In a message dated 12/19/2007 11:54:37 Pacific Standard Time, boyscout@gmail.com writes: >Yes, I understand all the math. I just don't know what to expect from >the 10 MHz references people will plug in. Clearly if they plug in a >Fury, I should use a wide bandwidth, but who knows what else they'll >use... >Thanks, >Matt Hi Matt, here is an idea: instead of switching the control voltage using a Relay or analog switch etc (which may introduce more noise, and instabilities etc), why not switch the 10MHz reference input using a low-jitter digital multiplexer? Then you could use a cheap 10MHz Oscillator with scaled phase-noise every so slightly better than your 100MHz oscillator to discipline the 100MHz oscillator in the absence of an external 10MHz source. No analog reference voltage required, and the overall noise performance will be better than the 100MHz oscillator by itself. By measuring the AC control energy in the PLL loop, and comparing this when using your known 10MHz source versus the external unknown 10MHz source you may be able to make a qualitative judgement about the external 10MHz source (similar to a 3-hat measurement). Then again it may be hard to figure out if the noise is coming from the 10MHz or the 100MHz oscillator.. A 10MHz oscillator that exceeds your 100MHz oscillator's (scaled) phase noise spec should be easy to find and inexpensive (DIP-14 type oscillator etc). bye, Said **************************************See AOL's top rated recipes (http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004)