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'[EE]: WWV frequency standard'
2002\12\22@141203 by Sean H. Breheny

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Hi all,

Can anyone tell me how accurate WWV's frequency is on the 10MHz signal? I
want to adjust a crystal oscillator to as close to 10MHz as I can and I was
hoping to listen to the beat frequency it makes with WWV to do so (what
gave me this idea is that a device that I have which has a 3ppm max
deviation 10MHz TXCO which generates a nice slow 4 or 5 Hz beat when I
listen to it and WWV at the same time in a receiver I have) However, I
can't seem to find specs on how close to 10MHz WWV is guaranteed to be. I
was under the impression that a secondary purpose of WWV was as a frequency
standard, but on NIST's site I can't seem to find that data.

Thanks,

Sean

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2002\12\22@151558 by Dave Dilatush

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Sean wrote...

>Can anyone tell me how accurate WWV's frequency is on the 10MHz signal? I
>want to adjust a crystal oscillator to as close to 10MHz as I can and I was
>hoping to listen to the beat frequency it makes with WWV to do so (what
>gave me this idea is that a device that I have which has a 3ppm max
>deviation 10MHz TXCO which generates a nice slow 4 or 5 Hz beat when I
>listen to it and WWV at the same time in a receiver I have) However, I
>can't seem to find specs on how close to 10MHz WWV is guaranteed to be. I
>was under the impression that a secondary purpose of WWV was as a frequency
>standard, but on NIST's site I can't seem to find that data.

Go to NIST's WWV web site and download NIST Special Publication
432, "NIST Time and Frequency Services", at:

http://www.boulder.nist.gov/timefreq/general/pdf/1383.pdf

You'll find what you're looking for on page 51.

At the transmitter, WWV frequency is said to be accurate to
within a few parts in 10^13, roughly a couple millionths of a
hertz.

HTH...

Dave

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2002\12\23@010307 by Brooke Clarke

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Hello Sean:

I've been working for a number of months on a precision clock ( and some variants). See
<www.pacificsites.com/~brooke/PClock.shtml>
If you have an oscillator with a separate crystal (or a unit oscillator) there are a number of sources of error:

(1) the initial frequency setting (what you are working on in your message).  Yes you can set the frequency to about 0.1 to 1
millisecond per day using HF time standards.  Better with LF standards like WWVB or LORAN-C and much better with GPS.

The HP 114BR Time Comparator <http://www.pacificsites.com/~brooke/TF_rack.html< generated a trigger pulse for a scope that could be
delayed with thumb wheel switches and the phase of the audio from WWV could thus be tracked.  You could get about 1E-8 with this
system being careful not to use data when the ionosphere was changing.  If you just listen with your ear (or better a meter) to the
zero beat you can get within a few Hz out of say 10 Mhz or 3E-7.

Remember a +/-3 ppm TXCO running at 10 MHz will have an error of +/- 30 Hz so a few Hz isn't going to matter.  What's included in
the 3 ppm value, initial set, temperature, aging, power supply?

(2) Temperature will change the frequency of any crystal oscillator.  For a normal OCO we're talking about 100 or 200 Hz variation
over temperature.

(3) Aging will also cause the frequency of a crystal oscillator to change with operating time.  If the temperature is held near the
turning point then the slope of the frequency vs. temperature curve is close to horizontal and so temperature effects are greatly
minimized.  When this is done you can see the aging.  On the 10 MHz unit oscillator I'm now calibrating this amounts to about -5 ppm
per year.  Notice that this is more than the spec you cited for your TCXO so I think your spec is only for the temperature
compensation.

What I'm trying to do is correct the time output of a unit oscillator to account for 1, 2 & 3 above and do it in a 12F675.

Have Fun,

Brooke Clarke
products I now make  http://www.prc68.com


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2002\12\23@091917 by Sean H. Breheny

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Hi Brooke,

Thanks for the info! Your message came thorough a bit corrupted. Your 2nd
link has a < at the end instead of a > so it make the entire rest of the
message a link.

What a great site you have! I like the page on your Rubidium standard. You
gotta love any piece of equipment whose block diagram has something labeled
"physics package" :-D

I really am just trying to take a break from all the other ambitious
projects that I've been working on to finally do something simple for a
change. My alarm clock (cheap off-the-shelf one) broke and I'm just making
a new one, based on a PIC, just for the heck of it. I'm giving myself the
restriction that I use only parts that I have on hand. The TXCO is not for
the clock, that is in my amateur weather radar system
( http://www.rocket-roar.com/BT/wsr.html  ). I only mentioned the TXCO
because it had given me the idea (not at all original, I realize) of using
the beat note with WWV to tune oscillators when I heard its nice stable
beat with WWV. I  am just going to use a good, but cheap 10MHz crystal for
the clock. I wanted to reduce the initial error as much as possible by
beating it with WWV. After that I am not going to worry about it too much.
Hoping for about 1 sec per day drift or so (comparable to a regular digital
watch).

Actually, you might want to have a look at the TXCO that I'm using in my
radar project. It sounds better than what you mention. I think the total
initial plus over temp error is +- 3ppm. Ageing is about 0.3ppm. Check out
the FOX801BE-10.0MHZ

Sean




At 10:02 PM 12/22/2002 -0800, you wrote:
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2002\12\23@091923 by Sean H. Breheny

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Thanks to all who responded. Exactly the info I needed.

