Source of accurate frequency

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 ?

Decades ago, the major TV stations kept the color subcarrier very
accurate (much better than 0.01 ppm), but that service has ben
abandoned. WWV needs a short-wave receiver, or at least a good
selective amplifier.
I am asking this smart group for some creative ideas. GPS ?
I indicate the frequency on a 9-digit display, so it would be
desirable to be able to guarantee better than 1 ppm (after calibration
for at least a few days. Temperature drift is not a big issue in a lab
instrument...)
Peter Alfke

>Decades ago, the major TV stations kept the color subcarrier very
>accurate (much better than 0.01 ppm), but that service has ben
>abandoned...

Some radio carriers are maintained to very high accuracy for use as a
frequency reference.  E.g. the BBC long wave transmission (previously
200kHz, now 198kHz) is controlled by a rubidium clock.  The audio
modulation is AM but there is also a low-bandwidth phase modulation
used to broadcast data to electricity meters (for control of overnight
load-shedding).  I don't know if that phase modulation would be a
problem for your application.

I'd guess that the signal is useable over all of western Europe. I
recall that when I stayed about 200km east of Moscow, I had forgotten
how to set my radio-controlled alarm clock to run only from the
internal crystal, but in the morning it had set itself from the 60kHz
transmitter in the UK.

Perhaps there is something similar in the USA.

http://en.wikipedia.org/wiki/Droitwich_transmitting_station
http://en.wikipedia.org/wiki/Radio_teleswitch

Mike

On Fri, 18 Jan 2008 14:49:46 -0800 (PST), 
Peter Alfke <peter@xilinx.com> wrote:


>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 ?

I don't know what happens in the USA, but here in the UK we have
a timecode broadcast service (MSF) that provides a very stable
60kHz carrier, modulated with some timecode stuff that keeps
clocks up-to-date.  Because the frequency is so low, coverage
is excellent and receivers very simple.  If you're happy to
do the calibration over many hours, that might be all you need.
An A-D converter and synchronous demodulation sounds like an
afternoon's work for you guys - you could use it as a demo
of a high-resolution delta-sigma A/D...  only a ferrite-rod
antenna needed.  Add a temperature sensor near your oscillator
and then you could, over time, calibrate its temperature 
coefficient, enabling you to keep it very accurate even if
you lose the time code signal.

I can easily imagine such a thing being subverted by nearby
CRT monitors or other sources of spurious-ness with components
close to 60kHz.  I still carry the scars of attempts to build 
readers for the TIRIS RF-ID tags, which worked at around 130kHz
and were pretty much inductively coupled.  That frequency was 
nicely at the second harmonic of the line scan rate of VGA 
monitors, which made the whole thing fail horribly if it 
was within a couple of yards of any cheap-and-nasty monitor.

Still, you could use synchronous demodulation to establish
a *very* narrow receiver bandwidth.
-- 
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
jonathan.bromley@MYCOMPANY.com
http://www.MYCOMPANY.com

The contents of this message may contain personal views which 
are not the views of Doulos Ltd., unless specifically stated.

Jonathan Bromley <jonathan.bromley@mycompany.com> wrote:
> On Fri, 18 Jan 2008 14:49:46 -0800 (PST), 
> Peter Alfke <peter@xilinx.com> wrote:


> >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 ?

> I don't know what happens in the USA, but here in the UK we have
> a timecode broadcast service (MSF) that provides a very stable
> 60kHz carrier, modulated with some timecode stuff that keeps
> clocks up-to-date.

In the US the same service comes from WWVB in colorado:

   http://tf.nist.gov/stations/wwvb.htm

and clock devices that use it as a time reference are fairly common,
though these typically only preform a time sync once a day (typically
at night when propagation is better).  In fact I'm wearing a Casio
watch that's both solar powered and gets the time nightly from WWVB
so I never have to think about it and I always have sub-second accurate
time on my wrist.

But I think GPS is generally the right solution for this problem these
days.  There's a whole sub-genre of the GPS community (and devices from
manufacturers) that only care about when rather than where.  Once you
get out of the consumer stuff, receivers that output a very accurate
1 pps (pulse per second) are common, and you can get any level of 
exotic time keeping beyond this that you have the money to pay for :-)

G.

