Re: Proposal for two additional environment parameter functions

Hi Geoffrey,
Yes I understand for an ac, the circuit is linearized around the op point. I 
never (at this stage at least) intended for $freq to cause G and C to be 
recomputed.
I have supported $freq for many years and it is mainly a debug function (It is 
helpful when used in conjunction with $debug()). I also use it in noise 
analysis, where we can have frequency dependent elements. It has proven helpful 
to designers when debugging model problems. I can detect models that are 
dependent on frequency to ensure that they are solved each point (even if that 
solve doesn't modify the jacobian).

Recently a model developer asked me how to load noise freq/pwr pairs from a file.
One solution is that you use $fscanf to load the values from the file into an 
array and then use noise_table(). However this is inefficient as you need to 
know how many elements are listed in the file so that your Verilog-A array is 
the correct size.

So the other way was to use $table_model() and $freq. This could do the 
interpolation of the freq/pwr pairs, and then you could use white_noise() to 
load that pwr value.
This worked beautifully, it allows you to have all the benefits of the 
$table_model interpolation/extrapolation algorithms (not just clamping at the 
bounds).
However it is breaking the definition of white_noise() since now my pwr value 
is frequency dependent.

A couple of ways to solve this:
1. Extend noise_table() and overload the first arg such that if it is a string 
then it represents a file name which contains the freq/pwr pairs.

2. Add $freq, and a new function 'colored_noise(pwr)' and use $table_model to 
do the interpolation.

I prefer 2. as I am not keen on overloading arguments to be different types. 
But I was still working through some of the finer points of the colored_noise() 
function, so wasn't quite ready to propose it. But I thought $freq was fairly 
simple, so thought I would send it through now for people to critique.

Cheers...
Dave

Geoffrey.Coram wrote:
> Arpad -
> $abstime works OK in a transient analysis, because the full model is 
> evaluated
> at each timepoint.
> 
> Spice's ac analysis is a different sort of animal: the operating point is
> computed, and then *the circuit is linearized about that op-pt*.  
> Generally,
> this means that the model is evaluated during the op-pt and all the 
> derivatives
> are stored off.  Then, for each point in the ac analysis, the 
> derivatives are
> used to compute J = G + j w C, where G=conductances and C=capacitances
> (derivatives of contributions involving ddt()); G and C are *not* 
> re-computed
> at the new frequency, because Spice assumes that they only depend on the
> v-vector (the operating point).
> 
> I agree with David (from Tiburon) about the hazards of $freq expressions
> for models that are supposed to work in transient, HB, etc.
> 
> -Geoffrey
> 
> 
> Muranyi, Arpad wrote:
>> Geoffrey,
>>
>> I don't understand the reason for why the IF statement
>> wouldn't get evaluated during the AC sweep.   Could
>> you please explain that to me?
>>
>> If we had this in the language, I would expect it to
>> work the same as $abstime.  Doesn't $abstime=0 in the
>> operating point analysis and greater than zero
>> thereafter?  If I remember correctly, I was able to
>> use if($abstime ...) before.  Why couldn't $freq do
>> the same?
>>
>> Thanks,
>>
>> Arpad
> 

-- 
=====================================
-- David Miller
-- Design Technology (Austin)
-- Freescale Semiconductor
-- Ph : 512 996-7377 Fax: x7755
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