Stairstep Generator

This is a throw-together project using junk box parts that can be useful whenever an interesting-looking waveform is needed for a demonstration or for the beginnings of a video test generator or curve tracer. It also uses some odd parts as an excuse to apply, for at least once in your life, a programmable-unijunction transistor (PUT), for instance. The circuit diagram is shown below.

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The clock generator that drives the rest of the circuitry is the programmable-unijunction transistor (PUT), Q1. The 2N6027 or equivalent is commercially available, though you can build one out of two BJTs instead. The equivalent circuit of a PUT is shown below, along with its companion, the equivalent SCR circuit.

The oscillator output is short pulses that are inverted for a logic high most of the time. The inverted pulse both clocks the 74AHCT76 counter and turns off Q4, allowing diode D3 to conduct the current from the source including Q5. R11, D2, R13 forms a temperature-compensated voltage divider that sets a voltage on the base of Q5. Its emitter resistance, R2, R12 then sets the current that charges C3, C4 to a new voltage step level. R2 adjusts the current and sets the step height or amplitude.

Meanwhile, the counter chain of 4 JK flops counts down by 16 as a ripple counter, then clocks Q3 through C2. Q3 turns on and discharges the stairstep capacitors. Just as the PUT can be replaced by two transistors, or even a 555 timer, the counter can be replaced by 74HCT74 flops instead, or by one four-stage binary or decade counter IC of the 7400 series. The two polypropylene capacitors can likewise be replaced by a single, cheap 2.2 μF ceramic capacitor.

The Q6, Q7 JFET buffer of matched transistors can also be eliminated by used a JFET input dual op-amp such as the TL072CP. The op-amp buffer drives an inverting amplifier to invert and reduce in amplitude the stairstep waveform, the size of which can be adjusted by the amplitude calibration pot. Finally, the output also drives two VCOs (U7) to provide 1 MHz and 10 MHz frequency-modulated outputs. The MC4024 dual VCO (not to be confused with the 4000-series CMOS logic family) is also a “legacy” part that can better be replaced with a 4000-series CMOS VCO such as the 4046 or 74HCT4046 (moved from the 4000 series into 7400 series).

The circuit, as given, was originally designed in August 1973 for use in Tektronix demonstrations of products at trade shows, where an interesting waveform was needed for the scope or spectrum analyzer displays. The stairstep generator is simple enough – a gated current source into a capacitor which is discharged by a transistor – that a μC implementation is nowadays a minimal parts-count solution. Use two-bit output lines, for current-source gating and stairstep reset, and retain the current source and output op-amp(s) for buffering and scaling. Calibrate the stairstep height by using a simple μC ADC to adjust the current source on-time duration, the frequency by setting an interrupt timer with the desired value, and the number of steps with a timer-driven counting loop.

With VCO, the unit could be used as a spectrum analyzer tracking generator, for generating frequency markers. It could also be applied to test scanning transceivers, where the stairsteps are calibrated to be at channel frequencies. Stairsteps are also found in the vertical interval test signal (VITS) of NTSC video waveforms and in curve tracers, for stepping base or gate drive as a parameter for each of the current-voltage curves.

1 comment on “Stairstep Generator

  1. nathandavidson
    October 3, 2018

    It really fascinates me just how a certain percentage of people is able to turn an old junk box full of parts into a complex circuit setup. I guess with growing experience over the years, it doesn't take much to build a new project by adding this and that. I would probably take someone with zero expertise to even build a single circuit. This isn't an industry that you can get away with just look and feel alone.

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