[time-nuts] Best way for generating 8994.03 MHz from 2899.00042272.....MHz?

Javier Serrano javier.serrano.pareja at gmail.com
Mon Aug 17 20:33:56 UTC 2009


Hi, I've been meaning to post a proper bird's eye view since John requested
it. It just took me some time because I am not the source of this
information. I have first to identify the application: low level RF control.
In particle accelerators, charged particles are accelerated in RF cavities
and it is very important to control the RF field inside the cavities
according to some set points. In this accelerator there are 23 S-band
cavities and 1 X-band cavity. Here's the S-band controller:



There is down-conversion of the S-input coming from the cavity, down to a
reasonable frequency for FPGA processing, then up-conversion to generate a
S-band signal to send back to the cavity, closing the loop.
And here's the X-band system:


As you see, it's basically a wrapper around the S-band system, so as to
reuse as much as possible from it. The X signal is at a frequency four times
that of the S signal. That's why he needs a 3*S, to use it in the
down-conversion and end up with 1*S to feed to the S-input. The box with a
question mark in the diagram is the subject of this thread. The 241.6 MHz is
generated remotely, i.e. no access to 2998.01 MHz in the X-system. There is
an X-reference (4*S) around, so taking that and multiplying by 3/4 is an
option, but he'd rather not touch that reference if possible.
He will be doing active calibrations so long term drift (something that
occurs at 10 Hz or slower) should be taken care of. High frequency jitter
(>1 MHz) is also not a big issue since he can average it out from his
measurements. What stinks is the phase noise around 10 kHz. That's a
problem. All these by the way apply to departures of the field in the cavity
wrt the x-reference.
I hope this gives some context, it's the least I could do for such good
help. This is a fantastic list. Thanks everyone!

Javier

On Mon, Aug 17, 2009 at 3:20 AM, Lux, Jim (337C)
<james.p.lux at jpl.nasa.gov>wrote:
....<snip>
>
> >> Is there actually a 2998.01 clock?  If so, why is a simple 3x PLL not
> the
> >> right answer?
> >>
> >> Perhaps the true master clock is actually S-LO at
> >> 2998.01*732/757, and it's
> >> just written that way to make all the ratios visible for reasons that
> are
> >> important when you look at some other part of the problem.
> >
> > These questions IMHO are exactly the right ones to ask, before
> speculating
> > on unconventional topologies and complicated block diagrams.  A
> birds'-eye
> > view of the overall conversion scheme would be helpful, assuming it's not
> > proprietary.
>
> You see that all the time, where you go through gyrations to locally
> optimize because of a perception that the other parts of the system are
> cast
> in concrete.
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/jpeg
Size: 26714 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20090817/903ca8d9/attachment-0002.jpeg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: image/jpeg
Size: 39597 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20090817/903ca8d9/attachment-0003.jpeg>


More information about the time-nuts mailing list