Sean

At 08:12 PM 12/22/2002 +0000, you wrote:
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2002\12\23@093336 by Olin Lathrop

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> My alarm clock (cheap off-the-shelf one) broke and I'm just making
> a new one, based on a PIC, just for the heck of it.
> ...
> I  am just going to use a good, but cheap 10MHz crystal for
> the clock.

Is this alarm clock pretty much in a fixed location in your house?  If so,
then I would use the power line cycles to measure time.  The frequency may
vary up and down a bit, but the long term average is deliberately kept
accurate (at least in most civilized places).  The only need for a crystal
would be to run from batteries during a power outage.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\12\23@170345 by Peter L. Peres

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On Sun, 22 Dec 2002, Sean H. Breheny wrote:

The 10MHz is probably very very 10MHz. The only way I know to do what you
want to do is to use a combiner and attenuator to feed the two frequencies
(LO and standard off the air) to a SSB or AM receiver and then tune the LO
to obtain zero beat on the S meter (!). This should get you well below 1
Hz which at 10MHz is better than 0.1ppm.

I did this a long time ago and do not remember all the details. I tuned to
the reference station, then I started the LO and swept it until the
S-meter rised. Then I set the attenuator until the LO (which I kept
switching on and off would rise the S meter by 2 divisions (I think! - it
was 6dB on that radio). The off air signal was regulated by shortening the
antenna (or shielding the ferrite antenna bar - I do not remember which).
The combiner was resistive.

Once I had each signal produce about two divisions by itself I left the LO
on and then tuned it until the S-meter indicated beat. On a AM receiver
you would hear a whistle etc descending in pitch towards zero beat. Fine
tuning involved several seconds of watching the needle and tweaking the
tuning of the LO.  Note that the LO stability of the radio does not play a
role in this game.

Most commercial AM shortwave receivers with some sort of fieldstrength
meter (even if it's a LED's brigtness that you have to watch) can be used
for this purpose. The 'combiner' can be simply another antenna attached to
the LO to be tuned. By adjusting the lengths of the receiver and LO
antennas you will be able to adjust the signal strength into a range where
the zero beat can be noticed easily.

I do not think that this works for a shortwave signal because fading will
mess your measurements up. I used a local station for reference. An AM
station will get you to within 1 ppm without trying hard (1 ppm is 0.5Hz
or more on the counter - see below). I used an AM station while it was
silent (they had silent hours during the night at the time).

I once read of a method to feed the direct amplified off air signal to the
input of a standard counter and to tune the counter timebase until the
correct frequency is indicated. You then have a secondary standard. My
counter has a 10MHz reference so I could use this method. It also allows
external 10MHz reference so I could use a LO to be tuned as that.

Of course the standard you will be using should allow a tuning accuracy at
least twice as good as what you need to tune with it.

Peter

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2002\12\23@185359 by Josh Koffman

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Cool project Sean. Keep us posted on the testing!

Josh
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"Sean H. Breheny" wrote:
> the clock, that is in my amateur weather radar system
> ( http://www.rocket-roar.com/BT/wsr.html  ). I only mentioned the TXCO

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2002\12\23@201557 by Larry Bradley

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Brooke, I read with interest the data on your site.

I've a project using a 16F877 with a 20 MHz xtal osc. in which I'm keeping
the time of day via in interrupt routine. It works juts fine, but I'd like
to be able to calibrate it - either us a trimmer, or compensate for any
error in code. Unfortunately, I don't have any HF gear at the moment, so
I've no way to use WWV or WWVB.


I'm interested in your comment on using GPS to set the frequency. I have a
GPS - a Garmin GPS 76 - and I have the cable to connect it up to a serial
port. How, in general, does one use a GPS to calibrate or set the frequency
of an oscillator?

Thanks

Larry VE3CRX



At 10:02 PM 12/22/2002 -0800, you wrote:
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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2002\12\24@020726 by Brooke Clarke

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Hi Larry:

To use GPS in timing applications you need to use a GPS receiver that has a 1 PPS output.
I don't know if the 76 has one or not.
TAPR often has group buys of suitable receiver boards.
http://www.tapr.org/tapr/html/gpsf.html

I have a Stanford Research Rubidium oscillator that accepts the 1PPS from a GPS receiver and uses that to steer the oscillator so that its long term stability is near zero.  It has both 10 MHz and 1
PPS outputs.  There was an article in QST by Brooks Shea that uses a PIC to do the same thing and is general in nature so can be used to steer about any oscillator with an electrical fine tune input.
<http://www.rt66.com/~shera/>

Have Fun,

Brooke Clarke, N6GCE
http://www.prc68.com


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2002\12\24@093357 by Sean H. Breheny

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Thanks, Josh.

I am in the process of making an updated page for that. I have tested it
out once so far. As expected, it didn't work :-) I found a bunch of
problems which I hope to fix and try it again in the spring when the
weather is warmer/rainier. The biggest problems are that the transmit power
amp generates noise which when received by the receiver, raises the noise
floor about 10dB, the low transmit power (20W), and some problems with the
dish rotation (especially with the slip rings for data transmission). My
plans are to use a more powerful amp which is actually off during receive
periods (to eliminate the noise feedthrough) and eliminate the slip rings
and use little RF modules instead to get the data from the rotating platform.

Sean

At 07:22 PM 12/23/2002 -0600, you wrote:
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