Simple Function Signal Generator


I needed simple and inexpensive signal generator, that can fit in the casing of an old multimeter, and does not use hard to get chips.
This project utilizes the connection between common base and common emitter circuits. The transistor Q1 is connected as a common base amplifier and has a low input impedance and high output one. Q2 is emitter follower with high input and a low output impedance. Both circuits have a 0 deg phase shift, so no Miller effect is present. All these, make the pair easy to couple and with high frequency range.
Capacitor C1 is the positive feedback, which makes the circuit an oscillator.
The same idea is used for an oscillator at 100MHz High Speed Multivibrator and above, using high speed transistors.
You can get a change of the frequency almost 20 times with a single capacitor and a potentiometer. The circuit can either be an VCO (Voltage Controlled Oscillator), or one, controlled by current.
The generator outputs two types of waveforms: a sawtooth and a square wave signal with 50% duty cycle.
You can easy upgrade the functions of the device by adding an integrator circuit after the square output. This way you get a triangle waveform. Adding second integrator after the first one, gets you a sine wave on its output.
From the sawtooth signal you can derive square wave with adjustable duty cycle, using a Shmitt trigger.
The generator also incorporates a high impedance, wide band attenuator at its output. Using a simple potentiometer to adjust the output amplitude will distort the waveform for the highest frequencies.


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These are different waveforms from the ready device.

waveform sawtooth 2Hz waveform sawtooth 4Hz waveform square 4kHz waveform sawtooth 8kHz waveform square 80kHz waveform sawtooth 160kHz waveform square 1MHz waveform sawtooth 2MHz