Low Frequency Oscillator

Circuit : Andy Collinson
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A low frequency test oscillator for testing tone controls and experimenting.

low frequency oscillator circuit

Circuit Notes
The circuit is a standard RC phase shift oscillator using a single bipolar transistor as the active element. When power is applied regenerative feedback is applied via C2 from collector to base of the transistor. The timing components, R1, R2, C1 and C3 dictate the oscillation frequency. In use preset RV1 is adjusted so that oscillation just begins. With values shown full amplitude oscillation takes about 4.8 seconds (see diagram below). Frequency Calculation
This oscillator is designed and simulated on LTSpice IV. Once simulated click the "probe" cursor on the output wire "Vo", the above waveform is produced. To calculate the frequency, place your mouse on the graph where oscillations have reached full amplitude and draw a rectangle, ensuring maximum and minimum amplitude is enclosed within the rectangle. The diagram below shows such a zoomed portion of the output waveform, starting around 5.6 seconds into the simulation. To add cursors left click on the name of the output waveform, this is called "V(vo)". A single cursor is added to the graph which can be moved with the mouse or keyboard arrows. Now right click the mouse on the Vvo waveform. In the window that appears click on the attached cursor menu and change to "1st & 2nd". Now two cursors will be visible and controllable. Make sure both cursors pass through the zero volt horizontal meridian and measure one output cycle. A sub window allows you to read the value as shown above, for this oscillator the time for one cycle is 127.2ms and frequency a little under 8Hz.

Downloadable Circuit
The LF oscillator may be downloaded here. Please note that you will also have to include a modified list of components to simulate the BC549B, see the LTspice Section for more details.

Output Attenuation
If desired, the output across RL can be attenuated. Simply replace RL with two resistors R1 and R2 according to the table below:
Output Attenuator Values
R1 R2 Output
47k 5.6k* 98 mV
47k 510 9.8 mV
47k 51 99.8 uV
100k 1k 9.5 mV
100k 100 950 uV
100k 10 95 uV
1 Meg 1k 990 uV
1 Meg 100 100 uV
1 Meg 10 10 uV
10 Meg 1k 100 uV
10 Meg 100 10 uV
10 Meg 10 1 uV

Using R2 values higher than 1k or so may cause inaccuracy due to the attenuator loading. Attenuator values can be calculated by the formula:

Vout = Vin * R2
4700 + R1 + R2
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