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Simulators >> RF Simulators >> ADS HB: Oscillator + Divider
https://designers-guide.org/forum/YaBB.pl?num=1241738571

Message started by essam on May 7^th, 2009, 4:22pm

Title: ADS HB: Oscillator + Divider
Post by essam on May 7^th, 2009, 4:22pm

Hello All,

I am trying to simulate 1.8GHz CMOS oscillator + divide-by-2 circuits. Both circuits are working fine separately (taking loading into account). When I put them together, the simulation does not converge at the divider output, but does converge at the oscillator output.

My simulation settings for HB:
Frequency: 1/2 oscillator frequency
Enable Oscillator: Yes

I also tried the Transient Assisted HB (TAHB), but still divider output is dead.

The following is the message I get from TAHB:
-------------------------------------------------------------------------
TAHB conversion started at t=1.105e-007 without reaching steady state.
The circuit did not reach steady state; the worst node was MOSFET44.d.
Final transient frequency = 1.799496029 GHz
t=1.111e-007 100%
---------------------------------------------------------------------------

Does anybody have a clue?

Thanks,
Essam

Title: Re: ADS HB: Oscillator + Divider
Post by Ken Kundert on May 7^th, 2009, 5:39pm

Quote:

the simulation does not converge at the divider output, but does converge at the oscillator output.

What does that mean? What do you mean when you say that the output either does or does not converge.

Perhaps you can try describing the problem using different words.

-Ken

Title: Re: ADS HB: Oscillator + Divider
Post by essam on May 7^th, 2009, 5:43pm

Hi Ken,

I mean that I am monitoring two output nodes. The output at the oscillator is good (I can see the waveforms). However, the output at the divider is dead, although I am sure that the oscillator can drive it properly. I tried that in a separate sim replacing the oscillator with a source that has similar output impedance and output swing.

I hope this is more clear now.

Thanks,
Essam

Title: Re: ADS HB: Oscillator + Divider
Post by pancho_hideboo on May 7^th, 2009, 7:27pm

essam wrote on May 7^th, 2009, 4:22pm:

When I put them together, the simulation does not converge at the divider output, but does converge at the oscillator output.

Partial covergence is never possible in simulator.
So if the divider output does not converge, simulation itself results in fail with convergence error.

essam wrote on May 7^th, 2009, 5:43pm:

I mean that I am monitoring two output nodes.
The output at the oscillator is good (I can see the waveforms).
However, the output at the divider is dead, although I am sure that the oscillator can drive it properly.

So your simulation itself does not fail with convergence error. Right ?

Show me version of ADS you use.
Show me netlist of ADS and full logfile.

Show me your Frequency setting for Fund[1].
Show me your HB_Order.
Show me your HB_Oversample.

Do you set "Convergence Mode" as "Advanced(Robust)" ?

Which method do you use for Oscillation Analysis using HB, "Specify Nodes" or "Use Oscport" ?

If you use "Specify Nodes", where do you specify in circuits ?

If you use "Use Oscport", where do you insert "OscPort" or "OscPort2" in circuits ?

Show me "Harmonic Number" in your setting for "Use Oscport" or "Specify Nodes".

Title: Re: ADS HB: Oscillator + Divider
Post by Ken Kundert on May 7^th, 2009, 8:40pm

This seems like a signal tracing exercise. The signal goes into the divider and does not come out. Can't you just follow the signal until it stops, and then determine why the signal stops?

-Ken

Title: Re: ADS HB: Oscillator + Divider
Post by essam on May 7^th, 2009, 8:49pm

HI,

Thank you for your help.

I am using the following:

Show me version of ADS you use: (ADS 2008 Update 2)

Show me netlist of ADS and full logfile.

Show me your Frequency setting for Fund[1]: 1/2*FRF where FRF is the oscillation frequency (I have divide-by-2 circuit)

Show me your HB_Order. (I tried 12, 20,31)

Show me your HB_Oversample. (4)

Do you set "Convergence Mode" as "Advanced(Robust)" ? (I tried auto and Advanced Robust)

Which method do you use for Oscillation Analysis using HB, "Specify Nodes" or "Use Oscport" ? (Oscport)

(I put it in the feedback path of the cross-coupled MOS diff-pair)

Thanks,
Essam

Title: Re: ADS HB: Oscillator + Divider
Post by pancho_hideboo on May 7^th, 2009, 8:52pm

Answer my question.

Your simulation itself does not fail with convergence error. Right ?

Show me full logfile.

essam wrote on May 7^th, 2009, 8:49pm:

pancho_hideboo wrote on May 7^th, 2009, 7:27pm:

Which method do you use for Oscillation Analysis using HB, "Specify Nodes" or "Use Oscport" ?

(Oscport)
(I put it in the feedback path of the cross-coupled MOS diff-pair)

Show me "Harmonic Number" in your setting for "OscPort".

Title: Re: ADS HB: Oscillator + Divider
Post by essam on May 8^th, 2009, 1:36pm

Thanks, you pointed out the problem. i was using Harm=1 in OscPort although I should use harm=2 because the oscillator oscillates at twice the HB frequency.

Thank you so much for your help. I really appreciate it.

Essam

Title: Re: ADS HB: Oscillator + Divider
Post by pancho_hideboo on May 8^th, 2009, 10:42pm

essam wrote on May 7^th, 2009, 8:49pm:

pancho_hideboo wrote on May 7^th, 2009, 7:27pm:

Show me your HB_Order.

(I tried 12,20,31)

In Agilent ADSsim(RFDE), you should increase HB_Order as 2**N-1, N=1,2,3,4,5,...
HB_Order=3,7,15,31,63,.....

So your HB_Order=12 is treated as HB_Order=15 internally.
Your HB_Order=20 is treated as HB_Order=31 internally.

essam wrote on May 7^th, 2009, 8:49pm:

pancho_hideboo wrote on May 7^th, 2009, 7:27pm:

Show me your HB_Oversample.

(4)

HB_Oversample=2**N, N=1,2,3,4,5,...
HB_Oversample=2,4,8,16,.....

See http://www.designers-guide.org/Forum/YaBB.pl?num=1237890768/10#10