PLL losing locking at low temperature

Hi,all
We recently design a pll using hmc704,but have some problems below low temperature(-45℃), the pll can?t lock.
the output frequency of vco is 6.49~6.9GHz and we want a frequency of 6.65GHz, the pll works proper at room temperature and high temperature
So we open the loop to debug，the structure is shown below.

we change the frequency by the VT,and test the output voltage(CP) by multimenter
when the VT =0 V (6.49GHz) the output voltage is 0 V (hmc704 Reg0Bh[4] =1)
normally , the output voltage is always 0 V below 6.65GHz
but, at low temperature(-45℃),
when the VT =0.2 V~0.4V (6.54GHz~6.56GHz) the output voltage is 5 V,it's abnormal.
at the same time ,we find the Noise of VCO get worse on the spectrum(more than 10dB, span 5MHz)
but if we turn off the reference signal(100MHz),the Noise of VCO return to normal

Now we can not find the reason. hmc704? VCO? others?
why?
Really want to get your help.

Your -45°C temp is outside of the operating range of the 704. Probably your VCO also.

maybe，but why？ if I test the vco or hmc704 independently，they are ok.
I can't find which index is abnormal ?

PLL circuits need a certain RF level at their inputs for a proper operation.If your VCO output level drops radically at low temperature ( it's usually so ) PLL wouldn't track the signal.
If you adjust the attenuator to augment the signal level, it may work.