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Frequency Standard - Rubidium or GPS?

B

bart

Jan 1, 1970
0
Hi all,

I'm looking for some sort of frequency standard (10 MHz ref.?).

Ebay has rubdium frequency standards for under $100.00
and there are also GPS disciplined OXCO frequency standards for
~$150.00 .

My question :
Is a 10-20 year old rubidium standard more accurate (even with aging
drift) that a newer GPS disciplined OXCO?

I can't afford a GPS corrected rubidium standard ( ~$700.00+).

I just want to recalibrate my so-so frequency counters .. to hopefully
within 10 Hz..?

Opinions?

Thanks for reading! :)

Cheers!
Bart
 
J

Jim Yanik

Jan 1, 1970
0
Think of the GPS disciplined ones as being like an old fashioned AC wall
clock--the phase can go all over the place, but over a long period the
frequency will be extremely accurate. Rubidium standards have really
good short term stability compared even with caesium ones. For
calibrating a counter, you're better off with the GPS one, but for
making really low phase noise signals, the rubidium is better.

Cheers,

Phil Hobbs

I'm surprised no one has cooked up an inexpensive WWVB receiver/freq
reference,for hobbyist uses.I used a Spectracom WWVB unit while at TEK.
Of course,that had a chart recorder to track the ref.osc. drift.
 
G

GregS

Jan 1, 1970
0
There have to be some construction projects in old ham radio
magazines.

After NIST upgraded the transmitter site for WWVB, its easy to get a
good signal in Central Florida. Recover the 60 KHz carrier. Divide it
by three to get 20 KHz. Divide that by two for a 10 KHz signal, then
phase lock it to a 10 MHz OCXO. I have a spare from a damaged HP 5245L
that I will use.

I used to look at the time changing from the Colorado station
compairing it to the stations Cesium standard at Goldstone Apollo.
That was for getting the right second. Ground Loran D was the prime
day to day reference. That was tricky and not very good anyway.
The station had Rubidium and Crystal backups, with the last being wristwatch.
When i first started working there there was a morse code chart on the
receiver console ?? I love backups. I really hated hearing the Sonalert
go off from the Collins time machine and figure out whats wrong.
That was when sonalerts were not common, but when I went
to McDonalds and that thing went off, I would jump.
I still have that frequency counter I calibrated from the Collins equipment,
and I added a crystal heater before that. Cheap counter at best. I'm sure the crystal aged.
The hams used to use the color subcarrier back then for calibrations broadcasted live.


greg
 
R

Rich Grise

Jan 1, 1970
0
I'm looking for some sort of frequency standard (10 MHz ref.?).

Ebay has rubdium frequency standards for under $100.00
and there are also GPS disciplined OXCO frequency standards for
~$150.00 .

My question :
Is a 10-20 year old rubidium standard more accurate (even with aging
drift) that a newer GPS disciplined OXCO?

I can't afford a GPS corrected rubidium standard ( ~$700.00+).

I just want to recalibrate my so-so frequency counters .. to hopefully
within 10 Hz..?

Opinions?

Well, for what it's worth, my $10.99 plus tax wall clock is sync'ed to
WWVB; I'm sure there's a 1 Hz pulse in there somewhere. :)

I'd imagine it would make a pretty good reference if you wanted to wait,
say, 60 seconds or so to get a good reading.

Cheers!
Rich
 
S

spamme0

Jan 1, 1970
0
Rich said:
Well, for what it's worth, my $10.99 plus tax wall clock is sync'ed to
WWVB; I'm sure there's a 1 Hz pulse in there somewhere. :)

I'd imagine it would make a pretty good reference if you wanted to wait,
say, 60 seconds or so to get a good reading.

Cheers!
Rich
I'd like to see the math on how much precision you're gonna get out
of sixty samples of a 1hz clock from a $11 wall clock.
 
D

David L. Jones

Jan 1, 1970
0
bart said:
Hi all,

I'm looking for some sort of frequency standard (10 MHz ref.?).

Ebay has rubdium frequency standards for under $100.00
and there are also GPS disciplined OXCO frequency standards for
~$150.00 .

My question :
Is a 10-20 year old rubidium standard more accurate (even with aging
drift) that a newer GPS disciplined OXCO?

I can't afford a GPS corrected rubidium standard ( ~$700.00+).

I just want to recalibrate my so-so frequency counters .. to hopefully
within 10 Hz..?

Opinions?

Thanks for reading! :)

Cheers!
Bart

The rubidium oscillator will likely give you the best short term stability,
which is really what you are after for your purpose.
If you get a rubidium oscillator (or any other reference source), you'd
simply leave it permanately connected to the external 10MHz reference input
on your counter. No need "recalibrate" and use your internal crystal any
more.
Rubidium aging would fairly negligable for your purposes (in the order of
10^-10/year IIRC)

Dave.
 
