A
Andrew Holme
Guest
I prefer to design PLLs with current output charge pumps, having equal
source and sink currents, where the loop filter is analysed as a
"trans-resistance" network i.e. current-in, voltage-out. I'm confused by
the 4046-style 3-state output. I find this far less intuitive than a
current output charge pump. It's not a simple voltage source, since part of
the time it's hi-Z. The source and sink currents are not equal, except when
the loop filter capacitor voltage is half-rail. Mathematically, how is it
justified to model this as a simple voltage source? Is there an assumption
about the VCO control voltage being half rail? Is the design really out by
a factor of 2 at the tuning range extremes?
TIA
Andrew.
source and sink currents, where the loop filter is analysed as a
"trans-resistance" network i.e. current-in, voltage-out. I'm confused by
the 4046-style 3-state output. I find this far less intuitive than a
current output charge pump. It's not a simple voltage source, since part of
the time it's hi-Z. The source and sink currents are not equal, except when
the loop filter capacitor voltage is half-rail. Mathematically, how is it
justified to model this as a simple voltage source? Is there an assumption
about the VCO control voltage being half rail? Is the design really out by
a factor of 2 at the tuning range extremes?
TIA
Andrew.