PFD 100MHz RF output:8-16GHz
when i use fractional mode there is many spurs,and the spur change is dynamic
1.why fractional mode have spur
2.why the spur is change,
The HMC704 synthesizes fractional frequencies using a Sigma-Delta Modulator (SDM). This is a digital finite state machine which is clocked at the comparison frequency rate. The DSM outputs a randomized sequence of control bits to increment or decrement the VCO feedback divider such that the effect over time is to synthesize a fractional frequency between two integer values. The output is not fully random, but rather pseudo-random, so it does have a repeating pattern. The length of the pattern varies with the specific programmed fractional divide setting. For example, the pattern may be [… N, N-1, N+2, N-3, N, N+1 …]. Averaged over time, the VCO feedback divider value is a fractional, non-integer multiple.
With Frac-N, the comparison frequency can be increased to a value much greater than the integer channel spacing. A higher comparison frequency results in lower VCO feedback divider (N) values which lowers phase noise (20Log(N) factor) improving spectral performance. Also, the higher comparison frequency means a wider loop BW can be used, which lowers the spectral (phase noise) requirement of the VCO’s (read integrated, cheaper VCO’s). Also, a wider loop BW means the system can have a much shorter lock time.
Fractional spurs are caused by…
Typically these spurs are static for a given configuration. There could be other interfering sources which cause the dynamic behaviour shown in your measurement.
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