Module 12—Modulation Principles
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AI-1 to AI-6, Index-1 to 2, Assignment 1 , 2
Figure 3-18K.—Gated-beam detector waveforms.
Advantages of the Gated-Beam Detector
The primary advantage of the gated-beam detector lies in its extreme simplicity. It employs only one tube, yet provides a very effective limiter with linear detection. It requires relatively few components and is very easily adjusted.
There are more than the three types of fm demodulators presented in this chapter. However, these are representative of the types with which you will be working. The principles involved in their operation are similar to the other types. You will now briefly study PHASE DEMODULATION which uses the same basic circuitry as fm demodulators.
Q-28. What circuit functions does the tube in a gated-beam detector serve?
Q-29. What condition must exist on both the limiter and quadrature grids for current to flow in a gated-beam detector?
Q-30. Name two advantages of the gated-beam detector.
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 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 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.
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.
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?
Another method of demodulating PDM is by the use of a low-pass filter. If the voltage of a pulse waveform is averaged over both the pulse and no-pulse time, average voltage is the result. Since the amplitude of PDM pulses is constant, average voltage is directly proportional to pulse width. The pulse width varies with the modulation (intelligence) in PDM. Because the average value of the pulse train varies in accordance with the modulation, the intelligence may be extracted by passing the width- modulated pulses through a low-pass filter. The components of such a filter must be selected so that the filter passes only the desired modulation frequencies. As the varying-width pulses are applied to the low-
pass filter, the average voltage across the filter will vary in the same way as the original modulating voltage. This varying voltage will closely approximate the original modulating voltage.
Pulse-position modulation (PPM), pulse-frequency modulation (PFM), and pulse-code modulation (PCM) are most easily demodulated by first converting them to either PDM or PAM. After conversion these pulses are demodulated using either peak detection or a low-pass filter. This conversion may be done in many ways, but your study will be limited to the simpler methods.
PPM can be converted to PDM by using a flip-flop circuit. (Flip flops were discussed in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.) Figure 3-22 shows the waveforms for conversion of PPM to PDM. View (A) is the pulse-modulated pulse train and view (B) is a series of reset trigger pulses. The trigger pulses must be synchronized with the unmodulated position of the PPM pulses, but with a fixed time delay from these pulses. As the position-modulated pulse is applied to the flip-flop, the output is driven positive, as shown in view (C). After a period of time, the trigger pulse is again generated and drives the flip-flop output negative and the pulse ends. Because the PPM pulses are constantly varying in position with reference to the unmodulated pulses, the output of the flip- flop also varies in duration or width. This PDM signal can now be applied to one of the circuits that has already been discussed for demodulation.
Figure 3-22.—Conversion of PPM to PDM.
Pulse-frequency modulation is a variation of PPM and may be converted by the same method.
Pulse-code modulation can easily be decoded, provided the pulse-code groups have been transmitted in reverse order; that is, if the pulse with the lowest value is transmitted first, the pulse with the highest value is transmitted last. A circuit that will provide a constant value of current without regard to its load is known as a current source. A current source is used to apply the PCM pulses to an RC circuit, such as that shown in figure 3-23, view (A). The current source must be capable of supplying a linear charge to C1 that will increase each time a pulse is applied if C1 is not allowed to discharge between pulses. In other words, if C1 charges to 16 volts during the period of one pulse, then each additional pulse increases the charge by 16 volts. Thus, the cumulative value increases by 16 volts for each received pulse. This does not provide a usable output unless a resistor is chosen that allows C1 to discharge to one-half its value between pulses. If only one pulse is received at T1, C1 charges to 16 volts and then begins to discharge. At T2 the charge has decayed to 8 volts and continues to decay unless another pulse is received. At T3 it has a 4-volt charge and at T4 it only has a 2-volt charge. At the sampling time, a 1-volt charge remains; this charge corresponds to the binary-weighted pulse train of 0001. Now we will apply a PCM signal which corresponds to the binary-coded equivalent of 7 volts (0111) in figure 3-23, view (A). View (B) is the pulse code that is received. Remember that the pulses are transmitted in reverse order. View (C) is the response curve of the circuit. At T1 the pulse corresponding to the least significant digit is applied and C1 charges to 16 volts. C1 discharges between pulses until it reaches 8 volts at T2. At T2 another pulse charges it to 24 volts. At T3, C1 has discharged through R1 to a value of 12 volts. The pulse at T3 increases the charge on C1 by 16 volts to a total charge of 28 volts. At T4, C1 has discharged to one-half its value and is at 14 volts. No pulse is present at T4 so C1 will not receive an additional charge. C1 continues to discharge until T5 when it has reached 7 volts and is sampled to provide a PAM pulse which can be peak detected. This sampled output corresponds to the original sampling of the analog voltage in the modulation.
Figure 3-23.—PCM conversion.
When the PCM demodulator recognizes the presence or absence of pulses in each position, it reproduces the correct standard amplitude represented by the pulse code group. For this reason, noise introduces no error if the largest peaks of noise are not mistaken for pulses. The PCM signal can be retransmitted as many times as desired without the introduction of additional noise effects so long as the signal-to-noise ratio is maintained at a level where noise pulses are not mistaken for a signal pulse. This is not the only method for demodulating PCM, but it is one of the simplest.
This completes your study of demodulation. You should remember that this module has been a basic introduction to the principles of modulation and demodulation. With the advent of solid-state electronics, integrated circuits have replaced discrete components. Although you cannot trace the signal flow through
these circuits, the end result of the electronic action within the integrated circuit is the same as it would be with discrete components.
Q-38. How does a low-pass filter detect PDM?
Q-39. How is conversion used in pulse demodulation?
Q-40. What is the discharge rate for the capacitor in a PCM converter?
Now that you have completed this chapter, a short review of what you have learned is in order. The following summary will refresh your memory of demodulation, its basic principles, and typical circuitry required to accomplish this task.
DEMODULATION, also called DETECTION, is the process of re-creating original modulating frequencies (intelligence) from radio frequencies.
The DEMODULATOR, or DETECTOR, is the circuit in which the original modulating frequencies are restored.
A CW DEMODULATOR is a circuit that is capable of detecting the presence of RF energy.
HETERODYNE DETECTION uses a locally generated frequency to beat with the CW carrier frequency to provide an audio output.
The REGENERATIVE DETECTOR produces its own oscillations, heterodynes them with an incoming signal, and detects them.
The SERIES- (VOLTAGE-) DIODE DETECTOR has a rectifier diode that is in series with the input voltage and the load impedance.
SHUNT- (CURRENT-) DIODE DETECTOR is characterized by a rectifier diode in parallel with the input and load impedance.
The COMMON-EMITTER DETECTOR is usually used in receivers to supply a detected and amplified output.
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