HMC701LP6CE Datasheet, PDF (19/46 Pages) Hittite Microwave Corporation – 8 GHz 16-Bit Fractional-N Synthesizer

v03.0709
HMC701LP6CE
8 GHz 16-Bit Fractional-N Synthesizer
(EQ 2)
11
Figure 18. VCO Calibration
The period of a single VCO calibration measurement, T_mmt, is given by:
(EQ 3)
Where
ncyc
= vcot_ncyc_rdiv Reg0A<5:2> Table 21
vcot

= Ref tune clock division ratio, vcot_xrediv_sel Reg0Ah<11:10>, Table 21
Note: Only vcot = 0 is supported currently
A 5-bit switched resonator VCO, for example, nominally requires 5 measurements for calibration, worst case 6, hence
total calibration time, worst case, is given by:
(EQ 4)
Normally we wish the absolute value of f_erro to be less than one half of the VCO switched resonator step size specified
for the switched resonator VCO.
VCO Auto Tuning Example
Suppose our target frequency is to operate at 2.3 GHz with a nominal 1.5V on the charge pump. Our example crystal
is 50 MHz and vcot_xrefdiv_sel divider is 1, hence the auto calibration clock = 50 MHz. Note, when using autocal, the
maximum input crystal cannot exceed 60 MHz.
We must decide how long, and how accurate we need to make the tuning. Suppose the switched resonator lsb steps
are 6.5 MHz nominal, 6 MHz worst case.
A reasonable calibration target would be to set the calibration frequency error to one half the VCO lsb steps, i.e. f_erro
<= 3 MHz. With some reasonable margin, lets target 2 MHz f_erro worst case. Using (EQ 1) we see that the minimum
calibration measurement time for a single measurement must be
(EQ 5)
Hence we must choose ncyc such that T_mmt >T_mmtmin
Solving for (EQ 3)
(EQ 6)
11 - 20
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