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