[time-nuts] How far can I push a crystal?

[Ed Breya]

I tried to send this message on Sunday, but for some reason it didn't go 
through, so here it is again. Please excuse any redundancy if the 
original shows up. I will have an update of the project soon.

Hi Bob L.,

Your suggestion of the 300/953 scheme was inspiration for what hopefully 
will be the simplest solution of all - I've started building it. First, 
I should clarify more, that the original scheme actually has three 
phase-locked loops - a 10.7 MHz, a 10.05944444 MHz, and the final one, 
1207.1333333 MHz. The last one is a PLO brick that just multiplies any 
RF input by any n within reason to phase lock the microwave output (to 
nth harmonic of input). I wasn't counting that one, since it's more or 
less a fixed function, but it's a variable (arbitrary n) in the numbers 
game. So, when I was referring to getting rid of one PLL, I meant not 
needing to produce the intermediate 10.7 MHz, since the "953" gives a 
rational number solution directly from 1 or 10 MHz - this is the "single 
PLL" scenario.

I tested the PLO and microwave section with 15.883333333 MHz = (10 
MHz/600)*953 from a synthesizer, and it worked just fine. The PLO is 
tuned near 1207 MHz, and uses whatever n lands it within lock range, so 
n=76 in this case. If you adjust the cavity, n can just as easily be 75 
or 77, with different output frequencies, or a number of numbers that 
satisfy the bounds of operation. So, the trick is to produce that one 
"correct" frequency from the 10 MHz reference, cleanly enough to get the 
job done, and feed it to the PLO - the n value takes care of itself.

The way it's partitioned now, I will have one can containing the 
15.8833333 MHz VCXO (74HC86 and a 16 MHz ceramic resonator), two LAN 
LPFs, a 74HC4020 feedback divider (1/953), and a CD4046B PLL. A second 
can, which is needed anyway for handling the various external and 
internal 10 and 1 MHz references, will not only route and scale, but 
will also include the divider to make the 16.66666 kHz (10 MHz/600) 
reference for the other box.

So, the overall synthesis chain is (10 MHz/600)*953*76=1207.133333 MHz. 
Pretty simple.

Ed