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DIY Frequency Generator

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DIY Frequency Generator

Postby maz » March 14th, 2014, 8:33 pm

My DIY Frequency Generator that I built some times ago.

A very cheap and useful project, powered on regulated 12Vdc, 1/4 and BNC output jacks, the big black knob at the top center is a 3 way selector to change frequency ranges (20Hz to 100Hz / 100Hz to 1Khz / 1Khz to 10Khz), toggle switches from left to right are: amplitude boost / square / sine & triangle, and the knobs at the bottom are: amplitude, coarse and fine adjust.

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Even if it's for troubleshooting DIY pedals, amplifier circuits or for tweaking your breadboarded projects, with this Frequency Generator it's easy to inject a sine wave, at an amplitude from a few mV up to 935mV.

The signal can be traced by using an oscilloscope, or it can be heard by using a DIY audio probe.

Some screenshot with my old Leader LBO-514a.

Sine Wave

Triangle Wave

Square Wave

DIY Frequency Generator - Schematic and Layout



:pdf: Download the complete pdf project file for direct PnP transfer and layout.

:txt: Download Bill of Materials in txt format.

Note that only one throw is used on the 3P4T rotary switch :



Note about the frequency offset

Ratio of both 5,1K resistors controls the frequency offset so it is recommended to use two 1% tolerance resistors. An external rotary B10K potentiometer can also be used instead of those resistors to control the offset.

Note about the calibration

The function generator can be calibrated by using an oscilloscope. If you don't have access to such a tool, do not install the B25K Symmetry Adjust trimpot, and trim the Sine Adjust trimpot to 200 Ohm before power up the circuit. With calibration sine THD is 0.5%, and 2.5% without.

Calibration procedure :

1. Set the Sine/Triangle SPST switch for sine wave output. (closed)
2. Set the Wave Selector SPDT switch for sine wave output.
3. Set the Frequency Range 3P4T rotary switch at position 2. (100nF)
4. Set the Amplitude potentiometer to the minimum.
5. Connect 12V regulated power supply and oscilloscope probe.
6. Power up the circuit.
7. Set the amplitude potentiometer to the maximum.
8. Set the frequency to 1KHz by using the coarse and fine potentiometers.
9. Trim B25K to the midpoint and adjust B500R for minimum distortion.
10. With B500R set as above, adjust B25K again to reduce distortion.
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