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Noise Figure of track and hold circuit

Alok110

Hi, I am trying to get Noise Figure in track and hold circuit with RC>>Ts/2. My ckt is like RC low pass filter {Port(with res=50) + switch + cap to gnd }

NF= 10logF,  where F is Noise Factor
and F = (Ni+Nsys)/Ni.

In my circuit Nsys = 0 (Sys include only switch and cap, there is no res(noise generator)). so, F should be =1, so the Noise figure should be = 0.
But, cadence giving me F = 1.96  and NF = 2.92 dB. (I am measuring it with Pss+Pnoise)


what I am doing wrong?

                           

Fullscreen netlist.txt.txt Download 
// Library name: PHD22
// Cell name: switching
// View name: schematic
W0 (Prt net02 clk 0) relay vt1=1m vt2=500.0m ropen=1T rclosed=1.0
C1 (net02 0) capacitor c=c
V1 (clk 0) vsource type=pulse val0=0 val1=500.0m period=1/f width=1/(2*f) \
        fundname="CLK"
PORT1 (Prt 0) port r=r dc=DC/2 type=sine freq=frf ampl=Amp/2 mag=Ac_mag/2 \
        isnoisy=yes
Fullscreen Input(scs).txt Download 
// Generated for: spectre
// Generated on: Aug 14 21:28:57 2021
// Design library name: PHD22
// Design cell name: switching
// Design view name: schematic
simulator lang=spectre
global 0
parameters r=50 frf=50M f=50000k c=1000n DC=0 Amp=1 Ac_mag=0

// Library name: PHD22
// Cell name: switching
// View name: schematic
W0 (Prt net02 clk 0) relay vt1=1m vt2=500.0m ropen=1T rclosed=1.0
C1 (net02 0) capacitor c=c
V1 (clk 0) vsource type=pulse val0=0 val1=500.0m period=1/f width=1/(2*f) \
        fundname="CLK"
PORT1 (Prt 0) port r=r dc=DC/2 type=sine freq=frf ampl=Amp/2 mag=Ac_mag/2 \
        isnoisy=yes
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
    tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
    digits=5 cols=80 pivrel=1e-3 sensfile="../psf/sens.output" \
    checklimitdest=psf 
pss  pss  fund=50M  harms=20  errpreset=liberal  autotstab=yes
+    saveinit=yes  method=gear2only  tstabmethod=gear2only  maxacfreq=50G
+    annotate=status
pnoise  (  net02  0  )  pnoise  start=1  stop=25M
+       maxsideband=1000  iprobe=PORT1  refsideband=0
+       noisetype=timeaverage  noiseout=[usb]  annotate=status
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
saveOptions options save=all

 

  • in reply to Andrew Beckett

    Dear Alok110,

    As Andrew did, I also simulated your netlist using a few of the suggestions I made. As Andrew found, i also found that the Noise Figure you quoted appears to be correct. As an attempt to illustrate the reason it is higher, I assembled a few pages detailing the unsampled and sampled spectrum in the hope they might provide some insight fir you. I also verified that if you move your filter capacitor before the sampling process occurs (i.e., this keeps the noise bandwidth to a value less than 1/2 the sample rate of 50 MHz), the noose is not aliased and the Noise Figure doesctske on your expected result.  Finally, two other items:

    1. I ran a conventional transient analysis and inspected and compared the pss solution with the conventional transient solution after it had reached its seady-state. With your settings, the pss solution did not have the same average DC value as observed in the conventional transient solution in steady-state. 

