Nice collection of articles they have there... lots of fun stuff if you navigate up a directory or two and look around.
I have a synthesizer that uses an LT1016 comparator to square up the 10.7 MHz signal coming out of a post-DDS crystal filter, prior to driving the reference pin of an Analog Devices PLL chip. The comparator tends to improve the loop's inband noise by a couple of dB by feeding the PLL a much stronger reference signal than it would otherwise get. After reading that article, I found myself wondering if there'd be any benefit to running an identical crystal filter between the comparator output and PLL reference pins. It seemed possible that the PLL's R-divider and phase detector might benefit from a band-limited input, since the comparator is a relatively-broadband high-gain device. It turns out not to help in this case ( http://www.thegleam.com/ke5fx/test.gif ). Seems to be little or no difference in the loop's inband noise regardless of the presence or absence of the post-comparator filter. I did verify that the (obviously mismatched) filter wasn't costing any signal amplitude. Because the PLL's input Z was so much higher than the comparator output Z, the peak-to-peak filter output voltage is actually slightly higher than its input voltage. So I guess this effect shows up only when you're dealing with an already-well-optimized divider chain, rather than a simple x10 R-divider and CMOS PFD... -- john, KE5FX > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > Behalf Of Dr Bruce Griffiths > Sent: Tuesday, February 06, 2007 1:32 PM > To: Discussion of precise time and frequency measurement > Subject: [time-nuts] Digital divider phase noise > > > For those who would like to improve the phase noise of the output of a > digital divider or who would like a good reason to using DDS instead see: > > http://tf.nist.gov/timefreq/general/pdf/1380.pdf > > Bruce > _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts