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Design >> RF Design >> Flexible Balance VS Shooting Method
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Message started by chase.ng on Jul 22^nd, 2006, 9:17pm

Title: Flexible Balance VS Shooting Method
Post by chase.ng on Jul 22^nd, 2006, 9:17pm

Hi all,

I have behavioral model of a transmitter system that I ran QPSS and Envelope simulation on it using Flexi and Shooting separately. However I found that the result is totally different. The number I get by using Flexi is much closer to my calculation. I was always told that the shooting method should give a more reliable result. It is true? Which result should I use as a reference? There are some signals in my circuit will tend to get very small, in the order of nV, so I believe the 3rd order harmonic will be even smaller. I wonder will arithmetic error in calculation cause the discrepency in the result. Thanks.

chase

Title: Re: Flexible Balance VS Shooting Method
Post by Andrew Beckett on Jul 24^th, 2006, 1:26pm

If the circuit is only weakly non-linear, you will tend to get better accuracy with flexible balance compared with shooting (at least, more cheaply) - if you're trying to resolve signals such as 3rd order intermodulation products which will be a long way down when you have small signal inputs. The numerical noise floor when representing a harmonic response with a small number of terms will be lower than when representing the same set of harmonics with a sequence of low-order polynomials.

If the circuit is strongly non-linear (e.g. switching circuits, dividers, circuits driven with square waves, circuits driven into compression), then shooting will tend to be more accurate as you would need hundreds of harmonics to represent the solution with flexible balance - it's much easier to represent something closer to a square wave with a set of low order polynomials.

The above are a bit of a generalisation, but maybe enough to answer your query.

Regards,

Andrew.

Title: Re: Flexible Balance VS Shooting Method
Post by chase.ng on Jul 24^th, 2006, 5:21pm

Thanks Andrew. Do you think that a transient followed by DFT will be able to provide more accurate result?

chase

Title: Re: Flexible Balance VS Shooting Method
Post by Andrew Beckett on Jul 24^th, 2006, 10:50pm

No. The transient solution is still a sequence of low-order polynomials trying to represent a sum of sinusoids, so it will have similar accuracy limits to shooting (not surprising, since shooting is transient-based).

Andrew.