OSC

  1. Model Examples
  2. OSC

OSC_LinearVCO8O

OSC_LinearVCO8O : A voltage-controlled oscillator (VCO) with a linear V-to-f characteristic and 8 outputs

A voltage-controlled oscillator (VCO) generates one or more clock signals out of which frequency varies with the control input in. In particular, this VCO model describes a VCO of which frequency varies linearly with the input in, as described by the following equation: 

frequency = freq0 + Kvco * (in - in0)

The parameter freq0 sets the nominal frequency of the VCO when the input is at its nominal value set by the parameter in0. And the parameter Kvco defines the VCO gain, defining the rate of change in the VCO frequency with respect to the input.

This VCO model is described using a poly_func primitive followed by a freq_to_clk primitive. The poly_func primitive calculates the frequency value from the input in and the freq_to_clk primitive produces clocks with the corresponding frequency. The phase noise characteristics of the VCO can be modeled by defining the phase noise parameters of the freq_to_clk primitive, such as PN_fcenter, PN_foffset, PN_dbc, PN_fcorner, and PN_floor. For more information on these parameters, please refer to the documentation on the freq_to_clk primitive.

Input/Output Terminals

Name I/O Type Description
out[7:0] output xbit output clock
in input xreal input voltage

Parameters

Name Type Default Description
in0 real 0.5 nominal input value (V)
freq0 real 16G VCO frequency at nominal input value (Hz)
Kvco real 1G VCO gain (Hz/V)
PN_fcenter real -1.0 phase noise center frequency (Hz)
PN_foffset real 0.0 phase noise offset frequency (Hz)
PN_dbc real -`INFINITY phase noise measured in dBc/Hz (dBc/Hz)
PN_fcorner real 0.0 1/f^3 phase noise corener frequency (Hz)
PN_floor real -`INFINITY phase noise floor measured in dBc/Hz (dBc/Hz)
RJ_kappa real 0.0 RMS accumulated random jitter after 1 second (second)
RJ_rms real 0.0 RMS independent random jitter (second)

List of Testbenches

tb_meas_tfdc : A testbench for measuring the voltage-to-frequency characteristic of a VCO

tb_meas_tfdc : A testbench for measuring the voltage-to-frequency characteristic of a VCO

This testbench measures the voltage-to-frequency (V-to-f) characteristic of a voltage-controlled oscillator (VCO) by applying a gradually-varying voltage to the VCO input and measuring its output frequency.

The measured frequency is expected to change with the input voltage according the V-to-f equation assumed by the VCO model. 

`include "xmodel.h"

module tb_meas_tfdc();
    
    xreal vctrl;            // input voltage
    xbit [7:0] out;         // output clock
    xbit out0;              // alias for out[0]
    
    // stimuli
    pwl_gen #(.data('{100e-9, 0.1, 900e-9, 0.9}))
            vctrl_gen(vctrl);

    // DUT (device-under-test)
    OSC_LinearVCO8O     DUT(.in(vctrl), .out(out));

    // measurements
    assign out0 = out[0];
    probe_xreal probe_vctrl(vctrl);
    probe_freq  probe_out(out0);
                        
endmodule

Simulation Results

the V-to-f characteristic of the VCO

Figure. the V-to-f characteristic of the VCO.