Phase Help - Page 1
Forum section: electronics / beginners

author: CpHarding
author_rank: Contributor
timestamp: March 03, 2012, 09:39:14 pm
content: Hi guys, I'm wondering if there is a device ( Either IC, or circuit) that can take a medium frequency (10KHz - 200KHz) signal, on a single phase, and give a three phase output. I already have a three phase amplifier made up, so even if its just at signal levels that will be suitable.I have an MBed which I know is capable of doing this ( by generating three phases) but I was having trouble finding a way to get it to work at the required frequencies. ( without going into custom libraries, I can only do microsecond delays, and three phases requires 6x the frequency of desired signal. ) I've tried with an external crystal oscillator, But the code isn't working as expected, probably due to timing issues with interrupts and such.Ideally I'd prefer to use the hardware version if I can. But if its not easily achievable I shall have to delve back into my weakness of micro controller programmingThanksChris

author: IanB
author_rank: Super Contributor
timestamp: March 03, 2012, 10:00:09 pm
content: Do you want three phases on three separate outputs?Basically the only difference between phases is the phase angle. You didn't say so, but presumably you want the three phases to be equally distributed around thirds of a circle (120 degrees apart)?So if your first phase is at 0 degrees, you need a circuit that provides a 120 degree lag, and then a repetition of that circuit to produce a 240 degree lag. Unfortunately a lag circuit will tend to attenuate the signal so you may need some gain control to re-amplify the signals all back to the same level.This is just the logical perspective from a non-electronics person. I can't give you practical circuit ideas, but the theory seems straightforward.

author: CpHarding
author_rank: Contributor
timestamp: March 03, 2012, 10:05:55 pm
content: Quote from: IanB on March 03, 2012, 10:00:09 pmYes, Ideally I need three independent outputs, each 120 degrees out of phase all taking an initial signal off a single frequency input.

author: Rerouter
author_rank: Super Contributor
timestamp: March 03, 2012, 10:11:20 pm
content: http://www.aleph.co.jp/~takeda/radio/phaser/indexE.htmlthen you would only need to deal with the phase offset of the actual op amp, however if you buffered your original signal through a third, technically they should all offset at the same rate, thus maintain a 120 degree shift,

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2012, 07:41:14 pm
content: How do you intend to keep the same phase shift over the entire frequency range?If you just want a variable three phase 10kHz to 200kHz digital synthesis is probably the best way to go. You could try a phase shift oscillator but it would require a triple ganged potentiometer or variable capacitor which is likely to be expensive. You could try adding varicap diodes to make a phase shift VCO but a 1:20 range is pushing it and there's bound to be plenty of distortion on the output.

author: CpHarding
author_rank: Contributor
timestamp: March 04, 2012, 09:34:56 pm
content: Quote from: Hero999 on March 04, 2012, 07:41:14 pmThis was the problem I was having with the analogue phase shift idea's. I was hoping there was a solution.Oh well, back to trying to get my coding to work!Thanks Guys, might end up asking for help with the code in a bit depending on how that goes!

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2012, 11:34:39 pm
content: I don't think you can do it without some digital processing.Think about it. All frequency components need to be delayed by the same amount which is dependant on the fundamental frequency. The signal processor would have to record one cycle to determine how long to wait before sending the signal to the other outputs.

author: Rerouter
author_rank: Super Contributor
timestamp: March 05, 2012, 01:30:13 am
content: not to sound pushy, but what about my idea was in-practical / wrong,

author: CpHarding
author_rank: Contributor
timestamp: March 05, 2012, 10:18:46 am
content: Quote from: Rerouter on March 05, 2012, 01:30:13 amNothing in essence, Because I need a large frequency range, I would get different phase shifts at different frequencies using the link you provided. For some reason OrCad couldn't simulate it either.