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Simulators >> RF Simulators >> PS ripple and harmonic balance simulators
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Message started by Eugene on Sep 20^th, 2005, 4:01pm

Title: PS ripple and harmonic balance simulators
Post by Eugene on Sep 20^th, 2005, 4:01pm

I recently saw a demonstration of a harmonic balance simulator that the salesman claimed simulated phase noise more accurately than SpectreRF. He claimed his simulator accurately captured the effects power supply ripple and SpectreRF did not. I did not buy the sales pitch for the following reasons and was wondering if my concerns were justified.

1. The power supply driving the VCO was a linear regulator supplied by an ideal source. I failed to see where ripple in a linear regulator would physically come from. If the ripple came from the VCO, SpectreRF should accurately capture it. The salesman claimed the LINEAR regulator had ripple independent of the VCO that SpectreRF missed. Did I miss something here? (Assuming the linear regulator was stable.)

2. Since harmonic balance is based on a truncated Fourier series, and a truncated Fourier series could have ripple, might harmonic balance introduce a fictitious ripple on the linear regulator output that would in turn falsely corrupt the phase noise simulation?

On different issue, the salesman claimed SpectreRF's noise floor was higher because the shooting Newton method uses a variable time step. If one strobes the output at the proper rate, doesn't that make SpectreRF's noise floor comparable to noise floor of a harmonic balance simulator?

Title: Re: PS ripple and harmonic balance simulators
Post by Ken Kundert on Sep 21^st, 2005, 11:43am

Jess,
1. I don't see any source for ripple either, though if it exists it should be easily visable in the supply voltage.

2. Harmonic balance results when viewed in the time domain often exhibit substantial ripple due to missing harmonics when the signal is abruptly discontinuous. However, it would not create ripple on an otherwise constant-valued signal.

The salesman was correct when he said the higher noise floor was due to the variable time step. In a sense, using a variable time step trades a lower total error for an error with a spectrum that is broadband. Thus it shows up as a noise floor. If the timesteps were uniform, the error would still exist, but it is somewhat concealed because it falls at the same frequencies where the signal already exists. In otherwords, with uniform time steps, the integration method appears to be LTI (linear, time invarient), but with nonuniform time steps it appars LTV (linear, time varying). An LTI system cannot move energy from one frequency to another, so the error is confined to fall at the same frequency as the signal. However, an LTV system can redistribute energy to other frequencies, and does, creating what appears as a noise floor.

-Ken

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Sep 21^st, 2005, 1:50pm

Thanks for the response Ken. I was hoping you would chime in on this one.

The example was an autonomous circuit and I believe the HB tool requires a special element that gives the oscillator a "kick start". Couldn't the kick start cause the ripple problem or does the discontinuity have to be periodic?

-Eugene

Title: Re: PS ripple and harmonic balance simulators
Post by Ken Kundert on Sep 21^st, 2005, 9:26pm

The "oscprobe" would not cause the ripple. Once the circuit has reached steady-state, the oscprobe has no effect on the circuit, or at least it shouldn't.

The way the oscprobe works conceptually is that it has an input and an output. The input and the output are shorted at all harmonics other than the fundamental. At the fundamental, the output looks like a voltage source and the input looks like a current sink. Initially it produces a sinewave at the output that is injected into the oscillator and the current drawn from the input is made to equal the current being sourced at the output. The amplitude and frequency of that sinewave at the output are adjusted until the amplitude and phase of the voltage at the input and output match exactly. In this way, the oscprobe enforces the Barkhausen condition (loop gain of one, phase shift of 360 degrees). At this point, the voltage and current at the input and output of the probe exactly matches, and the oscprobe is having no effect on the oscillator.

To see the ripple that results from truncating the number of harmonics, the signal must have a high harmonic content. Certainly, discontinuous signals fit this requirement. And of course since harmonic balance is constrained to a (quasi)periodic solution, the discontinuity would be (quasi)periodic.

-Ken

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Sep 22^nd, 2005, 8:31am

Thanks Ken.

