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Design Languages >> Verilog-AMS >> Turning on white_noise in a switch
https://designers-guide.org/forum/YaBB.pl?num=1208337794

Message started by DaffyDuck on Apr 16^th, 2008, 2:23am

Title: Turning on white_noise in a switch
Post by DaffyDuck on Apr 16^th, 2008, 2:23am

Hi All,

I am trying to model the white noise due to the switch resistance with the following statements:

if ( V(ps,ns) > thresh )
V(p,n) <+ ron * I(p,n) + white_noise(4*`P_K*$temperature*ron, "thermal"); // (A)
else
I(p,n) <+ 0;

Got the "error" message - analog operator embedded in conditionally executed statement or expression.

I tried to do an assignment to a real var, eg. noise = white_noise(4*..... ), and then add "noise" to the contribution statement (A) in place of the lengthy white_noise expression. But I got the error message to the effect that small-signal functions cannot be assigned.

What's the way to do it? :o

Title: Re: Turning on white_noise in a switch
Post by Stefan on Apr 16^th, 2008, 2:40am

Add white noise to an isolated electrical node.
Take the voltage from that node and add it to your desired variable/statement/whatsoever.

Title: Re: Turning on white_noise in a switch
Post by DaffyDuck on Apr 17^th, 2008, 1:22am

Hi Stefan,

Thank you! That works. :D

Title: Re: Turning on white_noise in a switch
Post by Geoffrey_Coram on Apr 17^th, 2008, 11:09am

Another approach would be this:

V(p,n) <+ white_noise(4*`P_K*$temperature*ron, "thermal"); // (A)
if ( V(ps,ns) > thresh )
V(p,n) <+ ron * I(p,n); // (A)
else
I(p,n) <+ 0;

Verilog-A's value-retention rules say that, if you contribute to the flow (I), then all potential contributions are discarded. Now your white_noise call is not inside the if() statement.

BTW: I personally believe the error from your simulator is incorrect. white_noise is *not* an analog operator along the lines of idt() or absdelay(); it appears in "4.5 Analysis dependent funtions" rather than "4.4 Analog operators" in the Verilog-AMS language reference manual (2.2).