In order to obtain a steady pattern on the screen of the CRO, the sweep generator has to commence its charging and discharging in step with the input signal. In other words the frequency of the time base generator must be equal to a whole multiple of the input signal frequency. If this condition is not satisfied the pattern on the screen is not stable and moves along the time axis. The circuit of the UJT relaxation oscillator explained above is of the free running type. There is no control over the generation of each new sweep. Such a sweep voltage will not produce stable pattern.
Synchronization is the process of interlocking the input signal and the time base generators signal. When the input signal and the sweep voltage are synchronized the pattern on the screen will be stable.
Synchronization can be done in several ways. The type of synchronization depends on le circuit used in the CRO.
The time base voltage produced by the relaxation oscillator using UJT as explained above an be synchronized with the input signal using the base two (B2) terminal as the synchronizing terminal. This is explained as follows.
What we do in synchronizing is simple. The run up ramp of the sweep voltage will be nade to close prematurely. This can be done by applying a train of negative pulses to the base two (B2) of the UJT, or by applying sinusoidal voltage to the base of the UJT (B2). When negative pulses are applied the run up ramp ends prematurely.
Application of the negative pulse voltage to base two (B2) will cause reduction in peak voltage Ep of the UJT at the instants where the pulse is occurring. The sweep generator produces sweep voltage that has its ramp amplitude determined by the peak voltage of the UJT. For the first few cycles of the sync pulses the time base ramp follows the free running mode. At one instant when the negative pulse reducing the peak voltage of the UJT is able to discharge the capacitor earlier to reaching the amplitude of the previous cycle, the synchronizing signal and the sweep signal are locked together. That is after this instant the capacitor discharges only at the peaks of the sync signal. This means that the sweep frequency has assumed the frequency of the sync signal, which is lower slightly than its original frequency. In order to obtain this action the following conditions are to be satisfied.
1. The period of the sync signal must be shorter than the period of the time base voltage.
2. The amplitude of the sync signal must be sufficiently large for proper operation of the circuit.
Alternating voltage can also be used to synchronize the sweep generators. When a sync voltage of alternating nature is applied to the base two (2) of the UJT, the peak voltage of the UJT, Ep is varied between a maximum and minimum. These limits depend on the amplitude of the sync voltage. When such a voltage is used for synchronization the period of ramp of the sweep generator may either be enhanced or reduced.
In the above figure we find that two sweep voltages of different frequency are shown. The sync. voltage is same for both. The sweep waveform with dotted line has its time period longer than the sync. period. The period to this sweep signal is shortened by the sync. signal. The solid line representing a sweep signal whose frequency is greater than the sync signal is modified to have a reduced frequency in synchronization with the sync. signal frequency. This is done by premature discharge in the first case and extended charging in the second case. In both the cases the sweep signal assumes the frequency of the sync. signal after passing through some cycles of the sync. signal.
In the cathode ray oscilloscopes the sync. signal can be obtained from the output of the vertical amplifier using the same signal for synchronizing the sweep, with the input signal. A low voltage derived from the supply mains can also be used. External sync. voltages can also be applied. Hence there will be a selector switch that connects the sweep generators sync. input terminal to either vertical amplifier, low voltage a.c or to external sync. voltage. These functions are termed as internal, line and external synchronization, and are obtained by the sync. selector switch. A diagram showing this sync. selector is shown below in figure.
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| Sync Selector Circuit |
The sweep circuits used in CRO must have excellent linearity to give accurate results of the measurements made. To improve the linearity of the sweep circuits the following methods can be used.
1. Constant current charging, using a constant current source.
2. The Miller sweep circuit.
3. The phantastron circuit which is a variation of Miller circuit.
4. The boot-strap circuit.
5. Compensating networks which are used to improve the linearity of Miller and boot-strap circuits.
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Cathode Ray Oscilloscope