On Jan 18, 4:49 pm, Peter Alfke <pe...@xilinx.com> wrote:
> 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 ?
>
> Decades ago, the major TV stations kept the color subcarrier very
> accurate (much better than 0.01 ppm), but that service has ben
> abandoned. WWV needs a short-wave receiver, or at least a good
> selective amplifier.
> I am asking this smart group for some creative ideas. GPS ?
> I indicate the frequency on a 9-digit display, so it would be
> desirable to be able to guarantee better than 1 ppm (after calibration
> for at least a few days. Temperature drift is not a big issue in a lab
> instrument...)
> Peter Alfke

You can get reference clocks that are locked to GPS.  Here are a few
links:

http://ngn.symmetricom.com/products/primary_reference_sources/index.asp
http://www.trimble.com/tmg_thunderbolt.shtml
http://cgi.ebay.com/Thunderbolt-GPS-Disciplined-Clock-Trimble-Precise-clock_W0QQitemZ300191164048QQihZ020QQcategoryZ25399QQssPageNameZWDVWQQrdZ1QQcmdZViewItem


If you want to Google for more, search on "primary reference clock" or
"telcom primary base reference".

It is pretty common to find them with a 10MHz reference clock output,
IRIG time code, and a one PPS signal.  They discipline an internal
oscillator to the GPS clock, which is locked to an atomic clock. 10
MHz is also a common reference clock for frequency generators.



Regards,

John McCaskill
www.FasterTechnology.com

On 18 Jan., 23:49, Peter Alfke <pe...@xilinx.com> wrote:
> 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)

Peter,

for our time to digital converters we use temperature compensated
oscillators  (TCXOs) with 1 ppm.
We pay a premium because we have custom frequencies cut for us, but
for standard frequencies
in higher quantities the price should be below 10$.
These devices are available from many manucatures in 7mm x 5mm SMD.

I recently saw a new TCXO that only has 100ppb temperature drift.

Below that you need OCXOs. The packages of these are a bit larger,
they are more expensive
and draw quite a lot of current (500mW to 5W). But you can get them
down to 5ppb.

Another option is to lock the clock to GPS. There are boxed solutions
for that available.

Have fun,

Kolja
www.cronologic.de

On Jan 18, 2:49=A0pm, Peter Alfke <pe...@xilinx.com> wrote:
> 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 ?
>
> Decades ago, the major TV stations kept the color subcarrier very
> accurate (much better than 0.01 ppm), but that service has ben
> abandoned. WWV needs a short-wave receiver, or at least a good
> selective amplifier.
> I am asking this smart group for some creative ideas. GPS ?
> I indicate the frequency on a 9-digit display, so it would be
> desirable to be able to guarantee better than 1 ppm (after calibration
> for at least a few days. Temperature drift is not a big issue in a lab
> instrument...)
> Peter Alfke

On Jan 18, 2:49=A0pm, Peter Alfke <pe...@xilinx.com> wrote:
> 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 ?
>
> Decades ago, the major TV stations kept the color subcarrier very
> accurate (much better than 0.01 ppm), but that service has ben
> abandoned. WWV needs a short-wave receiver, or at least a good
> selective amplifier.
> I am asking this smart group for some creative ideas. GPS ?
> I indicate the frequency on a 9-digit display, so it would be
> desirable to be able to guarantee better than 1 ppm (after calibration
> for at least a few days. Temperature drift is not a big issue in a lab
> instrument...)
> Peter Alfke

(I wish I didn't have to use google groups to access from work...)

Peter,

Your own austin has quite a background in stable timing so he can
probably provide some good lunchtime conversation.  The specs I've
seen on GPS timing references give 1 pulse per second outputs accurate
to within 100 ns.  Perhaps this accuracy is better than those many
years ago when I was actively looking at the specs but it gives you an
idea of the accuracies you'd need to work with.  GPS-trained frequency
sources use clean local oscillators to smooth out the uncertainty and
provide good accuracy under signal dropout conditions.  For very high
accuracy phase stuff, the Allan variance can come into play (again,
seek guidance from austin).

It may be that for measurement accuracy, the low Allan variance isn't
a necessity; I don't have an appreciation for the scale of the
problem, only the problem itself.  Heck - even rubidium oscillators
have close-in phase noise issues that are averaged out with external
help.  Just this week I've been demodulating jitter and watching cheap
oscillators changing frequencies on a whim, changing from one
relatively stable value to the next.