M

Martin Brown

Jan 1, 1970
0
There are several off air frequency standard designs that fairly
regularly get recycled in the various magazines. A bit like egg timers.
There's a *mostly* complete WWVB antenna/receiver project at
http://lakeweb.com/rf/wwvb/. It's a minimal receiver design that would
appear to work, although I've never known anyone other than the author who
actually built one. The antenna is the main focus of the site, although the
receiver is there in schematic form. The posted design uses a 6 MHz VXCO as
the disciplined element, but a 10 MHz unit could easily be substituted with
appropriate changes in the dividers.
There's another loop antenna design at
http://www.febo.com/time-freq/wwvb/antenna/index.html if you like to
experiment.

In the UK at least it is trivial to take a long wave 200kHz coil and
slug it with a bit more capacitance to tune for either 60kHz Rugby or
77kHz DCF77. Although monolithic chips are cheaply available a discrete
receiver is easy enough - even the original transistor radio would do.

A reasonably helpful generic page is online at:
http://www.timetools.co.uk/support/dcf-signal-coverage.htm

Using the Rugby off air signal to synchronise local Rb oscillators was
used in the late 1970's by Duffet-Smith et al to provide local
oscillators for remote VLBI stations at low frequencies.

ISTR they could easily detect the presence or abscence of dew at the
Rugby transmitter from the diurnal variation of phase errors.

Regards,
Martin Brown
 
R

Rich Grise

Jan 1, 1970
0
I'd like to see the math on how much precision you're gonna get out of
sixty samples of a 1hz clock from a $11 wall clock.

Just because it's cheap doesn't automatically make it crap. It keeps time
as well as any clock I've ever seen, and presumably synchronizes itself
to the WWVB signal; I could put it on my scope/freq. counter, but I'm not
that ambitious. It was just an idea.

Thanks,
Rich
 
G

GregS

Jan 1, 1970
0
Just because it's cheap doesn't automatically make it crap. It keeps time
as well as any clock I've ever seen, and presumably synchronizes itself
to the WWVB signal; I could put it on my scope/freq. counter, but I'm not
that ambitious. It was just an idea.

Something about 100 ms accuracy.

Back again to the Goldstone Apollo station. In back of the Collins timing system
was something that was not used anymore when I started working there.
A Marantz Model 9 mono tube amplifier to drive wall clocks ! You still had
to set the clock manually.

greg
 
S

spamme0

Jan 1, 1970
0
Rich said:
Just because it's cheap doesn't automatically make it crap.

I never implied that the clock was crap.
I implied that your concept of using it to calibrate a counter in a
minute was crap.

Sixty seconds of a one hz signal is...wait...let me do the math...
oh, yes, it's 60 counts.

You are NOT gonna get 10^-6 accuracy out of counting sixty counts...ever.

But wait...my counter has a time interval measurement.
Ok, what's the risetime of the signal, jitter, accuracy of
the time-interval function as specified by the counter manufacturer.
There's a reason you don't try to calibrate a counter's reference
in time-interval mode. Averaging over 60 clocks helps by a factor of 8
or so on a good day for random errors. Doesn't help much on
systematic errors.

Assuming the average accuracy is good, there are ways to
generate a stable reference, but not in 60 counts.

I just wanted to see some math supporting your assertion.

It keeps time
 
N

nospam

Jan 1, 1970
0
spamme0 said:
But wait...my counter has a time interval measurement.
Ok, what's the risetime of the signal, jitter, accuracy of
the time-interval function as specified by the counter manufacturer.

Risetime? jitter? wtf. If you get a stable reading it is a stable reading.
Crude count the 10MHz reference clock period measurement gives you 1 part
in 10 million resolution for a 1Hz signal.

I had no problem calibrating my counter/timer from a GPS pps output. Get as
close as you can with period measurement then scope the counter 10MHz
reference triggered by the pps. Trim for zero drift. The limiting factor
was resolution of the reference trimmer not jitter or observation of drift.
--
 
S

spamme0

Jan 1, 1970
0
nospam said:
Risetime? jitter? wtf. If you get a stable reading it is a stable reading.
Crude count the 10MHz reference clock period measurement gives you 1 part
in 10 million resolution for a 1Hz signal.

I had no problem calibrating my counter/timer from a GPS pps output. Get as
close as you can with period measurement then scope the counter 10MHz
reference triggered by the pps. Trim for zero drift. The limiting factor
was resolution of the reference trimmer not jitter or observation of drift.

