HMC701LP6CE Datasheet, PDF (19/40 Pages) Hittite Microwave Corporation – 8 GHz 16-Bit Fractional-N Synthesizer

v09.1112
HMC701LP6CE
8 GHz 16-BIT FRACTIONAL-N PLL
Charge Pump Phase Offset
Either of the UP or DN charge pumps may have a DC leakage or “offset” added. The leakage forces the phase detector
to operate with a phase offset between the reference and the divided VCO inputs. It is recommended to operate with
a phase offset when using fractional mode to reduce non-linear effects from the UP and DN pump mismatch. Phase
noise in fractional mode is strongly affected by charge pump offset.
DC leakage or “offset” may be added to the UP or DN pumps using cp_UPoffset_sel and cp_DNoffset_sel (Reg08
Table 19). These are 4 bit registers with 28.7uA LSB. Maximum offset is 430uA.
As an example, if the main pump gain was set at 1mA, an offset of 373uA would represent a phase offset of about
(392/1000)*360 = 133 degrees. For best spectral performance in Fractional Mode the leakage current should be
programmed to:
Required Leakage Current (µA) = (2.5E-9 + 4xTvco) x Fcomparison (Hz) x CP current (µA)
Leakage Current should never exceed 25% of the programmed CP current.
Figure 14. Charge Pump Gain, Trim and Phase Offset Control
Frequency Programming
The HMC701LP6CE can operate in either fractional mode or integer mode. In integer mode of operation the delta
sigma modulator is disabled. Frequency programming and mode control is described below.
Fractional Frequency
The fractional frequency synthesizer, when operating in fractional mode, can lock to frequencies which are fractional
multiples of the reference frequency.
Fractional mode is the default mode. To run in fractional mode ensure that dsm_integer_mode Reg12h<3> Table 29
is clear and dsm_rstb Reg01<13> Table 12). Then program the frequency as explained below:
The output frequency of the synthesizer is given by, f_vco, where
Fractional Frequency
of VCO
(EQ 13)
19
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