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hmc704 losing lock at low temperature

Question asked by hgb1648 on Sep 6, 2017
Latest reply on Sep 11, 2017 by dyoung1
Hi
   We recently design a pll using hmc704,but have some problems at low temperature(-10~-55℃), the pll can‘t lock.
the output frequency of vco is 6.4~6.9GHz and we want a frequency of 6.6GHz, 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 by multimenter
when the VT =0 V (6.49GHz)  the output voltage is 0 V   (Reg0Bh[4] =1)
normally , the output voltage is always 0 V below 6.6GHz
but, at low temperature,
when the VT =0.2 V~0.4V (6.45GHz~6.5GHz) (changes a little as temperature) 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?
we want to know that the hmc704  works proper?
or the requirements for input signals of hmc704,the power?,wave form?

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