[time-nuts] Re: HP 10811-60111 Oscillators {External}

[Lux, Jim]

On 1/11/23 8:55 AM, Bob Camp via time-nuts wrote:
> Hi
>
> There always has been a pretty wide range of ADEV’s on the 10811’s. One of the fun things
> they did at Santa Clara was to screen them for ADEV. Once they found enough “good ones”
> for that month, they stopped screening. The ones that made the grade got this or that part number
> sufix on them and headed into gear that needed them.


Cherry picking oscillators is a "standard thing" (although one that you 
hope you'd not have to do).

My project is in the middle of looking at oscillators in an X-band 
transmitter for phase noise/short term ADEV.

It turns out that DSN only has narrow symbol loop bandwidths (up to 25 
Hz) and these transmitters (2 Mbps) weren't designed with that in mind, 
so they'll lose symbol lock occasionally. In a 400 Hz loop bandwidth 
there's no problem.   But DSN was designed and built for 10 bits/second, 
insanely tight loop bandwidth to get the noise down.

And, of course, it's just on the ragged edge.  Some work, some don't. Of 
course, the crystal mfr (I have no idea who it is, that's 3 subcontract 
tiers down) probably doesn't specify anything - maybe they have an 
overall jitter spec with some wide bandwidth.  And all the usual things 
- how do you measure it without opening up the transmitter? How do you 
set up a test jig? What is the "real requirement"?

Had we realized this earlier, it's conceivable we could have flowed a 
requirement down to the radio mfr, who would then flow that to the board 
designer, who would flow that to the crystal company.  And turn $20 
crystals into $2000 crystals, because the mfr would probably have to do 
the cherry picking, and the yield would be terrible. (Of course, they 
could sell the other ones per usual, but when you're ordering 10-20 
crystals, and you have to screen 1000 to get the good 10, it might not 
be worth even saving the ones that don't make the cut. They're now out 
of the usual flow.

Another strategy would be for the radio mfr to make lots of boards, with 
crystals as they come, and sort those.  That might actually be cheaper - 
I'll bet the "as mfr" cost of the boards is <$150, which is a lot less 
than $2000. and the "reuse" is easier.

This whole oscillator business is MUCH more complicated than a lot of 
people think.