HMC700LP4 Datasheet, PDF (16/36 Pages) Hittite Microwave Corporation – 8 GHz 16-Bit Fractional-N PLL

v07.0411
HMC701LP6CE
8 GHz 16-Bit Fractional-N PLL
PFD Lock Detect
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lkd_en (Reg01h<11> Table 12) enables the lock detect functions of the HMC701LP6CE.
The Lock Detect circuit in the HMC701LP6CE places a one shot window around the reference. The one shot window
may be generated by either an analog one shot circuit or a digital one shot based upon an internal ring oscillator timer.
Clearing lkd_ringosc_mono_select (Reg1Ah<14> Table 36) will result in a nominal ±10nsec ‘analog’ window of fixed
length, as shown in Figure 11. Setting lkd_ringosc_mono_select will result in a variable length ’digital’ widow. The
digital one shot window is controlled by lkd_ringosc_cfg (Reg1Ah<16:15>). The resulting lock detect window period is
then generated by the number of ring oscillator periods defined in lkd_monost_duration Reg1Ah<18:17> (Table 36).
The lock detect ring oscillator may be observed on the GPO2 port by setting ringosc_testmode (Reg1Ah<19> Table
36) and configuring the gpo_sel<3:0> = 0111 in (Reg1Bh Table 37). Lock detect does not function when this test mode
is enabled.
lkd_wincnt_max (Reg1Ah<9:0> Table 36) defines the number of consecutive counts of the VCO that must land inside
the lock detect window to declare lock. If for example we set lkd_wincnt_max = 1000 , then the VCO arrival would
have to occur inside the selected lock widow 1000 times in a row to be declared locked. When locked the Lock Detect
flag ro_lock_detect (Reg1Fh<0> Table 41) will be set. A single occurrence outside of the window will result in clearing
the Lock Detect flag, ro_lock_detect.
The Lock Detect flag ro_lock_detect (Reg1Fh<0> Table 41) is a read only register, readable from the serial port. The
Lock Detect flag is also output to the LD_SDO pin according to lkd_to_sdo_always (Reg1Ah<13>) and lkd_to_sdo_
automux_en (Reg1Ah<12>), both in Table 36. Setting lkd_to_sdo_always will always display the Lock Detect flag
on LD_DSO. Clearing lkd_to_sdo_always and setting lkd_to_sdo_automux_en will display the Lock Detect flag on
LD_SDO except when a serial port read is requested, in which case the pin reverts temporarily to the Serial Data Out
pin, and returns to the lock detect function after the read is completed.
Figure 11. Normal Lock Detect Window
Lock Detect with Phase Offset
When operating in fractional mode the linearity of the charge pump and phase detector are more critical than in
integer mode. The phase detector linearity is worse when operated with zero phase offset. Hence in fractional mode
it is necessary to offset the phase of the reference and the VCO at the phase detector. In such a case, for example
with an offset delay, as shown in Figure 12, the mean phase of the VCO will always occur after the reference. The
lock detect circuit window can be made more selective with a fixed offset delay by setting win_asym_enable and
win_asym_up_select (Reg1Ah<11> Table 36). Similarly the offset can be in advance of the reference by clearing
win_asym_up_select while leaving win_asym_enable Reg1Ah<10> set both in Table 36.
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