15.3 Synchronization of Other Relaxation Circuits – Pulse and Digital Circuits

15.3 SYNCHRONIZATION OF OTHER RELAXATION CIRCUITS

Frequency synchronization and division is also possible using other relaxation circuits such as astable multivibrators and monostable multivibrators. We will consider the blocking oscillator circuits and conventional astable and monostable multivibators.

15.3.1 Synchronization of Astable Blocking Oscillators

Synchronization of the output of an astable blocking oscillator with frequency division by a factor of 5 using positive sync pulses is illustrated in Fig. 15.5(a). Q2 acts as an inverter. The positive sync pulses that appear at the base of Q2, after amplification and polarity inversion by the CE configuration, appear as negative pulses at the collectors of Q1 and Q2. Because of the polarity inversion by the pulse transformer, these negative pulses appear as positive pulses at the base Q1 as per the chosen dot convention on the windings. Consequently, the base current of Q1 increases, its collector current further rises, the voltage at the collector falls still further, the voltage at the base increases further and so on. A regenerative action takes place, the transistor Q1 is quickly driven into saturation, and a pulse of duration tp is generated. During this period of pulse generation, the capacitor C1 charges, the voltage across the capacitor at t = tp being V1, as shown in Fig. 15.5(b). As this voltage reverse-biases the base – emitter diode of Q1, Q1 now goes into the OFF state. As a result, the charge on C1 discharges through R1 and when the voltage across the capacitor terminals falls to VBBVγ, then Q1 is again ON and C1 charges and this process is repeated. In the absence of sync pulses, a new sweep would have started at the voltage (VBBVγ), at which voltage Q2 would have normally gone into the ON state. The output would have a time period To, as shown in Fig. 15.5(b).

FIGURE 15.5(a) The sync signal applied to an astable blocking oscillator

FIGURE 15.5(b) 5:1 synchronization in an astable blocking oscillator

However, the sync pulses appear as positive pulses at the base Q1 (after polarity inversion in Q2 and further polarity inversion in the pulse transformer). Pulses numbered 1, 2, 3 and 4 do not have sufficient amplitude to terminate the sweep prematurely. However, pulse 5 occurs at such an instant and has sufficient amplitude that it prematurely terminates the cycle as Q2 goes into the ON state at the instant of occurrence of the 5th pulse, as regenerative action again takes place. C1 again charges and so on. Thus, the cycle is prematurely terminated at Ts, and a new cycle starts. Synchronization with 5:1 division is accomplished. If, on the other hand, the amplitude of the sync pulses is increased, it could result in synchronization with 3:1 division, as shown in Fig. 15.5(c). Thus, we can say that with the proper spacing between the sync pulses (proper choice of pulse repetition frequency) and proper choice of amplitude for these pulses, it is possible to achieve synchronization with desired frequency division.

15.3.2 Synchronization of Transistor Astable Multivibrators

Synchronization with frequency division in a transistor astable multivibrator can be accomplished by applying either positive or negative pulses to both the transistors or to any of the transistors. Figs. 15.6 (a) and (b) depict the circuit and the waveforms to achieve synchronization with a frequency division of 6:1. Here, positive pulses are applied at the base B1 of Q1.

FIGURE 15.5(c) 3:1 synchronization in an astable blocking oscillator

FIGURE 15.6(a) An astable multivibrator with sync pulses applied at B1

FIGURE 15.6(b) The waveforms of an astable multivibrator with positive sync pulses applied at B1

In the absence of sync pulses, the astable must have had a time period To (= T1 + T2) when the cycle would have naturally terminated at VB1 or VB2 = Vγ. However, with sync pulses connected, the positive pulses applied at B1 are amplified and inverted and appear as negative pulses at B2. During T2, the positive pulses at B1 have no effect on the time period as Q1 is already ON. Further the negative pulses 1, 2 and 3 appearing at B2 will not be able to change T2. Hence, T2 remains unchanged. However, during the time period T1, the pulses numbered 4 and 5 do not have sufficient amplitude to drive Q1 into the ON state and terminate the time period T1 prematurely. However, the 6th pulse has sufficient amplitude to prematurely terminate the time period T1 as this pulse drives the base of Q1 positive; and hence, Q1 goes ON. The new time period for which Q1 is OFF is and the new sweep period is . In this arrangement, the multivibrator completes one cycle for every six sync pulses. Although the complete period is synchronized, the individual time periods are not synchronized. T2 is the same as without synchronization.

15.3.3 Synchronization with Division of an Astable Multivibrator by Applying Negative Pulses at both the Bases (B1 and B2)

If an astable multivibrator is required to be synchronized during both the time periods T1, T2 and also for T, then the negative pulses can be applied to both the bases B1 and B2 of transistors Q1 and Q2. Let it be assumed that both the time periods are required to be synchronized with a division of 3:1 so that the total period is synchronized with a frequency division of 6:1, as shown in Fig. 15.7.

The negative pulses applied at B1 get amplified, inverted appear as positive pulses at the base B2. Similarly, the negative pulses applied at B2 get amplified and inverted and appear as positive pulses at the base B1. Thus, the positive pulses superimposed on the exponential portion of the waveform at B2 during T2 are a combination of negative pulses applied directly and the inverted and amplified negative pulses from the other transistor which appear as positive pulses. The pulses marked 1 and 2 do not have sufficient amplitude. However, the pulse marked 3 has an amplitude that can terminate T2 earlier, resulting in a new time period . Similarly, during the period T1 when Q1 is OFF, pulses marked 4 and 5 will not have any influence on the time period T1. However, the pulse numbered 6 will terminate T1 prematurely, resulting in a new time period . Each of these time periods are individually synchronized with a frequency division of 3:1 as the third and the sixth pulses prematurely terminate the time periods T2 and T1. Hence, synchronization with a division of 6:1 occurs for the entire time period T of the astable multivibrator.

15.3.4 Positive Pulses Applied to B1 Through a Small Capacitor from a Low-impedance Source

Synchronization with the division of both the time periods of an astable multivibrator can be achieved by applying the positive pulses to only one base instead of at both the bases, say, B1. During the period when Q1 is ON, as its input resistance is very small, the time constant of the pulse input is also very small. This RC circuit behaves as a differentiator and the pulse is quasi-differentiated. The negative spikes in this differentiated signal at B1 of Q1 appear as the positive spikes during the exponential variation at B2. Pulses 1 and 2 may not be able to drive Q2 ON and terminate T2 prematurely. However, the positive spike appearing at the trailing edge of the pulse numbered 3 will prematurely terminate the OFF period T2 of Q2. The new time period for which Q2 is OFF is T2′. During the exponential variation of the voltage at B1 during T1, the positive pulses are superimposed and at the leading edge of the pulse numbered 6, the OFF period of Q1 is prematurely terminated. The new time period for which Q1 is OFF is T1. The original time period of the astable multivibrator was T (= T1 + T2). Whereas, the new time period after synchronization is T (= + ). Thus, not only the entire cycle with time period T of the astable is synchronized with a frequency division of 6:1 but the individual time periods and are also synchronized with a division of 3:1, as shown in Fig. 15.8.

FIGURE 15.7 The synchronization of individual periods and the total period with a division of 6:1