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Accuracy of CROSS() function in Verilog-A

RFStuff

Dear All,

I am using cross() in verilog-A as below:-

 

vtol=1e-12;

@(cross(phase-10u,+1,ttol,vtol)) begin
       $strobe("TIME = %rs \n", $abstime);
     end

But when I checked the abstime, it is NOT exactly at 10u of phase but at 10.541u of phase. This error is much larger than my vtol value.

But when I chnaged to strobeperiod of tran analysis to  1ns, abstime shows up at phase value of 10.002u.

I wonder why such descrpancy and why vtol  is NOT playing any role here.

Kind Regards,

 

 

Parents
  • Andrew Beckett

    The conventional way of handling this would be to convert the input voltage to the expected instantaneous frequency, and then integrate that to produce the phase. In this example:

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

          phase = idtmod(inst_freq,0,1);
          V(vout) <+ amp * sin (2 * `PI * phase);

    The phase will ramp up from 0 to 1 and the automatically reset. In this case we're using a sin() to produce a sinusoidal output, but you could also use an @cross or similar to produce a logic output.

    Regards,

    Andrew.

  • atulkumar245
    in reply to Andrew Beckett

    real iout,vd,temp;

    analog initial
    state = 0 ;


    analog begin
    //temp=0;

    vd=V(vc,out);
    @(cross(vd-3,1))
    state=1 ;
    @(cross(vd-2,-1))
    state=0 ;

    $strobe("Vds=%e",vd);

    if (state==1)
    begin
    temp=0.5+0.5*tanh((vd-2)/0.01);
    iout = temp*(vd/500)+(1-temp)*(vd/7000);
    end
    else
    begin
    temp=0.5+0.5*tanh((vd-3)/0.01);
    iout = temp*(vd/500)+(1-temp)*(vd/7000);
    end


    I(vc,out) <+ iout ;
    end

    endmodule

    I want to implement a hysteresis loop in I_V_characteristics at V=3 and V=4 being transition points. Is there any alternative to cross-function? Please help me out in this 

Reply
  • atulkumar245
    in reply to Andrew Beckett

    real iout,vd,temp;

    analog initial
    state = 0 ;


    analog begin
    //temp=0;

    vd=V(vc,out);
    @(cross(vd-3,1))
    state=1 ;
    @(cross(vd-2,-1))
    state=0 ;

    $strobe("Vds=%e",vd);

    if (state==1)
    begin
    temp=0.5+0.5*tanh((vd-2)/0.01);
    iout = temp*(vd/500)+(1-temp)*(vd/7000);
    end
    else
    begin
    temp=0.5+0.5*tanh((vd-3)/0.01);
    iout = temp*(vd/500)+(1-temp)*(vd/7000);
    end


    I(vc,out) <+ iout ;
    end

    endmodule

    I want to implement a hysteresis loop in I_V_characteristics at V=3 and V=4 being transition points. Is there any alternative to cross-function? Please help me out in this 

Children
  • Andrew Beckett
    in reply to atulkumar245

    Why do you need an alternative to the cross function? Surely that does what you want? (the code above appears to have transitions at 3V and 2V).

    Andrew.

  • atulkumar245
    in reply to Andrew Beckett

    Actually, I have to implement a hysteresis with the transition happening at 2 and 3 V . The circuit made with this device used should oscillate but this implementation is showing an error. So I thought maybe it is due to cross so I asked for an alternative.

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