EMRFD Message Archive 765
Message Date From Subject 765 2007-05-12 14:53:03 Ed Almos Switching Time of Phased Lock Loop If I have a transceiver that uses a PLL for frequency control then it
will need to change frequency very quickly. As an example, If I work
'split' on full break in then the PLL needs to change from the receive
frequency to the transmit frequency within the time period of a morse dot.
A DDS such as the AD9851 will do this very quickly but a PLL will be
slower because of the loop filter.
How can I measure the amount of time a PLL will take to change from
one frequency to another and what sort of switching speeds should I
expect to see?
Edward Almos HA6SST
766 2007-05-13 15:33:18 Allison Parent Re: Switching Time of Phased Lock Loop 767 2007-05-13 18:46:33 Andy Re: Switching Time of Phased Lock Loop > How can I measure the amount of time a PLL will take to change fromPut scope on VCO control voltage, trigger scope on the digital inputs that
> one frequency to another
change the PLL's frequency.
Do it several times, and use various frequency steps, in both directions.
The loop is nonlinear, and the response is not necessarily 100% repeatable.
You could also look at the phase error (phase detector's output), to see
when the phase error (which is related to frequency error) has settled to
> what sort of switching speeds should IThat could be anything! It depends on the loop's characteristics, which in
> expect to see?
turn are usually related to the reference frequency (input to phase
detector). The latter might be anywhere from Hz to MHz, so the switching
speed could be anything over several orders of magnitude.
> If I workInstead of one PLL, you could have two and switch between them.
> 'split' on full break in then the PLL needs to change from the receive
> frequency to the transmit frequency within the time period of a morse dot.