The jitter generator I produced a decade ago went with a small OCXO
from www.mti-milliren.com that had nice close-in phase noise
characteristics within a reasonably small package but it was still a
$50 instrumentation solution.

Both a GPS reference and an OCXO will probably be a chunk of the
device cost and consume more power than you'd want for batteries.

A quick discussion with an apps engineer from someplace like MTI might
get you more precise information that from the lovely engineers that
frequent this group.

Let us know when we can order one of your devices!
- John_H

"Peter Alfke" <peter@xilinx.com> wrote in message 
news:76bcd2dd-330e-47c8-9813-00fea9a4d6d1@v67g2000hse.googlegroups.com...
> Decades ago, the major TV stations kept the color subcarrier very
> accurate (much better than 0.01 ppm), but that service has ben
> abandoned. WWV needs a short-wave receiver, or at least a good
> selective amplifier.
> I am asking this smart group for some creative ideas. GPS ?
> I indicate the frequency on a 9-digit display, so it would be
> desirable to be able to guarantee better than 1 ppm (after calibration
> for at least a few days. Temperature drift is not a big issue in a lab
> instrument...)

Other posters have mentioned the excellent GPS-based solutions available, 
and/or WWV(B) options. Here are two others:
1) The TV carriers are still often dead-on in frequency, as they are now 
locked to GPS.
2) The Loran-C system (100 kHz pulses) is being recapitalized with precise 
(GPS-locked) references as a backup to GPS against potential jamming.

I've been very happy with my Symmetricom frequency references; for a 
small/cheap solution, SigNav is now selling a GPS receiver based on the old 
Motorola Oncore series (interface compatible) that provides a precise 10MHz 
output. Trimble also sells something called the Micro-T with similar 
capability. Both I believe are in the ~$200 range for an OEM, but prices 
have been dropping. When locked to GPS, these jobbies can give you better 
than 10 ppb accuracy.

Marty 

On Jan 18, 8:02=A0pm, "Marty Ryba" <martin.ryba.nos...@verizon.net>
wrote:
> "Peter Alfke" <pe...@xilinx.com> wrote in message
>
> news:76bcd2dd-330e-47c8-9813-00fea9a4d6d1@v67g2000hse.googlegroups.com...
>
> > Decades ago, the major TV stations kept the color subcarrier very
> > accurate (much better than 0.01 ppm), but that service has ben
> > abandoned. WWV needs a short-wave receiver, or at least a good
> > selective amplifier.
> > I am asking this smart group for some creative ideas. GPS ?
> > I indicate the frequency on a 9-digit display, so it would be
> > desirable to be able to guarantee better than 1 ppm (after calibration
> > for at least a few days. Temperature drift is not a big issue in a lab
> > instrument...)
>
> Other posters have mentioned the excellent GPS-based solutions available,
> and/or WWV(B) options. Here are two others:
> 1) The TV carriers are still often dead-on in frequency, as they are now
> locked to GPS.
> 2) The Loran-C system (100 kHz pulses) is being recapitalized with precise=

> (GPS-locked) references as a backup to GPS against potential jamming.
>
> I've been very happy with my Symmetricom frequency references; for a
> small/cheap solution, SigNav is now selling a GPS receiver based on the ol=
d
> Motorola Oncore series (interface compatible) that provides a precise 10MH=
z
> output. Trimble also sells something called the Micro-T with similar
> capability. Both I believe are in the ~$200 range for an OEM, but prices
> have been dropping. When locked to GPS, these jobbies can give you better
> than 10 ppb accuracy.
>
> Marty

Thank you, guys, for all the advice arriving in just a few hours.
I remember Droitwitch 200 kHz from my parents' (highly illegal)
listening to the BBC during WW2.
It covered most of Germany (!) very well, but is obviously silent here
in the US.
The 60 kHz timing transmissions in Europe and here from Fort Collins,
Colorado are rather slow, and burdened with their data modulation.
I like the GPS-based solutions from SigNav and Trimble best.
I have no intentions of burdening the box with a >>$10 precision
oscillator, I rather provide a separate calibration capability, for
manufacturing and for ultra-demanding users, where a few hundred
dollars do not matter.
The nicest solution would be over the internet, but I have not heard
about anything like that (yet).
I will keep you informed about our progress.
Peter Alfke