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Simulators >> Circuit Simulators >> PSS PSTB for SC simulation
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Message started by aaron_do on Nov 27^th, 2013, 6:25pm

Title: PSS PSTB for SC simulation
Post by aaron_do on Nov 27^th, 2013, 6:25pm

Hi all,

I'm trying to check the loop gain of my unity-gain SC amplifier. Just like in continuous time circuits, I figured I should not break the loop. So I tried PSS + PSTB. However, the loop gain I get is garbage (less than 0 dB). So my first question is:

Is it correct to use PSS + PSTB to measure loop gain, and if so, how do I setup PSS and PSTB properly?

So instead, I added a 1 V AC source in series with the loop, and checked the gain around the loop. This time my loop gain was 13 dB. The actual gain of the op-amp is around 23 dB, so I know this number is wrong too. It occurred to me that the reason is that my input source is being modulated by the switching action of the circuit, and so it may not be correct to look at the 0th sideband at the output. I guess the 1st sideband is more correct, but it will be attenuated by a sinc function. Does that sound correct? However, I guess there is something fundamentally wrong in the way I am simulating this circuit. Could anybody point me in the right direction?

thanks,
Aaron

For the PSS + PAC method, I added an ideal S/H as per the sc-filters.pdf document on this website. The loop gain looks more correct now...still wondering about the pstb analysis. BTW, I believe the PSS + PAC method is not the right way to accurately determine the loop gain as I am only injecting a voltage. Any thoughts on this are also welcome.

Title: Re: PSS PSTB for SC simulation
Post by Frank Wiedmann on Nov 28^th, 2013, 2:31am

As you correctly noted, pstb analysis examines sideband 0, so it can only be used if you have an unmodulated signal somewhere in the loop (like for example in switched-mode power supplies).

For your pac analysis, I suggest you use sampled pac, so you don't need the additional ideal S/H.

Title: Re: PSS PSTB for SC simulation
Post by aaron_do on Nov 28^th, 2013, 4:33am

Hi Frank,

thanks for the reply. I guess Sampled PAC is a different analysis? I will try it tomorrow.

I think I get your point about PSTB. So in this case, I need to use PAC in order to check my loop gain/phase to check stability. In that case, is there an accurate way to measure loop gain/phase? The method I'm using is to embed an AC voltage source in series in the loop and check the gain across the voltage source terminals. However, this doesn't seem to be too accurate at higher frequencies (there are some discontinuities in my response).

thanks,
Aaron

Title: Re: PSS PSTB for SC simulation
Post by Frank Wiedmann on Nov 28^th, 2013, 5:08am

Sampled pac is one of the "Specialized Analyses" in the pac setup form.

I suggest that you simulate voltage loop gain, which seems to be what you are doing already. This is a good approximation for the loop gain if the conditions given in http://www.omicron-lab.com/fileadmin/assets/customer_examples/Bode_Info_LoopGain_V1_0.pdf are satisfied.