. - Peak detector. PEAK DETECTION

Pulse-Duration Modulation

The peak detector circuit may also be used for pdm. To detect pdm, you must modify view (A) of figure 3-21 so that the time constant for charging C1 through CR1 is at least 10 times the maximum received pulse width. This may be done by adding a resistor in series with the cathode or anode circuit of CR1. The amplitude of the voltage to which C1 charges, before being discharged by the negative pulse, will be directly proportional to the input pulse width. A longer pulse width allows C1 to charge to a higher potential than a short pulse. This charge is held, because of the long time constant of R1 and C1, until the discharge pulse is applied to diodes CR2 and CR3 just prior to the next incoming pulse. These charges across C1 result in a wave shape similar to the output shown for pam detection in view (C) of figure 3-21.

Q.34 In its simplest form, what functions must a radar detector be capable of performing?
Q.35 What characteristic of a pulse does a peak detector sample?
Q.36 What is the time constant of the resistor and capacitor in a peak detector for pam?
Q.37 How can a peak detector for pam be modified to detect pdm?