April 24, 1997

Simple circuit provides timebase calibration

William Whitehead, Lafayette, CO

The circuit in Figure 1 provides an inexpensive and quick way to check the timebase speeds and linearity in vintage oscilloscopes. Component aging in the oscilloscopes sometimes alters the timebase parameters enough to yield grossly erroneous results. You can make timing adjustments in the oscilloscope by using the timing waveforms from the circuit in Figure 1. You can power the circuit from any suitable lab supply or from a 9V battery if you need a stand-alone calibrator. One section of a 74HCT04 (IC₁) is configured as a 1-MHz, crystal-controlled oscillator. A 74HCT74 (IC₂) provides two divide-by-2 counters that provide the 500- and 250-kHz outputs.

You could buffer outputs TP-1 through TP-3 by using three unused sections of IC₁ if you need to connect loads other than a 10X oscilloscope probe. After approximately 5 minutes of warmup (at room temperature), you adjust the oscillator via the 4- to 60-pF trimming capacitor. While you monitor TP-1 (1 MHz), tweak the trimming capacitor to obtain minimum pulse width and amplitude. An off-the-shelf crystal in the breadboard produced a frequency of 1.000005 MHz using this trimming procedure. Commercial (non-TCXO) crystals are available with ±50-ppm tolerance at room temperature. (DI #2027)

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