Function generators built around the XR2206 have always had an excellent price/performance ratio, and the IC although ‘obsolescent’ is still available. If your generator does not have built-in sweep (‘wobbulator’) capability, a small external circuit is all you need. You can fit the circuit in place of the frequency adjustment potentiometer if you don’t already have a sweep input. The circuit is a classic sawtooth oscillator based on a unijunction transistor (UJT), which switches when its base voltage reaches the trigger level. This allows the capacitor connected to the base to discharge rapidly. To obtain a linear charging characteristic and thus a linear sawtooth ramp, the capacitor is charged by a constant current source built around a BC557. The output signal is buffered by a FET (BF256C) to minimize the load on the oscillator.
The output stage built around the BC547 provides the interface to the XR2206 function generator, with trimpot P1 serving to adjust the sweep amplitude. To make it easier to see the signal on an oscilloscope during alignment, it’s a good idea to remove jumper JP1 near capacitor C2 in order to increase the sawtooth frequency. Fit the jumper again after completing the alignment. With the 100 µF electrolytic capacitors (C2) back in the circuit, the sweep frequency will be significantly lower.
If necessary, you can use a different value to obtain a different sweep rate. You can probably find all the necessary components in your parts bin, but if not, the UJT is still available (for little money from RS Components), and the FET can be replaced by most other small-signal N-channel JFETs. For the UJT, you can also use a 2N2646 or a 2N2647. If you also want to put together an XR2206 function generator, check reference [1] for free instructions for assembling a tried and tested Elektor circuit.
