A spectre newbie question

Hi everyone,

I am seeking your advice here. Have been trying to figure this out by
myself and looking around here for answers, but could not find anything
yet.

Here's a simple question. I am using Spectre now in Analog Design
environment in Cadence, and I am wondering how do you plot a graph of
gm/Id vs. Vgs - Vt (i.e. overdrive)?

I am kind of stuck as although I sort of know how to plot gm/id vs. Vgs
(there's a gmoverid intrinsic parameter there), I don't know how I
could plot vs. Vgs - Vt.

I am a Hspice user. In the past, I simply use .probe and create a
variable and use the command PAR to group Vgs - Vt into a single
variable.

I wonder if there is anything like this in Spectre? I search through
the manual but couldn't find a way to deal with this. Please help.

David.

 

Hi David,

Spectre doesn't support expression in save statement because it can be
done in Waveform tools at most of situations. Do you want to use
'Vgs-Vt' as X axis variable?

Yuchun

anointing 写é“：

 

Hi Yuchun,

Thanks for your reply. Yes, I would like to use 'Vgs-Vt' as X axis
variable. However, I dont know how to sweep Vgs-Vt instead of just Vgs
(or change the axis variable using the waveform calculator....). Please
help.

Thanks in advance.

David.

 

Hi David,

Sorry for the late response! I think Spectre can't sweep parameter
expression, and it is not easy for the Waveform tool to use expression
as X Axis. I may be wrong.

Is it necessary to use 'Vgs-Vt'? I mean that it should be enough to
sweep one parameter at most of situations. For your requirement, it is
the same for using 'Vgs' or 'Vgs-Vt' as X Axis if 'Vt' is a constant.

Actually, you may define a new parameter 'Vgs_new' and replace the old
'Vgs' with 'Vgs_new+Vt'. Then you can sweep 'Vgs_new' and set it as X
Axis.

Regards,
Yuchun

anointing 写é“：

 

SImply save all the operating point data for the device by adding an include
file containing:

save M1:all

(for example) - there are several previous posts on this in this forum - check
on google groups.

Then use the results browser to access the vgs and vt of the device and
subtract one from the other. Plot this. Do the other calculation you want to
plot, and plot that. Then by double clicking on the x-axis in the waveform
tool (AWD or wavescan) you can change the x-axis to one of the plotted
expressions.

Regards,

Andrew.