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A question about PAC plot setting in Direct Plot Form (Read 754 times)
Bisharp
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A question about PAC plot setting in Direct Plot Form
Sep 25th, 2023, 3:59am
 
what is the difference between "spectrum" sweep and "sideband" sweep in the PAC Direct Plot Form?
I thought that the only difference is that "spectrum" plots mag. in whole spectrum. "sideband" plots each frequency band.
but it makes me confused that the absolute values in "spectrum" result and "sideband" result are not same.
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Ken Kundert
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Re: A question about PAC plot setting in Direct Plot Form
Reply #1 - Sep 25th, 2023, 5:07pm
 
First of all, this is not a design question, and so should not be posted in the design section of this forum.  I will move it to the RF simulation section, where it belongs.

Second, ADE tends to needlessly use a lot of confusing terminology with RF simulations.  For example, on the PSS analysis form it requests the Beat Frequency, which naturally assumes the presence of multiple fundamental frequencies.  But PSS analysis only works if you have a single fundamental frequency.  When it requests the beat frequency, it really is requesting the fundamental frequency.  Rarely, people apply PSS analysis to circuits that contain drivers at more than one frequency.  In this case the drive frequencies must be co-periodic (they must be integer multiples of a single fundamental frequency).  When they do, the fundamental frequency also happens to be the beat frequency.  But what makes this needlessly confusing is that while the fundamental frequency is defined and well understood in both situations (with a single drive frequency and with multiple drive frequencies), the concept of a beat frequency makes no sense in the more common case of a circuit that has only a single drive frequency.

Now, concerning your question. The term spectrum in the direct plot form means "all sidebands", where as sideband means "an individual sideband".  What makes this needlessly confusing is that the collection of all sidebands does not make up a spectrum.  Something else that contributes to the confusion is that spectrum is the default choice, but is almost never desired.  When used it tends to put up a whole slew of transfer functions that most people do not really want or understand.

Imagine that you want to measure the frequency response of a high-side down-conversion fundamental mixer.  That implies that the -1 sideband is of interest.  So you would select sideband and then specify -1.  If you instead selected spectrum and specified maxsidebands=7 you would get the transfer functions from the input to the output at each of the first 7 positive and negative harmonics.  Nobody wants that because all sidebands except the -1 sideband are all removed by the filter that always follows the mixer.

Having said that you are given the same choice on the PXF direct plot form, and there it is useful.  For example, imagine analyzing a clocked circuit such as a sample & hold or a switched capacitor filter.  Here you sweep the PXF analysis over the normal output range of the circuit, from DC to the Nyquist frequency.  Now observe the transfer function from Vdd to the output and select spectrum. In this case each sideband displayed is useful.  For example, the -3 sideband gives the transfer function for signals on Vdd near the third harmonic of the clock to the output at baseband.  Very useful.  On PXF analysis it makes sense that spectrum is the default, where it doesn't for PAC analysis.

Bottom line:
- when you see beat frequency on the PSS form, thing 'fundamental frequency',
- when you see spectrum on the PAC or PXF direct plot forms, think 'all sidebands'
- when you see sideband on the PAC or PXF direct plot forms, think 'individual sideband'
- when you use PAC, you should be careful to pick sideband rather than spectrum, then specify the sideband that makes sense in your situation.
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Bisharp
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Re: A question about PAC plot setting in Direct Plot Form
Reply #2 - Sep 25th, 2023, 6:23pm
 
Thank you for your teaching. I recently started trying to use SpectreRF simulation.

I may not have clearly expressed the points of confusion I had. The issue I encountered was that when I used PAC in Specialized Analysis - Sampled to simulate.

When I plot the result, I try to use two different settings that I mentioned. 1. Sweep - spectrum - select eventtime. 2. Sweep - sideband - choose specific sideband

I found that the magintude value difference between these two settings is about 97 times on the same frequency. I don't know why. Later on, I used Sweep - spectrum - time averaged, the magnitude values are same with 2.

The beat frequency I set in PSS is 100MHz. I don't know what operation caused this approximately 97x difference in the "time average"
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smlogan
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Re: A question about PAC plot setting in Direct Plot Form
Reply #3 - Oct 13th, 2023, 1:42pm
 
Dear Bisharp,
Quote:
The issue I encountered was that when I used PAC in Specialized Analysis - Sampled to simulate.

When I plot the result, I try to use two different settings that I mentioned. 1. Sweep - spectrum - select eventtime. 2. Sweep - sideband - choose specific sideband

I found that the magintude value difference between these two settings is about 97 times on the same frequency. I don't know why. Later on, I used Sweep - spectrum - time averaged, the magnitude values are same with 2.


I do not know the specifics of your circuit nor the simulation settings, but a PAC Specialized sampled analysis is used to compute the instantaneous conversion gain (for example) at one or more specific times in the corresponding PSS waveform - or at a specific voltage threshold crossing in the PSS waveform. Without the sampled option, the conversion gain represents the gain computed using all of the PSS timepoints (or "average conversion gain"). Hence, I would expect there to be a significant difference. If you were to choose the sampled option and record its value for all the timepoints of the PSS waveform and then take the time average of the gains, it should be close to the "average conversion gain".

Secondly, the instantaneous conversion gain defined at a specific timepoint does not represent the gain due to a single mixing product - which is what you are selecting and plotting when the "sideband" radio button is chosen in the Direct Plot form.

I hope I understood your question correctly and this is somewhat helpful to you Bisharp!

Shawn
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Bisharp
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Re: A question about PAC plot setting in Direct Plot Form
Reply #4 - Oct 18th, 2023, 6:56pm
 
smlogan wrote on Oct 13th, 2023, 1:42pm:
If you were to choose the sampled option and record its value for all the timepoints of the PSS waveform and then take the time average of the gains, it should be close to the "average conversion gain".


Dear smlogan,

Thank you very much for your answer. But I still have a little confusion regarding the quote.

If I chosen the sampled option and there was only one point within the PSS period that meets the sampled threshold condition, will there still be a difference in the result of the "time average"?

Bisharp
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