clk_gen :
A digital clock generator.

The clk_gen primitive generates an xbit-typed, periodic clock signal with a frequency and initial phase specified by the parameters freq and init_phase, respectively. The primitive can also add sinusoidal jitter, random jitter, or phase noise to the generated clock.

As for the sinusoidal jitter (SJ), the parameters SJ_amp, SJ_freq, and SJ_phase set the amplitude, frequency, and phase of the sinusoidal jitter, respectively. This SJ generation capability is useful when performing jitter transfer or jitter tolerance simulations.

The phase noise of the output clock can be specified using the phase noise parameters PN_fcenter, PN_foffset, PN_dbc, PN_fcorner, and PN_floor, assuming the phase noise spectrum of a typical oscillator consisting of 1/f^3 noise, 1/f^2 noise, and noise floor (ref: D. B. Leeson, “A Simple Model of Feedback Oscillator Noise Spectrum,” in Proceedings of the IEEE, vol. 54, Feb. 1966, pp. 329–330). These phase noise parameters take effects only when the parameter PN_fcenter has a positive value (default: -1.0).

  log(PN) | \
          |  \ 1/f^3 noise
          |   \
          |    \
          |     `` 1/f^2 noise
          |       ``          noise floor
          |         ``---------- (=PN_floor)
          +----+------------------ log(f)
           PN_fcorner

The parameter PN_fcorner defines the 1/f^3 phase noise corner frequency, i.e., the offset frequency at which the 1/f^3 noise and 1/f^2 noise have equal levels (default: 0.0). The parameter PN_floor defines the phase noise floor, i.e. the minimum phase noise level in dBc/Hz at all frequencies (default: -`INFINITY). When the parameters PN_fcorner and PN_floor have their default values (0.0 and -`INFINITY, respectively), the output clock has only 1/f^2 phase noise.

The 1/f^2 phase noise characteristics can be defined by specifying its phase noise level in dBc/Hz (PN_dbc) measured at one offset frequency (PN_foffset) from the center frequency (PN_fcenter, which is equal to freq by default) using the parameters PN_dbc, PN_foffset, and PN_fcenter, respectively. To avoid confusion, it is recommended to use the PN_foffset value greater than the PN_fcorner value and the PN_dbc value greater than the PN_floor value to define the 1/f^2 phase noise level. Otherwise, the total phase noise level produced by the primitive at the offset frequency PN_foffset might not be equal to the specified PN_dbc due to the additional 1/f^3 phase noise or phase noise floor.

Alternatively, the phase noise of the output clock can be specified using the random jitter parameters RJ_kappa and RJ_rms. The parameter RJ_kappa defines the RMS value of the accumulated random jitter, which increases as a square-root function of time due to the 1/f^2 phase noise. On the other hand, the parameter RJ_rms defines the RMS value of the independent random jitter, which has fixed statistics regardless of time. Defining these random jitter parameters is equivalent to defining the phase noise parameters satisfying the following relationships:

        RJ_kappa = 10^(PN_dbc/20) * PN_foffset / PN_fcenter.
        RJ_rms = 10^(PN_floor/20) / (2π*sqrt(PN_fcenter)),

Note that the random jitter parameters RJ_kappa and RJ_rms cannot specify the 1/f^3 phase noise.

Input/Output Terminals

Name	I/O	Type	Description
out	output	xbit	signal output

Parameters

Name	Type	Default	Unit	Description
freq	real	1.0e9	Hz	clock frequency
init_phase	real	0.0	radian	initial clock phase
SJ_amp	real	0.0	UI	sinusoidal jitter amplitude
SJ_freq	real	0.0	Hz	sinusoidal jitter frequency
SJ_phase	real	0.0	radian	sinusoidal jitter phase
PN_fcenter	real	-1.0	Hz	phase noise center frequency
PN_foffset	real	0.0	Hz	phase noise offset frequency
PN_dbc	real	-`INFINITY	dBc/Hz	phase noise measured in dBc/Hz
PN_fcorner	real	0.0	Hz	1/f^3 phase noise corner frequency
PN_floor	real	-`INFINITY	dBc/Hz	phase noise floor measured in dBc/Hz
RJ_kappa	real	0.0	second	RMS accumulated random jitter after 1 second
RJ_rms	real	0.0	second	RMS independent random jitter