In a message dated 3/1/2007 13:30:26 Pacific Standard Time, [EMAIL PROTECTED] writes:
I had never thought about relative performance issues of using a VCXO locked with a really narrow band PLL to a lower frequency reference versus a multiplier with a narrow band cleanup filter at the output... other than to realize that unless one uses a more complex PLL design really narrow band loops implemented extremely straightforwardly (perhaps to the point of idiocy) require the higher frequency VCXO to be accurately on frequency within the low pass bandwidth else the loop won't capture. This gets a bit dicey if one is talking 1 Hz or less at 100 MHz. Hi Dave, this is not necessarily as complicated as you mention. If you use a simple Exor gate as the phase comparator with the 100MHz (or 1GHz) divided down to 10MHz, then there is no dependency between the loop bandwidth to the frequency of the VCXO. It will simply lock, no matter how slow the loop bandwidth (of course the bandwidth has to be faster than thermal, and aging effects on the VCXO since this is what you would like to compensate for with the PLL). The Exor gate won't have a problem in locking onto harmonics etc of the reference since the VCXO has a very small control frequency range. You do have to make sure that the VCXO adjustment range is sufficient to allow it to always lock to the reference, taking long-term crystal aging etc into account. For a sample VCXO that can be driven by a simple Exor gate as the phase comparator see also: _http://www.edn.com/contents/images/112703di.pdf_ (http://www.edn.com/contents/images/112703di.pdf) The cleanup filter is a simple second-order 25Hz cut-off RC, and it only has to remove the 10MHz phase comparison output from the Exor gate. No relationship between loop bandwidth and output frequency. bye, Said <BR><BR><BR>**************************************<BR> AOL now offers free email to everyone. Find out more about what's free from AOL at http://www.aol.com. _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts