A test signal generator is invaluable when making or repairing electronic equipment. Although a fully-fledged, accurate (and expensive!) sig gen is desirable, there are many occasions when a simple, non-calibrated, highly portable signal source will suffice for a quick test such as signal tracing. To that end I've designed this circuit which selectively generates square, triangle and pseudo-sine waves in three frequency ranges covering 15Hz to 2MHz.
The core of the generator is a Schmitt-trigger oscillator U1 whose frequency is determined by the setting of a 100k pot Rt and one of three switch-selectable capacitors Ct1-Ct3. These passive components should have short connecting wires to minimise spurious signal pick-up. Unused Schmitt gate inputs in the CD4093 or CD40106 IC should be grounded.
The U1 gate output is a square wave, and the voltage on the Ct capacitor is a triangular wave. The triangular wave is shaped by diodes D1, D2 into a reasonable approximation of a sine wave. Any one of these waves is switch-selectable.
The selected wave may be used as is, to feed a high-impedance load, but here it is passed to an optional buffer amplifier with a gain of 3. The amplifier comprises an LT1007 op-amp. Other op-amps would be suitable provided they have an adequate gain-bandwidth product. Capacitors C3 and C4 aid amplifier stability. C4 should be wired close to the op-amp supply pins. Resistors R2 and R3 provide a synthesised ground reference (half the supply voltage) for the op-amp. R4 and R5 set the op-amp gain. The op-amp output is AC-coupled to a 10k pot for output-level control.
The op-amp frequency response is practically flat up to ~ 20kHz and then tails off, but the output directly from U1 stays flattish up to ~ 200kHz. There is still a very usable output up to 2MHz.
The sig gen can be run from a PP3 battery and housed in a small box. Current consumption is about 8mA.
Footnote: Feedback welcome on circuit performance and suggested improvements.