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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 |
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