Re: noise_table question

Geoffrey,
    It seems like a nice little enhancement. If I remember correctly, very
little thought went into the details of the noise_table() function. The simplest
thing possible was done with the assumption that it could be fixed later based
on use/need.

While we are on the topic, there is a real problem with the small-signal
stimulus functions, of which noise_table is an example (along with ac_stim,
white_noise and flicker_noise). The problem is with at least one of the major
commercial implementations of these functions rather than with the LRM.
Currently Cadence requires that these functions be contributed directly to
a signal. So,
    I(out) <+ noise_table(...);
is okay, but
    I(out) <+ f(noise_table(...));
is not. This is problematic because it makes it difficult to build models with
modulated noise sources. For example, the following simple model for a vco that
exhibits phase noise is not supported in Cadence (at least last time I checked,
which admittedly was some time ago):

module vco (out, in);
voltage out, in;
parameter k = 1;
parameter f0 = 0;
parameter real pn=1;
real phase, freq;

    analog begin
        freq = k*V(in) + f0;
        phase = idtmod(freq, 0, 1);
        V(out) <+ cos(2*`M_PI*phase + flicker_noise(pn,2));
        $bound_step(1/(10*freq));
    end
endmodule

If someone is going to update the LRM to include logarithmic interpolation in
noise_table, I think at the same time we should add the above model as an
example in the flicker_noise section of the LRM to make it clear
1. how to build a model that exhibits modulated noise, and
2. that the intent of the LRM is that there should be no restrictions on the
   placement of the small-signal stimulus functions within an expression.

-Ken

On Wed, Apr 04, 2012 at 03:55:49PM -0400, Geoffrey Coram wrote:
> Was there ever a proposal of how to request log-log interpolation
> for noise_table ?
>
> Does any simulator support it?
>
> The current syntax is:
> noise_table ( input [ , name ] )
>
> where input can be a vector or a string (filename), and name is a string.
>
> We could require the noise have a name to request interpolation:
> noise_table ( input [ , name [, interp ] ] )
>
> Or we could let the type of the 2nd argument determine what it is:
> noise_table ( input [ , name ] )
> noise_table ( input [ , log_interp ] [, name ] )
>
> where log_interp would be 0 or 1 (an integer).
>
> -Geoffrey
>
>
>
> Oskar Leuthold wrote:
> >Geoffrey,
> > you might also question the wisdom of linear interpolation here.
> >Noise generally varies as the log of the frequency, see section 4.5.4.2,
> >thus interpolation in the log-log domain would be more useful.
> >Regards
> > Oskar
> >
> >----------------------------------------------------------------------
> >Oskar Leuthold, Corporate Applications Engineer
> >Silicon Engineering Group Synopsys, Inc. 700 E. Middlefield Rd.
> >Mountain View, CA 94043 650 584 5887
> >oskar.leuthold@synopsys.com
> >
> >>-----Original Message-----
> >>From: owner-verilog-ams@eda.org
> >>[mailto:owner-verilog-ams@eda.org] On Behalf Of Geoffrey.Coram
> >>Sent: Monday, March 20, 2006 9:18 AM
> >>To: VerilogAMS Reflector
> >>Subject: noise_table question
> >>
> >>Section 4.5.4.3 on noise_table specifies that linear
> >>interpolation is used (presumably for frequencies inside the
> >>range of frequencies specified in the vector).
> >>
> >>What happens outside the vector?
> >>1) value is zero
> >>2) if the first pair is (fmin,val1), then for frequencies < fmin,
> >> the value is val1; similarly if (fmax,valn) is the last pair,
> >> then the value is valn for frequencies > fmax.
> >>
> >>I propose (2).
> >>
> >>
> >>Also, the LRM does not specify what a "vector" is.
> >>
> >>-Geoffrey
> >>
> >
>
>
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