Pulse Circuits - Synchronization

In any system, having different waveform generators, all of them are required to be operated in synchronism. Synchronization is the process of making two or more waveform generators arrive at some reference point in the cycle exactly at the same time.

Types of Synchronization

Synchronization can be of the following two types −

One-to-one basis

All the generators are operated at a same frequency.

All of them arrive at some reference point in the cycle exactly at the same time.

Sync with frequency spanision

Generators operate at different frequency which are integral multiples of each other.

All of them arrive at some reference point in the cycle exactly at the same time.

Relaxation devices

Relaxation circuits are the circuits in which the timing interval is estabpshed through the gradual charging of a capacitor, the timing interval being terminated by the sudden discharge (relaxation) of a capacitor.

Examples − Multivibrators, sweep circuits, blocking oscillators, etc.

We have observed in the UJT relaxation oscillator circuit that the capacitor stops charging when the negative resistance device such as UJT turns ON. The capacitor then discharges through it to reach its minimum value. Both these points denote the maximum and minimum voltage points of a sweep waveform.

Synchronization in Relaxation Devices

If the high voltage or peak voltage or breakdown voltage of the sweep waveform has to be brought down to a lower level, then an external signal can be appped. This signal to be appped is the synchronized signal whose effect lowers the voltage of peak or breakdown voltage, for the duration of the pulse. A synchronizing pulse is generally appped at the emitter or at the base of a negative resistance device. A pulse train having regularly spaced pulses is appped to achieve synchronization.

Though the synchronizing signal is appped first few pulses will have no effect on the sweep generator as the ampptude of the sweep signal at the occurrence of the pulse, in addition with the ampptude of the pulse is less than V_P. Hence the sweep generator runs unsynchronized. The exact moment at which UJT turns ON is determined by the instant ofoccurrence of a pulse. This is the point where the sync signal achieves synchronization with the sweep signal. This can be observed from the following figure.

Where,

T_P is the time period of the pulse signal

T_O is the time period of the sweep signal

V_P is the Peak or breakdown voltage

V_V is the Valley or Maintaining voltage

To achieve synchronization, the pulse timing interval T_P should be less than the time period of sweep generator T_O, so that it terminates the sweep cycle prematurely. The synchronization cannot be achieved if the pulse timing interval T_P is greater than the time period of sweep generator T_O and also if the ampptude of the pulses is not large enough to bridge the gap between the quiescent breakdown and the sweep voltage, though T_P is less than T_O.

Frequency Division in Sweep Circuits

In the previous topic, we have observed that synchronization gets achieved when the following conditions are satisfied. They are

When T_P < T_O

When the ampptude of the pulse is sufficient to terminate each cycle prematurely.

Satisfying these two conditions, though synchronization is achieved, we may often come across a certain interesting pattern in the sweep with regard to sync timing. The following figure illustrates this point.

We can observe that the ampptude V’_S of the sweep after synchronization is less than the unsynchronized ampptude V_S. Also the time period T_O of the sweep is adjusted according to the time period of the pulse but leaving a cycle in-between. Which means, one sweep cycle is made equal to two pulse cycles. The synchronization is achieved for every alternate cycle, which states

$$T_o > 2T_P$$

The sweep timing T_O be restricted to T_S and its ampptude is reduced to V’_S.

As every second pulse is made in synchronism with the sweep cycle, this signal can be understood as a circuit that exhibits frequency spanision by a factor of 2. Hence the frequency spanision circuit is obtained by synchronization.