3 phase oscillator

. donderdag 10 februari 2011
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To drive my turbo pump (on it's way! found it on ebay) I need a 3 phase power source.
Started with a simple oscillator, this is the concept:

You have to click the image to see the schematic.
It's just 3 times a phase shift of 60 degrees + inverter (180 deg) gives me 120 degrees.
Simple isn't it?
Now I need to add amplitude controlled gain (two diodes, another fet with a cap on it's gate will control gain of one of the stages).
The gain per stage should be 4 for perfect oscillation (at 60 deg phase shift absolute damping of RC circuit is 2, there is another divide-by-two because I use two capacitors per stage).
Adding frequency control is easy, when I insert a switch in the feedback loop, the whole thing acts as a sample-and-hold circuit, so frequency can be controlled by a PWM signal of about 100kHz (max oscillator frequency is 1.2kHz).
Then I need to add an output driver which takes the 3 signals and converts it to a high current.
I need voltage to current, because a voltage source would drive very high currents to the turbo pump at low frequencies.

Update: I have a turbo pump controler now, so I don't need the 3-phase oscillator. But if anyone wants to use this design as a frequency controllable 3-phase sine oscillator, you can add 3 switches from a CD4066 to the circuit between the drains of the fet's and the capacitor to the next fet. When driving the 4066 switches with a PWM signal (e.g. 100kHz) all the fet stages behave as a sample-and-hold during the off part of the PWM signal. By adding a capacitor across every drain resistor you can filter the 100kHz part of the signal. (if your max 3-phase frequency is 1000Hz, put the -3db point of the drain RC filter on 2kHz and you won't see any 100kHz in the output). I've tried and it works, gives a great 3-phase sine form 10Hz to 1kHz. (I did add some gain control with an extra fet between source and ground for one of the oscillators' fets).

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