Now I am just curious. Getting back to the linear regulator ripple issue, suppose the regulator had an internal instability that produced a small amount of ripple at its output. Assume this poorly designed regulator drove a VCO. In this case, the linear regulator contains an unwanted oscillator. Would either harmonic balance or shooting Newton methods accurately simulate the problem?

For the regulator/VCO circuit, the user might not know to place a second oscprobe (if it is even possible.) But for some circuits, the user would know to place a second oscprobe. Some frequency plans call for two VCOs operating at different frequencies. Would either simulator have a chance at simulating multiple VCOs? If both could do it, would one do it better? Envelope methods would probably work for transients but what about phase noise?

Title: Re: PS ripple and harmonic balance simulators
Post by Ken Kundert on Sep 23^rd, 2005, 10:49pm

I believe only transient would handle this situation correctly. With harmonic balance the parasitic oscillation would be supressed, while with shooting methods it would likely build up during the tstab interval, and then the shooting method would not converge because it could not find a periodic solution.

-Ken

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Sep 26^th, 2005, 7:59am

Thanks Ken. This discussion was very informative.

-Eugene

Title: Re: PS ripple and harmonic balance simulators
Post by vborich on Oct 7^th, 2005, 3:50pm

Hi Eugene,

If the HB simulator you saw supports multi-tone oscillator simulation, the analysis is both valid and rigorous. The first tone is at the unknown oscillator frequency and the other is at the AC supply frequency. The ripple then shows as two sidebands of the VCO frequency at +- (I guess) 60 Hz.

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Oct 8^th, 2005, 10:08am

Hi Vborich,

Thanks for the response but I was not referring to 60 Hz ripple. The ps ripple I was referring to was due to an unstable linear regulator running off a battery. In this case there are two unknown frequencies, the power supply oscillation frequency and the VCO frequency. I doubt any RF simulator to date is ready for this challenge.

-Jess

Title: Re: PS ripple and harmonic balance simulators
Post by sheldon on Oct 8^th, 2005, 6:40pm

Eugene,

If the comment was that SpectreRF does not support
autonomous envelope-following analysis , then the
information is out of date. Autonomous envelope-following
support was added in MMSIM60 USR1. Look in, "Virtuoso
SpectreRF Simulation Option User Guide, Product Version
6.0", Appendix M. There is an example of a 1MHz signal
riding on top of the 3.46GHz carrier, shown by plotting
instantaneous frequency. It seems like this is a useful
capability. BTW, one thing I don't understand about the
original comment, why would you want simulate the
phase noise of an unstable circuit? Wouldn't you want
to find and eliminate the problem before simulating the
phase noise?

Best Regards,

Art Schaldenbrand

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Oct 8^th, 2005, 10:50pm

Art,

It is true that one would eventually want to eliminate the unwanted oscillation and proceed with a stable power supply. However, with ICs, we want to be as sure as we can that we've correctly identified and solved a problem before we tell our management that the next tape out will be OK. Before one can prove that a simulated solution works, one should simulate all observable symptoms of the problem and phase noise might be one of them. Furthermore, many circuits, like superhet transceivers, actually have two VCOs and one might want to simulate the phase noise at the final output. Even with direct conversion, if one wants to simulate the entire link, the transmit and receive VCOs will be independent and often at different frequencies. My point is that there are times when we need to simulate total phase noise from two or more independent sources of oscillation.

Thanks for the update on SpectreRF. I did not know that Cadence released an autonomous version of envelope following. That is indeed a welcomed step in the right direction.

-Eugene

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Oct 9^th, 2005, 5:36am

Before we spend too much energy here I should confess that I've never actually had an occasion to simulate two independent VCOs or a VCO supplied from an unstable power supply. If you look back through this exchange, you'll see that the hypothetical problem arose in comparing HB and shooting methods.

-Eugene

Title: Re: PS ripple and harmonic balance simulators
Post by vborich on Oct 11^th, 2005, 1:19pm

Hi Eugene,

In principle, you can perform the signal analysis of multiple oscillators using multi-tone envelope simulation. ADS/RFDE features multi-tone envelope analysis. (As a disclaimer, I work for Agilent).