    2. To my surprise, i did not observe a significant difference between the pss solution using your "liberal" accuracy setting and one with "liberal" changed to "conservative".

    i hope this adds a bit to your understanding snd Andrew's similar findings....nnoise_figure_nose_figure_sampled_sml_081521.pdf

    Shawn

  • in reply to ShawnLogan

    Shawn,

    1. I didn't see that. If I run the tstab for 1m and add saveinit=yes, the settled net02 is at 619mV, which is what you get from the PSS time domain solution - as it should. PSS doesn't need a long tstab if the circuit has normal settling - the shooting method can find that. So for me, it matches well. Sure, I didn't run tran - I just looked at the tstab, but I wouldn't expect a massive difference.
    2. This doesn't surprise me - I would expect liberal to give reasonable results here. The circuit does not have high large-signal dynamic range - it only has the clock - and there will also be a fair number of time points (especially with maxacfreq=50G) which would reduce much chance of their being any significant error in such a simple circuit. Liberal can often be much better than people think - and in this case where there's no smaller signal to be resolved (other than in the pnoise analysis), it's not going to make a huge difference to the accuracy.

    Andrew

  • in reply to Andrew Beckett

    Hi Andrew,

    Thank you for reading my comments and your insights (totally unnecessary but appreciated!)!

    Andrew Beckett said:
    I didn't see that. If I run the tstab for 1m and add saveinit=yes, the settled net02 is at 619mV, which is what you get from the PSS time domain solution - as it should. PSS doesn't need a long tstab if the circuit has normal settling - the shooting method can find that. So for me, it matches well. Sure, I didn't run tran - I just looked at the tstab, but I wouldn't expect a massive difference.


    Just for your amusement to indicate what I observed as  differences between the pss converged solution and the end of a 4 ms conventional transient simulation, I updated the prior Adobe Portable Document to include pages 5, 6, and 7. Pages 5 and 6 illustrate the pss solution and last 4 periods of a 4 ms conventional transient analysis for the output voltage on Alok110's circuit (net02 - output capacitor following sampling switch) respectively. The ranges of the two y-axes are identical for a comparison of the DC values of each. To further illustrate the differences, page 7 compares the two waveforms on an identical x-axis by time shifting the last period of the 4 ms conventional transient analysis.

    Shawn

    (Note: prior Adobe Portable file with added 3 pages is apparently too large to post and hence I am providing a URL to the updated file. If you are interested and happen to try to open file, please let me know if you experience any issue with the link.)

    ent.box.com/.../fbxt680s2cpop64y9ksqqquj57wvxi82

  • in reply to Alok110

    Dear Alok110,

    Alok110 said:
    Thank you.

    Absolutely  - but I just hope it adds something to your insights to allow you to complete your simulations!

    Shawn

  • in reply to ShawnLogan

    Shawn,

    OK, I thought you were talking about a massive difference in the DC level! This is fairly small, especially with a circuit where the settling time is considerably longer than the PSS interval and the circuit is so small too. In essence, it's unsurprising that the shooting method can get a slightly different result given that it works by detecting small error in each of the voltages and branch currents in the circuit and ensuring it is smaller than a tolerance (involving reltol and steadyratio) - small accumulation of errors can be missed. That said, even the transient is rather sensitive to whether you run that with liberal or moderate (I didn't try the tran with conservative because I thought I'd spent long enough on it).

    Anyway, it's clear now. Thanks for your additional info.

    Andrew

  • in reply to Andrew Beckett

    Dear Andrew,

    Thank you for even taking your very limited time to view, comment, and provide your insight! The latter is very helpful!

    I do agree that the difference between the DC averages of the pss converged solution and that of the apparent steady-state response from a 4 ms conventional transient simulation. For your information, the spectre transient command I added to the netlist we were provided was:

    tran tran stop=4m write="spectre.ic" writefinal="spectre.fc" \
    errpreset="conservative" annotate=status method=gear2only maxstep=0.2e-09

    and hence it did use the spectre errpreset of "conservative".

    I did not change the pss command from that in the netlist we were supplied and hence its accuracy setting was "liberal".

    Perhaps I should just be impressed at how close the two solutions are!

    In any case, once again, my sincere thanks for "wasting" some of your limited time on me to add your comments to my responses.

    Shawn

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