Great Voltage Stable Oscillator

Question

i'm want to produce an AC signal an +-3.3V (or 3.3v-0) voltage supply with a small frequency around 1kHz-2KHz and a low output voltage too(200mV,for example), where the frequency stability is not important but the wave amplitude stability is.

I'm having some doubts about deciding which solution can suit better my needs. I was looking for a stability within the mV magnitude.

My first focus was on the Wien Bridge Oscillator, using diodes as Amplitude Gain Control conductors[fig2], but i'm afraid its forward voltage drift with temperature and current could compromise significantly the output signal. I also found Wien Bridges Oscillators using a FET[fig1]. And lastly i looked at Phase Shifts Oscillators which looked as good option as only element that has a significant drift is the Amplifier and i could minimize it using a low drift OpAmp.

Can someone please help me choosing the solution which has a higher voltage stability or advice me if there is better solutions with similar complexity.

Thank you in advance.

Answer 1

At such a low frequency (2 kHz), you can generate the sine wave by streaming values into a D/A. 256 samples per cycle only requires 512 kHz sample rate, which is quite easily doable.

The harmonic content will be so low in amplitude and high in frequency that it can be filtered out with little effect on the intended signal, and that effect can be calculated up front. For example, two poles of R-C low pass filtering after the D/A at maybe 10 kHz should yield quite low harmonic content.

The amplitude can also be easily adjusted with the right kind of D/A. It will produce a voltage proportional to something you give it. To adjust the amplitude, you only need to adjust the reference voltage being fed to the D/A. The same micro that is feeding the D/A can produce a PWM signal in hardware that you low pass filter to create the D/A reference signal.

Alternatively, you can use a high resolution D/A. Arrange the full amplitude to be a bit more than the maximum you ever want, then multiply the samples by some number a bit below 1 to digitally scale down the amplitude of the resulting waveform. This scale factor would be automatically adjusted by frequency to account for the slight frequency-dependent attenuation of the fundamental by the output filter.

This method will produce a highly stable output waveform. The shape is a function of the digital samples, and the amplitude a function of the A/D reference voltage.

Answer 2

Sine wave oscillators need amplitude stabilization and you have focused on two different types but you haven't focused on the root of the problem and that is the accurate measurement of amplitude.

If you really need "great" amplitude stability then use an RMS to DC converter to measure the sine wave amplitude with a much higher precision. Then feed the RMS/DC output to an op-amp circuit to produce an error voltage. The error voltage is the actual RMS amplitude minus the demanded amplitude (set via a simple pot fed from a stable voltage reference).

The error now tells you how far the RMS value is from the set point.

Next, integrate the error so that long term errors (in one direction) produce a large correcting voltage output.

The correcting voltage output then feeds the JFET in your first diagram.

This will give you accurate stability and a somewhat cleaner sine wave shape.

Answer 3

You don't mention whether you're looking for ~1% or ~0.01% or whatever stability so there is some guesswork here.

You can replace the diode with a precision full wave rectifier circuit and heavily low-pass filter it, which will improve the stability. You might have to add some more gain in the feedback loop.

There's an inherent trade-off between distortion and amplitude stability and the amount of time it takes the oscillator to stabilize.

Another option is to produce the sine waves by filtering a DAC output, either from a microcontroller or by using a dedicated DDS (Direct Digital Synthesis) chip such as those from Analog Devices. Then your amplitude stability will be primarily determined by how good the DAC (and the reference for the DAC) is. That kind of circuit will happily operate from 0/3.3V (+/-1.65v) where JFETs may not be that happy.

Answer 4

I have had that need too; an amplitude stable sine wave; I did the DAC generation; I used the DAC (12b) from a MCU (STM32F103) whose power supply was a stable precision 0.24% reference of 2.5V ( IC supplied from a 3.7 battery); The stability of the generated sine wave by the DAC was loose; +/- 30mV for a full swing 0...2.5V sine wave; I needed maximum 0.5mV. Although the DAC was outputting very stable singe constant voltages; e.g. 1.5V DC ( nonsinusoidal , but constant voltage) had a far better amplitude stability than the 1.5V sine wave.