Re: Source of accurate frequency
- From: hal-usenet@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (Hal Murray)
- Date: Sat, 19 Jan 2008 03:05:19 -0600
In article <76bcd2dd-330e-47c8-9813-00fea9a4d6d1@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Peter Alfke <peter@xxxxxxxxxx> writes:
This is slightly off-topic:
I have mentioned before that I am in the process of designing (and
manufacturing in limited volume) a clock-generator box (1 Hz to 1.5
GHz in 1 Hz increments). We debated some jitter issues...
It's tough to get a very accurate reference xtal (2 ppm should be
possible, 20 ppm is easy)
I might include a simple calibration mechanism (any reasonable amount
of logic is free in the Virtex-5 FPGA), but from where do I get a very
accurate reference frequency input of any value ?
My two cents worth...
I assume your target market is pretty broad, some combination
of professionals/geeks with serious cash, hackers with time but no cach,
and professors/students and whatever in between.
A connector for an external clock is the obvious first step.
That lets you use whatever is available at the local environment.
Serious labs will have 10 MHz available from the wall next to the
110 V AC and ethernet.
Lots of lab equipment (for example counters) have a BNC jack
on the back for input of a reference clock andor output of
their internal oscillator. That sort of lab gear has good to
very good internal oscillators.
That's all pretty fancy/expensive. But it lets the geeks go
for the bleeding edge.
Most low cost digital crystal oscillator packages have 2
interesting error components. One is manufacturering offset.
The other is temperature. (Supply voltage is probably third,
but I'm not calibrated on that.)
The ballpark for tempertaure is 1 ppm per C. The ballpark for
manufacturing (at room temp) is 1/2 of the spec.
I'd expect you could get pretty close to a few ppm if you did
a calibration run at a known temperature on your final checkout
line and recorded the fudge factor.
You could do even better if you included a temperature sensor
near the local oscillator package and made a few test runs to get
a simple linear correction. (Maybe non-linear via table lookup
would be better. Mumble, TBD.)
I don't think you will get much from WWVB or WWV and friends
without a stable local clock to use as a refernce. They are
great for the long term, but you need a place to stand.
As others have suggested, ask Austin for high end ideas.
DDS type approaches are notorious for close-in spurs.
I think they are not interesting for longer term measurements.
(But lots of people are interestedin short term measurements.)
GPS is a good (low cost) straw man.
You can get GPS receivers with a PPS (pulse per second) output
for under $100. They are basically low cost commerical units
with an extra output pin.
You can get various boxes with a good crystal, a GPS input,
a pile of software, and ... They tend to be expensive
on the hobbyist scale.
An example:
http://jackson-labs.com/products_fury.html
Symmetricom (by way of Agilent) has Borged HP's old Cesium
clock business area and friends and lots of others.
http://www.symmetricom.com/Products
(I didn't run into their Flash crap until I got there.)
GPSDO is a good magic word: GPS Disciplined Oscillator.
GPS provides the long term stability. The local (good
crystal) provides the short and medium stability. Usually
there is a lot of software filtering involved.
HP pioneered that technology many years ago.
For more than you wanted to know:
http://www.hpl.hp.com/hpjournal/96dec/dec96a9.pdf
The Global Positioning System and HP SmartClock
December 1996 Hewlett-Packard Journal
http://www.realhamradio.com/GPS_Frequency_Standard.htm
(Z3801As were available cheap for a while as cell towers upgraded.)
HP app notes:
HP SmartClock Technology Application Note 1279
The Science of Timekeeping Application Note 1289
GPS and Precision Timing Applications Application Note 1272
(Ask me if google can't find them for you.)
Just for fun, and this needs a serious time-sink warning,
here is my all time favorite paper about time:
http://www.physicstoday.org/vol-59/iss-3/p10.html
Daniel Kleppner
Physics Today, March 2006, page 10
[Don't say I didn't warn you.]
--
These are my opinions, not necessarily my employer's. I hate spam.
.
- References:
- Source of accurate frequency
- From: Peter Alfke
- Source of accurate frequency
- Prev by Date: Re: Source of accurate frequency
- Next by Date: Re: Source of accurate frequency
- Previous by thread: Re: Source of accurate frequency
- Next by thread: Re: Source of accurate frequency
- Index(es):
Relevant Pages
|