You set the tone frequencies as close as possible to the VCO and the parasitic oscillation frequency and it'll likely pick up the correct multi-tone oscillatory regime. If it doesn't, tickler sources may be needed to excite it.

It is also possible to run multi-tone envelope simulation with one unknown frequency, in which case a probe is needed and it virtually guarantees convergence to the correct frequency of the oscillator to which it is attached.

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Oct 11^th, 2005, 1:34pm

vborich,

Thanks for the response. An envelope analysis with two unknown frequencies is definitely an impressive distinction. Is there a way to also get phase noise power spectral densities from envelope simulations without resorting to home-brewed time domain noise generators and post-run Fourier analysis?

Title: Re: PS ripple and harmonic balance simulators
Post by vborich on Oct 12^th, 2005, 9:22am

Hi Eugene,

You can run an HB noise analysis at the end of an envelope simulation. The operating point is taken to be the last envelope time sample. (You can also run a Monte-Carlo envelope noise analysis, where noise sources are represented as noisy AM/PM modulation of the HB carriers. Envelope noise is a Monte Carlo type analysis and so it carries with it many of the same disadvantages.)

I've never attempted a phase noise simulation with two unknown frequencies. For reasons that I can't get into in a public forum, I'd expect it to work under limited circumstances. One instance involves an arbitrary oscillator and another that is very high-Q.

There are no fundamental obstacles to multiple oscillator HB, but there are practical considerations that make it unappealing. Oscillator analysis is inherently less efficient than standard HB and it really shows in large systems where the whole thing becomes one big, usually multi-tone, oscillator. The simulation would get slower with multiple oscillators and the slowdown wouldn't scale in linear proportion to the number of oscillators. In fact, even with a single oscillator, most users choose to simulate it separately and use a behavioral representation to simulate the link.

I am nearly certain, after seeing a white paper describing what you saw in the sales presentation, that you didn't see a two-oscillator noise simulation. Rather, I am quite sure that the "ripple" was represented as an equivalent noise source, peaking at the fundamental frequency of the ripple. This can be done routinely in most HB simulators I am aware of.

Vuk

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Oct 12^th, 2005, 10:17am

Vuk,

Thanks for the information. I have not seen this white paper. I am curious: where did the ripple come from in a linear regulator? And why did the SpectreRF phase noise differ so much?

The salesman claimed SpectreRF "failed to average out the ripple effects". I am still not convinced SpectreRF gave incorrect results. I still wonder if the HB simulator falsely introduced the ripple. Without any lab data, it is hard to say which simulator was right. Again, I did not see the white paper but the entire example seemed contrived and the conclusions struck me as a salesman's wishful thinking. Another problem I had with the results is that it appeared that the HB analysis was performed by an HB expert while the SpectreRF version of the analysis was performed by a SpectreRF novice. The difference in levels of expertise hardly make the comparison fair.

Title: Re: PS ripple and harmonic balance simulators
Post by vborich on Oct 12^th, 2005, 10:38am

Eugene,

I can't think of a context in which the statement "failed to average out the ripple effects" makes sense. I am also curious how the salesman knew the results that another simulator calculated.

Vuk

Title: Re: PS ripple and harmonic balance simulators
Post by Eugene on Oct 12^th, 2005, 11:33am

Vuk,

I believe the saleman's party had a copy of SpectreRF. That's why I don't think the comparision was fair. They are HB experts. Based on their misunderstanding of other SpectreRF features, I can only assume they are also not experts at simulating phase noise with SpectreRF.

As for averaging the ripple, I think he was trying to say that HB assessed phase noise of the VCO over a time long enough to average over the low frequency ripple, while SpectreRF only averaged over one cycle of the VCO. Of course, I may be mis-stating his claim. He was pretty vague at this depth.

Title: Re: PS ripple and harmonic balance simulators
Post by vborich on Oct 12^th, 2005, 11:42am

Eugene,

>>I believe the saleman's party had a copy of >>SpectreRF.

Just what Connections is designed for :).