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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'm designing feedback for an inverter. The feedback is a voltage sine wave. Is there a simple way to make the switch happen an adjustable time before the sine wave crosses zero, without using a PLL?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Hi Jan, Yes you can do what you require without using a PLL. At first it appears that the required circuit must be able to "see into the future" to predict the zero-crossings before they actually happens...
Fortunately you can actually achieve this effect by adding a small amount of the derivative of the sinewave to the sinewave before feeding it to a normal comparator. Because the derivative of a sinewave leads it by 90 degrees you are actually introducing a small phase-lead by adding in a derivative component. This will make the comparator toggle early.
Or alternatively you can add a small amount of the integral of the sinewave to the reference level defining the comparators switching threshold. Since the integral lags by 90 degrees but is effecting the threshold instead of the signal the net effect is the same.
Both of these techniques will achieve the desired result although the actual amount of phase lead or lag may need to be fine-tuned to get the comparator to toggle the desired number of ns early that you want at the chosen operating frequency.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks Richie, that's something along what I was thinking. However I'm not sure how to implement it. The problem is that the feedback frequency is not constant. A simple phase shift could be done with a RC element, but the shift would be heavily dependent on frequency.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Yes, as you said you can achieve a +90 degree phase-lead well below Fc with a first order high-pass filter, but with a dramatic drop in signal amplitude. Or alternatively a -90 degree phase-lag well above Fc for the 1st order low-pass filter.
If you need to process a signal to introduce variable phase-lead or lag WITHOUT altering the amplitude there is also something called the all-pass filter. This will give you an adjustable phase shift without the accompanying low-pass, band-pass or high-pass amplitude characteristic:
This circuit is used in guitar phaser pedals to shift the relative phases of harmonics without messing with the amplitude. Since it doesnt cause an amplitude change, you can cascade allpass filters to get as much phase shift as you want.
I guess it is as good a "see into the future" circuit as you can get with analogue electronics! If anyone knows of a better one, let me know as I have a plan involving such a circuit, a cheap radio, and next week's lottery results.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Going back to this... I think it would be best to make it switch adjustable time before the zero crossing, but this "pre-delay" should not be frequency dependent. Is this possible with a PLL or in any other way?
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Switching before zero cross
