Signal Generators

Triggered Sawtooth Generator Circuit Diagram

The Triggered sawtooth generator shown in the diagram may be used with an oscilloscope. It is linear retriggerable and is enabled automatically in the absence of a trigger signal.

When a positive pulse is applied to its input B, monstable IC-3a, generates a sawtooth, which, owing to the ‘standard’ RC network is not linear. If, however, the resistor in that network is replaced by a current source, consisting here of T1, R4, P2, linearity returns. The period of the sawtooth depends on the settings of S1 and P2.

The sawtooth signal is buffered by T2 to prevent its quality being affected adversely. The voltage level at the gate of T2 is 0-3.5 V that at the source is slightly lower. The rectangular signal at the Q output of IC-3a, is used to suppress the electron beam during the flyback.

The trigger signal is provided by Schmitt trigger IC-1a, which has the small drawback that the input signal must be at least 1 V r.m.s. Preset P1 provides compensation for the d.c. level at the input. Some experimentation with the values of R1 and R2 is advisable since these resistors determine the operating range of P1.

Triggered Sawtooth Generator Circuit Diagram

Triggered Sawtooth Generator

As long as trigger signals are input, 1C-3a generates sawtooth signals. Since 1C-2a receives the same trigger signals, it is enabled continuously. There is, however, a fundamental difference between the two circuits: the 74HCT123 is retriggerable, whereas the 74HCT221 is not; it is enabled only after the current period has been processed; all intermediate trigger pulses are ignored.

In practice, the Q output of IC-2a is high as long as trigger pulses are input. When these pulses cease, the Q output goes low which results in IC-3b being started via the Q output. After its mono time has elapsed, IC-3b enables IC-3a and this in its turn re-actuates IC-3b, so that, even in the absence of input trigger signals, the sawtooth generator continues to operate.

The flyback is suppressed by network R6-C8, provides a slight delay to ensure that the beam is suppressed at the right moment. Since these components have a fixed value, it may happen at very low frequencies that small portions of the sawtooth are not suppressed. This may be obviated by switching capacitors cf different values in the C8 position via a second wafer on S1.

The range of the generator may be refined by adding capacitors in the time base whose ratio is 1:2:5; the capacitors in the diagram, C2—C6, have a ratio of 1:10. With values as shown, the period of the sawtooth in the upper position of the switch can be set between 1 µs and 6 µs; periods below 1 µs cannot be obtained.

The circuit as shown draws a current of about 7.5 mA from the 5 V supply. The negative supply is not critical and may be between 5 V and 12 V.


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