In the Phase frequency detector of PLL, we should get up and down
signal --narrow/broad depending on the phase / freq dfference... but I
am getting spikes at a constant interval in the output up and down
signals... what should I make of it??
any suggestions??
Thanks
In practice any real charge pump can only produce pulses down to a certain
minimum width. Below that width, the peak current of the pulse is never
achieved, and the relationship between pulse width and integrated charge in
the pulse becomes non-linear. It is desirable always to operate in the
region where the charge pump is linear, particularly in Fractional-N
synthesisers. In order to achieve this, a certain minimum time is often
applied to both the up and down currents. Provided the up and down
currents are well matched, the only disadvantage of the scheme is slightly
increased noise, but this is usually small in comparason to the thermal
noise in the loop filter resistors etc.
Chris