Originally posted by beenthere@Feb 26 2005, 05:12 PM
You are correct that the overall phase shift will change as the capacitance changes in a twin-T oscillator. However, as the frequency out of the oscillator must track the frequency change, it will not reflect any of the phase shift. You would need a reference frequency to track such a change, unless the change was of short duration and occurred very rapidly. You simply have to have a reference point to measure phase shift.
I agree that the phase shift due to the capacitor change will null out once the frequency stabilizes.
The problem with my oscillator is that the frequency of the oscillator does not stabilise. If the oscillator is supposed to oscillate at 120 KHz. The frequency of the output signal is in the range 119.4 - 120.5 KHz for some reason unknown.
Let me explain to you in detail what I actually want. It'll be a little lengthy but I would appreciate if you could read it and give me ur inputs.
I am building a capacitive sensor system, where the capacitance varies as a result of certain parameters. Now, these sensors form the capacitors of a Twin T oscillator ckt. and are hence responsible for the o/p signal frequency.
If the initial capacitance is say 20 nF in the system. Due to the parameter variations the capacitance varies by say 0.1 nF. Now, that should cause a variation in the frequency of oscillations. However, the frequency does not stabilize at a particular value. It keeps oscillating at about +/- 1 KHz. Now, the change in capacitance should cause a frequency variation of < 1 KHz. So effectively, even when my capacitance changes I'm not sure if it is due to the initial drift or due to the capacitance change. I need to stabilize this frequency of the oscillator. If you have suggestions on how to do this, please help me out.
As, an alternative I was thinking probably, measuring phase change in the output signal might be a better option. I wud also appreciate if you think it is relevant to think of measuring the phase change. If yes, how ?
waiting for ur inputs.
Originally posted by beenthere@Mar 2 2005, 09:08 PM
I'm not able to answer your last question for certain. As far as I know, the phase shift causing the oscillation is constant. Adjusting the value of a capacitor will cause a transient phase shift, but as soon as the new frequency stabilizes, the phase shift will be effectively nulled out. Tracking the frequency change with a pll will give you an indication of the phase shift - if you can get at the error voltage signal. You would need to be able to record this transient, as the error will go to zero as the oscillator steadies on the new frequency. Changing capacitance as a step function rather than as an adjustment will make the brief error voltage easier to see.
I'm mostly guessing here. I've used twin-T's as filters and oscillators, but have never been concerned about actual phase shift. I was just happy if I got my result. Technicians are like that.
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by Jake Hertz
by Jake Hertz