Variable Hold-Off and Time-Bases on Oscilloscopes


Suppose a series of double pulses must be displayed. The end of the first time-base ramp is reached after pulse 5 and the end of the hold-off period is reached before pulse 6. The second sweep will then be triggered at pulse 6. This means that the following pulses appear at the screen. By making the hold-off period longer, the second sweep will be triggered by pulse 7. As a result, the waveforms during the first and second sweeps will coincide, and the proper picture will be obtained. The same result could have been obtained by shortening the time­ base sweep by means of the Vernier control, but then the time scale would no longer be calibrated. For this reason, a variable hold-off control is sometimes built into an oscilloscope. However, relatively few oscilloscopes possess this feature, such as the Philips PM 3260 and PM 3265. The hold-off time must be related to the time-base sweep speed. If not, at high sweep speeds the hold-off time would be too long, and the successive sweeps will appear only after a relatively long time. Consequently, a fast sweep would be displayed at a low repetition rate, which would reduce the light output (brightness) to a large extent. For this reason, the range of the hold-off time is automatically set appropriately with the TIME/mv switch.

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