Hello,

I experienced some problems with lock detect at -20C to -40C for PLL HMC840LP6CE.

The lock detect window after the calculation have to be near 7ns (by old user guide) to answer for all temperature span but PLL is not locked , even if I move the window to 7.1[nS] the pll is locked.

I found that in HMC832 hittite suggest Eq for LD window in fractional mode and it looks that this Eq working more precisely at HMC840 too, please suggest if this Eq can be used cross all the family for Hittite PLL , specially at HMC840.

I added picture with equation for HMC832.

Thank you.

Both parts use the same lock detect circuit. The lock window size must be set mid-way between a minimum value and 1/Fpd (where Fpd is the phase detector frequency). In integer mode the min value is 0. In fractional mode the min value is:

(Ioff/Icp)*(1/Fpd) + 4*(1/Fvco).

where Ioff is the offset current and Icp is the charge pump current.

You can use the HMC832 equation to set the min value for the HMC840. This is exactly the above equation but with Fvco=1500MHz (HMC832 fundamental VCO range is 1500MHz to 3000MHz) which results in "large" min value for the HMC840 (fundamental VCO range is 2620MHz to 2830MHz). The mid-way point is set slightly larger with the HMC832 equation which is fine as long as the window does not exceed 1/Fpd.