Enhancing Function Generators with XR2206
Function generator built using the XR2206 IC have consistently offered an outstanding balance between cost and performance. Despite the IC’s ‘obsolescent’ status, it remains accessible. If your generator lacks built-in sweep, often referred to as ‘wobbulator’ capability, a straightforward external circuit is all that’s required. This circuit can be seamlessly integrated in place of the frequency adjustment potentiometer, in case your generator lacks a pre-existing sweep input.
The circuit itself is a classic sawtooth oscillator, primarily reliant on a unijunction transistor (UJT). The UJT switches when its base voltage hits the trigger threshold, ensuring rapid discharge of the capacitor linked to the base. Achieving a linear charging profile, and consequently a linear sawtooth ramp, is accomplished by charging the capacitor with a constant current source built around a BC557. The output signal is then buffered by a FET (BF256C) to minimize the load on the oscillator.
Connecting XR2206 to BC547 Output Stage
In this setup, the BC547 forms the output stage that interfaces with the XR2206 function generator. Trimpot P1 plays a key role in adjusting the sweep amplitude. During the alignment process, it’s advisable to enhance signal visibility on an oscilloscope by removing jumper JP1 near capacitor C2. This temporary removal increases the sawtooth frequency, aiding alignment. After completion, reattach the jumper. When the 100 µF electrolytic capacitors (C2) are back in the circuit, the sweep frequency significantly decreases.
Customizing Sweep Rate and Component Substitutes
If necessary, altering the sweep rate is feasible by employing capacitors of different values. Most of the required components are likely available in your parts collection. However, if not, the UJT remains accessible, obtainable inexpensively from RS Components. Additionally, the FET can be substituted with various small-signal N-channel JFETs. For the UJT, alternatives such as 2N2646 or 2N2647 can also be utilized. If you intend to assemble an XR2206 function generator, consider referencing  for detailed instructions on constructing a proven Elektor circuit, ensuring a successful build.