Built this about two weeks ago. Took a while to get the right values I needed for the opamps. It generates a perfect sine or square wave from 1Hz to 800kHz. It's possible to get it up to 1MHz but with a bit of voltage loss on the output. However anything between 1Hz to 800kHz gives a completely voltage stable output and a perfect looking sine wave. Square wave has a bit of slope when going down to a few Hz at some output draw. Either way it makes a nice signal generator for audio testing, transmitter modulation testing, and just great for the bench.
Schematic as built:
Front:
Inside:
Closeup:
Good luck finding the antique selector switches I have lol. They came from junk parts out of an old Gatesway radio station console. You can substitute the opamps for something more common but at a loss of high frequency range. The better OP audio chips seem to have a much broader bandwidth. If anyone is curious to build yourself one I am open for questions. Having a signal generator is a must have for the bench. This little bugger worked out way better than I expected.
Next project is a quality microphone audio compressor. That's my current project and oddly enough this signal generator is helping a lot for testing the compressor circuit I am currently building.
Using square waves you can really test for high and low frequency response of amplifiers. Also for distortion artifacts in amps and modulators. Sine waves are great for general audio testing.
Quick description of the circuit in case anyone is interested..
The power supply is very simple. Puts out 20 volts DC after the transformer and diode bridge. Using two 1kOhm dropping resistors it gets its -10 and +10 volts DC for the IC chips and the transistor T1 buffer output transistor. The ground of the whole circuit floats across the two 1k resistors. All grounding is relative to the floating ground output of the power supply.
FET transistor T2 is for negative feedback leveling. Without it the sine oscillator IC1 and first half IC2 would produce distortion because of building positive feedback. T2 reduces as much feedback voltage it detects as the free running oscillator creates thus sustaining a clean sine wave.
IC2 2/2 converts sine to square. The power selector switch simply selects either center OFF position, or takes output from the OP square output or the OP sine output to the AF control and then to RCA output jack. The second section of the switch simply controls actual power, either on/off/on to circuitry.
T1 is a buffer transistor. It doesn't actually amplify anything, but it protects the output of the OP amplifiers so that current can be drawn w/o distortion or output failure.
The complex capacitor selection section can be simplified using only two identical capacitors but will limit frequencies able to be tuned to relative to the value in uF you choose.
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