aaron_do
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Hi all,
i'm designing a tuning circuit for my bandpass filter. I have a reference frequency which i feed into a replica of the filter, Vref. I take the output, Vout, and use a PFD (phase frequency detector) to compare its phase to Vref. Then I have a charge pump and a loop filter. I'm simulated extensively in the open loop, and it seems the loop should work. Unfortunately i noticed that if i input a pulse into the control voltage of the filter, the filter takes about 2 us to start up. I figure the filter only works around the bias point of about 600 mV. i.e. from say 550 mV to 650 mV. So i reduced the loop bandwidth to around 10 kHz and it seemed to work. The frequency was offset by about 60 kHz for the 2 MHz filter. Does this result seem correct?
Any better ideas on a tuning circuit?
thanks, aaron
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