I don't really understand why you'd want to use idt rather than idtmod - it's cleaner, and probably more accurate to use idtmod (because you need a timestep with the assertion to reset the phase). Here's an example:

Code:

`include "discipline.h"
`include "constants.h"

module vco(vin, vout);
input vin;
output vout;
electrical vin, vout;
parameter real amp = 1;
parameter real center_freq = 1K;
parameter real vco_gain = 1K;
parameter integer steps_per_period = 32;

   real phase;
   real inst_freq;
   integer resetph;

   analog begin

	inst_freq = center_freq + vco_gain * V(vin);
	$bound_step (1.0 / (steps_per_period*inst_freq));

	phase=idt(inst_freq,0,resetph);
	resetph=0;
	@(cross(phase-1.0)) begin
	  resetph=1;
	  end
	//phase = idtmod(inst_freq,0,1);
	V(vout) <+ amp * sin (2 * `M_PI * phase);
   end
endmodule
 

Note that the 5 lines of code from the idt onwards could be replaced with the single idtmod line.

So why do you want to make life difficult for yourself?

Andrew.

I don't believe that making the modulus a function of the input is what you want to do. Remember that the output is a phase, which is a point on a circle. You already have the argument (inst_freq) being a function of the input, this controls the rate at which the point moves around the circle. Now if you also make the modulus of a function of the input, you are effectively varying the circumference of the circle as well. Meaning that the period of the sawtooth will be constant as a function of the input because, as the input increases the rate, it also increases by the circumference by an equal amount. So the period of the sawtooth is independent of the input, but its amplitude will increase with the input. This is problematic, because with low input levels, the output amplitude will not be large enough to trigger the cross statement.

There are two ways to generate a pulse with a controlled width. The easiest is to put in two thresholds, say at 0.25 and 0.75 (assuming modulus is 1), and have one set the output high and the other reset the output low. This will give you a controlled 50% duty cycle. The other thing to do is to use timer event to produce a fixed width pulse (in the body of the cross event you set the output and then save the time you would like to trigger the timer, and in the body of the timer you reset the output). In this case, you must assure that the pulse width is not longer than the minimum period.

-Ken

You all make excellent points.
I intended to have the phase period independent of the input magnitude.
The only thing changing is the slope of the phase with changing input magnitude. By counting how often the phase crosses m*PI, I can obtain a "quantized" value of the input signal. The quantization error would then be the remainder of the last m*PI crossing till the end of the period before the phase resets. Eventually, I would like the quantization error to be the initial phase for the next period. There might be another way of getting the same result more sophisticated using idtmod, I just don’t know how.

Thanks