Hi expert ,
I had design a PFD(phase frequency detector) and Charge Pump(CP). PFD is conventional structure of three state PFD. The Charge Pump is also conventional structure which include a dummy current branch and actual current branch with unit gain buffer in order to reduce switch noise. Charge and discharege current is 100uA.
When simulation of PFD+CP , and load with 10pF Cap(CP's output port connect 10pF Cap to Ground), the function is right, even added with bond wire model(two cap and one inductor).
But ,we taped out the chip. During test the chip (PFD+CP) with 10pF Cap, the function(charge and discharge) is not right.(output DC level is not change with frequency difference change), CP's output only show some irregular sine waveform ,and amplitude is about 20-100 mV. DC level is fixed.
If excepted the bias problem for CP, do you advice any other problem for this result?
any suggestion, thanks a lot