Various diagram

This circuit of a voltage and frequency calibrator provides simultaneous voltage and frequency calibrations by generation of a precision squarewave.
The 555 timer IC is used in a slightly unusual configuration, having the advantage that an exact 50:50 mark/space ratio may be attained by trimming R1. The frequency of oscillation may be set between l0 kHz and 1 kHz by switching timing capacitors C14. C5 decouples the internal ref- erence potential-divider of the 555 from supply·transients. The squarewave output from pin 3 of the IC, while stable in frequency, is not stable in peak-to-peak voltage as this depends on the supply voltage. This is used to switch on and off a temperature compensated constant- current source O1. R2 ensures that the current·source turns off completely when pin 3 goes high. The current- source output, trimmed by R3 to be exactly 1 mA, drives a resistor ladder network so that a series of precise squarewave voltages are generated. The advantage of current drive rather  than voltage drive for this sort of net- work is that calibration is much easier. A simple ladder network is shown by way of example, and more complex ones may simply be constructed to give a wider variety of output voltages.  The non-standard component values used were obtained by paralleling standard values. For the timing capacitors several in parallel had to be used, and only the resultant value is shown on the diagram.