Well...it's been 30 years since I was the hardware design manager for a
counter company....and I like to learn new things.
While what you say is spot-on theoretically, there are practical issues
to deal with.

My input was related to:
_______________________
I just want to recalibrate my so-so frequency counters
and
Well, for what it's worth, my $10.99 plus tax wall clock is sync'ed to
WWVB; I'm sure there's a 1 Hz pulse in there somewhere. :)

I'd imagine it would make a pretty good reference if you wanted to wait,
say, 60 seconds or so to get a good reading.
_______________________

I asked about the math that allowed the specified source to calibrate
the specified counters....vague as those specs are.


So, the first question to you is,
"with your method, how many reference frequencies exist that produce
a zero drift display on your scope?"
How do you tell which is the right one?
 
N

nospam

Jan 1, 1970
0
spamme0 said:
So, the first question to you is,
"with your method, how many reference frequencies exist that produce
a zero drift display on your scope?"

As many as there are Hz in the counter reference trimming range.
How do you tell which is the right one?

You measure the pps period to get 'on' the right Hz first, you can measure
period after to confirm.

--
 
S

spamme0

Jan 1, 1970
0
nospam said:
As many as there are Hz in the counter reference trimming range.


You measure the pps period to get 'on' the right Hz first, you can measure
period after to confirm.
Got it...
Assume the counter can measure period with more resolution
than is implied by the reference clock rate.

I use the period function to measure the 1Hz. and adjust the 10MHz
oscillator
to better than a part in 10^-7.
Then I set my oscilloscope sweep speed to 1uS/division...oughta be
able to
easily resolve 10-cycles/division...and trigger the scope on the 1Hz.
I get a display that sweeps once/second and I dial it right in.
I just tried it...
I don't think the trigger jitter spec on my scope is anywhere near
a part in 10^7, but it don't matter, cause I couldn't see anything.
Your oscilloscope has a LOT better writing rate than mine!!

While your approach is theoretically sound, I maintain that the
average person with average equipment can't take a so-so counter,
a $11 wwvb-disciplined wall clock and (assuming they even have) an
average oscilloscope
to implement it in SIXTY seconds or so of measurement time as
suggested by the original suggestion.

The devil is in the details.
 
N

nospam

Jan 1, 1970
0
spamme0 said:
easily resolve 10-cycles/division...and trigger the scope on the 1Hz.
I get a display that sweeps once/second and I dial it right in.
I just tried it...
I don't think the trigger jitter spec on my scope is anywhere near
a part in 10^7, but it don't matter, cause I couldn't see anything.
Your oscilloscope has a LOT better writing rate than mine!!

Digital scope, you are only looking at 10MHz doesn't have to be a fast one.
Jitter from the trigger and pps wasn't enough to cause any problem
observing drift. The pps jitters was maybe 30 or 40ns, and it was 'zero' or
a whole 30 or 40ns, obviously synced to some about 30MHz clock in the GPS
module.
While your approach is theoretically sound, I maintain that the
average person with average equipment can't take a so-so counter,
a $11 wwvb-disciplined wall clock and (assuming they even have) an
average oscilloscope

Pulling a pps signal from an old hand held GPS and using test equipment I
already had let me calibrate my counter/timer with no cost.

I doubt a wwvb wall clock would contain any useful signal. I doubt they
lock on to anything, just occasionally correct their time from the time
code signal.
--
 
R

Rich Grise

Jan 1, 1970
0
I never implied that the clock was crap. I implied that your concept of
using it to calibrate a counter in a minute was crap.

OK, let it run all day or all week - eventually you'll get a usable
reading.

My point was merely that it's doable, if you want to wait long enough. :)

Thanks,
Rich
 
J

JosephKK

Jan 1, 1970
0
Build yer own.

A older rockwell/conexant "Jupiter" gps has 10 khz outputs when locked
that align with the hydrogen clock in the satellites.

Most surplus Jupiters do just fine, the T model is highly desirable ,
but not readily available

This worked for me, and there are other articles using similar
techniques, some using the 1 PPS signal instead of the 10 khz from a
Jupiter.

Trimble Thunderbolts are highly popular as well.

See http://www.rabel.org/archives/General_GPS_Info/A Simple GPS Stabilized 10 MHz Oscillator.pdf
for one of many sites.

Since this is mainly used for phase locking 10 Ghz amateur radio, a
good search string is

"GPS 10 Ghz Ham GPSDO "

Mine cost 45$ in ebay parts. 10 Mhz VCXOs are all over ebay.

Steve

The last i heard the GPS sats had cesium and rubidium clocks, not
hydrogen. Where do you get hydrogen frequency references?
 
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