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Old 10th November 2006, 07:52 PM   #1
xuhaoz is offline xuhaoz  Sweden
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Default Question about the feedback

Hi, all

In phase shift modulation method, there are two types of feedback, one of them is that the feedback is added before the output filter, the other is that the feedback is added after the output filter.
Does anybody know what's the difference and result or advantage or disadvantage between two method ?

Thank you

Jim
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Old 11th November 2006, 12:35 AM   #2
Gorilla is offline Gorilla  United Kingdom
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Post filter feedback accounts for the non-ideal properties of the output filter/inductor.

But notably the output filter causes an almost 180 degree phase shift of high frequency components (the modulation carrier), which is why post-filter feedback is difficult to implement without parasitic oscillation. Unless of course, you make a UcD.

With pre-filter feedback, your poles can be placed wherever you like, of course avoiding the above issue, but at the cost of greater THD. There has been skepticism of manufactures reporting very good THD specs for their (pre-filter feedback) chips, but not specifying the make of their output inductor.
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Old 12th November 2006, 09:58 AM   #3
xuhaoz is offline xuhaoz  Sweden
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Hi, Gorilla,

I am confused about your explaination. In self-oscillating method, we don't need a carrier, the circuit will oscillate itself.

.
Quote:
But notably the output filter causes an almost 180 degree phase shift of high frequency components (the modulation carrier), which is why post-filter feedback is difficult to implement without parasitic oscillation
Does it mean if we add feedback after the output filter, there will be one more pole in our loop gain, if the other feedback structure(before output filter) has been designed with two poles, and phase shift 180 degree, for the new feedback structure(after ouput filter) , it is impossible to have phase shift 180 degree with three poles? (Sorry for the poor english)

Jim
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