Synchronous Modulation and Demodulation – Low dc current may be transformed into a high voltage dc by simple chopper action. Although an inductive type transformation process is required, the output dc may be obtained without rectifying devices.
Figure 14.39 shows a circuit function of a synchronous vibrator. Contacts a and b connect the input dc to the primary side of the transformer in an alternating fashion, thus producing an ac product. By adding another set of points to the normally composite vibrator reed, the output of the transformer can be rectified whenever a and b close the circuit in unison with the primary side contacts a’ and b’. The output product will be unfiltered direct current. This principle of coinciding phase switching may be used in dc amplifier chopping circuits.
A functional diagram of a synchronous chopper is shown in Fig. 14.40. Using this chopper it is possible to obtain an amplified output signal of the original dc input from the ac amplifier. In this circuit no rectification is necessary to produce the dc signal. The proper polarity of the signal is always maintained in the proper phase.
Solid State Modulator/Demodulator Circuit
A solid state modulator/demodulator circuit is shown in Fig. 14.41. The transformer T is driven by an ac source and couples each secondary connected to the two transistor pairs, Q1 — Q2 and Q3 — Q4.
During the first half cycle, point X is positive with respect to point Y. This forward biases transistors Q1 — Q2 which are driven in full conduction. The resistance of the transistors falls. The current flow through the path is shown in Fig. 14.41 (arrow A). The other transistor Q3 — Q4 is reverse biased and its resistance increases. Therefore the output voltage increases.
During the second half cycle, the polarity reverses. making Y positive with respect to X; now transistor Q3 — Q4 conducts fully, reducing its resistance, and output. Transistor Q1 — Q2 are reverse biased. Its resistance increases, hence the output developed across it increases. Thus, it is seen that Q1 — Q2 act as a modulator which feeds an ac signal to the ac amplifier and Q3 — Q4 act as a demodulator, which demodulates the amplified ac signal to obtain dc again. The dc output obtained is an amplified value of the dc input.
Photo Optical Modulator
The principle of an optical dc chopper is illustrated in Fig. 14.42. Devices of this type have been used widely in infrared signal detectors, whose output is a slowly varying dc product. The properties of the parallel connected photo resistor are such that the device produces a decrease in its internal resistance when struck by light. The photo resistor may have a dark resistance of several Meg-ohms, but has a dynamic resistance of 6 Ω or less when light falls on it. (In order to make use of this effect in a chopping system, a motor driven chopping disc supplied with a slit is rotated in front of the photo resistor. The latter’s chopping frequency is determined by the number of the slots in the disc, and by the speed at which it is being rotated. A 1 slot disc, being rotated at 3600 rpm, would thus produce a chopping frequency of 60 Hz. Two slots with rpm constant give 120 Hz; and with 10 slots the frequency would be 600 Hz.) The bulb may be driven by a simple relaxation oscillator in place of the motor.