John,

Elsewhere in this thread there seems to have been some debate on the
operation
of the phase frequency detector. IMHO if the phase detector has a
tri-state
output then the loop over time must lock with no phase difference between
the
reference and controlled signals, ie 0 degrees.

Theoretically, the edge controlled PFD locks the signal & reference with 0
degrees phase error. When the loop is locked, the output pulses from the PFD
are theoretically infinitesimally small. However, neither the PFD chip nor
the external charge pump/loop filter are perfect. Any leakage in the charge
pump and/or loop filter causes the PFD to continually output wider than
normal pulses in order to supply the additional charge current necessary to
make up for the current leakage. This results in a non-zero phase error at
the PFD inputs.

I am struggling to understand the comment above that the AD9901 is not
suitable
for use in frequency synthesizers because of the large spurious sidebands
arising from its use. What causes the additional sidebands ?

As mentioned, the PFD output pulses approach zero width at lock, making it
easier to filter the pulses down to pure DC in the charge pump/loop filter.
The XOR PD will output a 50% duty cycle pulse train at lock. This is much
more difficult to filter down to pure DC, resulting in modulation of the
VCO, and thus sidebands.

Joe
W3JDR



"John Crabtree" wrote in message
...
Hello All

Rick Karlquist. N6RK, on 5/28/04 wrote:
Everyone please read the 11C44 datasheet at:

http://ira.club.atnet.at/rd/11c44/11C44.html

before declaring there is no dead zone. See figure 11.

Rick N6RK

"Avery Fineman" wrote in message
...
In article B_Htc.4677$pt3.1214@attbi_s03, "Rick Karlquist N6RK"
writes:

There are various fixes for the dead zone problem.
In the mid-1970's, Fairchild (the original company)
sold an "11C44" phase detector that got rid of the
dead zone by injecting a narrow pulse so that the
phase detector pulses would never have to try to
go to zero width. Eric Breeze holds the patent
on this technique; if interested read his patent.
Analog Devices makes that AD9901 phase detector
which gets around the dead zone by first dividing
the frequency by 2. However, it is not suitable
for a frequency synthesizer because of the large
spurious sidebands resulting from this technique.
SNIP

See also 'The PLL Dead Zone and How to Avoid it', A. Hill & J. Surber, RF
Design, March 1992, pp131-134. The authors were with Analog Devices and
compared the performance of the AD9901 with that of the MC4044.

My understanding of the AD9901 is that it behaves as a edge controlled
frequency detector, until it gets close enough in frequency, when it
switches
to being an EXOR phase detector.

I am struggling to understand the comment above that the AD9901 is not
suitable
for use in frequency synthesisers because of the large spurious sidebands
arising from its use. What causes the additional sidebands ?

Elsewhere in this thread there seems to have been some debate on the
operation
of the phase frequency detector. IMHO if the phase detector has a
tri-state
output then the loop over time must lock with no phase difference between
the
reference and controlled signals, ie 0 degrees.

73 John KC0GGH