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Phase noise of frequency multiplier

MTP3

Hi!

I am trying to simulate a frequency multiplier. The oscillators are simple ring oscillators oscillating at 2GHz made form inverters.What the multiplier circuit is that it cancels the fundamental tone and retains/preservs the 3rd harmonic(3,6,9...) and cancels the other harmoics like 1,2,4,5.......  

The problem;

I am interested in looking at the Phase noise at the combiner output oscillating at 6GHz. The output signal has a voltage swing of around 900mV. The setup does not have any ideal blocks in it. In the PSS options form I donot check the oscillator option (I read a post suggesting this in case of a driven circuit, which I assume this multiplier to be)

When I run pss using shooting method it does not converge. I have tried changing the tstab (increasing it), changing the method from trap to gear2 etc. But I can't get the pss to converge.

When I run PSS using the HB method the pss does converge but the Phase noise at the comes out to +150dbc/Hz which is a unrealistically large value  (opposed to the theroratical results).

Running a pss+pnoise for a single ring oscillator works fine. I would be grateful if someone could guide me what could be the problem or if this is the right way of simulating this kind of circuit,

Regards,

MTP 

I cannot post the image so here is a link to the circuit diagram

www.dropbox.com/.../FrequencyMultiplier.jpg

  •  Hi MTB,

     

    A couple of thoughts came to mind - perhaps you have already explored these.

    1. Are all three ring oscillators operating at exactly 2 GHz? The concern is that if they are not, one of the intermodulation products may have a very low relative frequency (i.e., for example 2 GHz/1.999 GHz => 0.001 GHz) which could make pss think the circuit has not settled.

    2. Are you using an errpreset of "conservative" or setting the maximum integration timestep to a low value consistent with the expected maximum frequencies of interest? Since your circuit appears to be very sensitive to the creation of intermodulation products - whose frequencies will be higher and less than your fundamentalof 2 GHz - I was thinking you may need to force a maximum integration timestep to get the proper waveform fidelity.

     3. Have you examined the converging HB waveforms and do they look at all realistic?

    Shawn

     

     

     

  • Many thanks for the reply Shawn. So here goes.

    1)I have verified that the ROs are all oscillating at the same frequency (although there is some variation in their oscillation frequency due to device noise etc) but generally all of them oscillate in the same frequency range with a relatively small offset.

    2) As recommended in the spectre RF manuals and other resources I have run the simulation both at conservative errpreset and varying the tolerances etc. I have not used the max. integration step. Just read vaguely that maxstep < 1/(200*OscFreq) but I will look more in to how to set the value of this parameter but it would be great if you could suggest a good starting point.

    3) I have not looked at this but let me get back to you on this.
     

  •  Hi MTP3,

     

    Thank you for your detailed update!

    [1] - Great - this a good thing!

    [2] - I've never needed to use anything less than a maxstep of 1/100 of the period of interest for stiff oscillators in pss in the MHz range. Hence, I suspect with your 2 GHz design a maxstep value of 1/100 is more than sufficient. Perhaps you might look at the step size being used in the pss transient analysis to determine how close to the minimum steps are to 1/100 of the the 2 GHz period.

     

    [3] - Sounds good - this might provide some insight (hopefully!)

     

    Shawn

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