LED alarm clocks all lose accuracy over time

In article
<998017860359162824.429035zekor-comcast.net@news.eternal-september.org>,
gregz <zekor@comcast.net> wrote:

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote:
Many years ago I reviewed Heath's "Most-Accurate Clock" for one of Ed Dell's
magazines. It used the Bureau of Standards' shortwave time signals. Sync was
a bit touchy (I eventually replaced the carbon calibration pots with
ceramic), but it otherwise worked very well. It even had an interface that
allowed your computer to reset its clock each time the machine restarted.

When I needed money a few years back, I sold it on eBay for something like
$400, without anyone questioning the price.

If Heath wants to come back as a kit company, it needs to design products
that have no commercial equivalents.

Heath had an fm tuner with direct frequency entry push button.
The digital frequency synthesizer used for tuning was not exactly
well-designed, but the tuner and stereo decoder part (which is what most
folks actually wanted an FM radio for) was a real POS.

Isaac
 
Klaatu <whichway@today.org> wrote in message
news:Gs-dnZcc7uhR0SXSnZ2dnUVZ_hmdnZ2d@giganews.com...
"Bill Proms" <none@anywhere.us> wrote in message
news:jp850p$fk2$1@dont-email.me...
I have 3 Intelli-Time LED alarm clocks around the house, just like the
one
here:


http://www.acurite.com/clock/alarm-clock/intelli-time-digital-alarm-clock-13
027a2.html
I initially bought these due to them keeping time when the power goes
off
and auto resetting for DST. There is a problem, however. Each of the
clocks becomes inaccurate over time. If I set them all manually to the
same time, within a few months, each one will be off by 3-5 minutes.

So I ask, what is the problem and is there any way to repair it?

Thanks in advance,
Bill



http://www.ebay.com/itm/Westclox-70026A-LCD-Atomic-Digital-Alarm-Clock-/2610
19821915?pt=LH_DefaultDomain_0&hash=item3cc5fe675b
or, if you want to see the display at night


http://www.ebay.com/itm/Atomic-Projection-Alarm-Clock-/310396561330?pt=US_Cl
ocks&hash=item484513a7b2
I've used one of the projection alarm clocks for 4 years now, with the
projector light on 24/7.
No problems. Its has battery back-up, but no projection if the power
fails.
All the alarm and wall clocks in the house are now atomic. So is my
wristwatch.
IMHO, If a clock doesn't show the correct time, its not a clock...its a
timer.

Confucius , he say, but even a stopped clock tells the right time twice a
day
 
Jim Yanik <jyanik@abuse.gov> wrote in message
news:XnsA058E21BE509Ajyaniklocalnetcom@216.168.3.44...
"Ian Field" <gangprobing.alien@ntlworld.com> wrote in
news:9bTtr.549288$yJ4.364502@fx07.am4:


"gregz" <zekor@comcast.net> wrote in message
news:1124369627359136873.854687zekor-comcast.net@news.eternal-september
.org...
"N_Cook" <diverse@tcp.co.uk> wrote:
Bill Proms <none@anywhere.us> wrote in message
news:jp850p$fk2$1@dont-email.me...
I have 3 Intelli-Time LED alarm clocks around the house, just like
the one
here:


--
Jim Yanik
jyanik
at
localnet
dot com
OT:
Have you seen the recent thread on rec.antiques.radio+phono ?
 
Heath had an FM tuner with direct frequency entry.
I never saw another for home use, but there may
have been another for commercial use.
Not quite correct.

AFAIK, Heath made the world's first digitally tuned stereo FM tuner. The kit
was appallingly expensive -- around $550 35+ years ago. It used little cards
you notched for a particular station. I saw it at a hi-fi show, with the
Heath rep explaining how you could use it to monitor the station's broadcast
frequency. (In my best Menckenesque manner, I set him straight.)

Tuners with direct frequency entry were and are uncommon, because it
requires a keypad, plus a decoder to output the digital value needed to set
the local oscillator. As the tuner would have a station memory anyway, which
most people would use to store their favorite stations, direct entry has
little advantage (except during initial setup).

GE made at least two clock radios with direct entry. (Yes, I have one.)
Because there's no overlap between AM and FM frequencies (in kHz and MHz),
you didn't need to specify the band.

Component tuners with direct entry are virtually unheard-of. Toshiba had
one, I believe, and my Parasound T3 permits direct entry from the remote
control.

Does anyone know of any others?
 
On Sun, 20 May 2012 00:41:44 -0700, isw <isw@witzend.com> wrote:

The 1pps output can be used to run a digital clock, but I would hate
to see the final cost.

Why use the 1 pps? Any cheap GPS you get on eBay will output NMEA
"sentences" in ASCII that tell you the precise time. Just use those.
The NEMA sentences are not synchronized to GPS time and add delays in
the decoding process. The time might be off a fraction of a secod.
However, using the NEMA sentence is probably adequate for a consumer
alarm clock. $GPZDA give the time as:
$--ZDA,hhmmss.ss,xx,xx,xxxx,xx,xx
hhmmss.ss = UTC
xx = Day, 01 to 31
xx = Month, 01 to 12
xxxx = Year
xx = Local zone description, 00 to +/- 13 hours
xx = Local zone minutes description (same sign as hours)

The 1pps output has the advantage of simplicity and easy of
integration with existing digital clock designs.

Say, $15 for the GPS, another $15 for an Arduino, $5 for an LCD, and
whatever crystal you have on hand, stuck in a home-made oven. Maybe
another $20 for all the "glue", and the rest, as they say, is just
software.
The cheapest GPS board or module that I could find is about $40.
<http://www.sparkfun.com/categories/4?sort_by=price_asc&per_page=50>
Chips seem to run about $8/1000. Using your prices, a commercial
digital alarm clock product would retail about $175-$200. Meanwhile,
WWVB controlled "atomic" alarm clocks are selling for $15.

This one decodes NEMA sentences, and has some compromises due to
running on battery power:
<http://www.siliconchip.com.au/cms/A_111192/article.html>
To conserve the battery, the GPS module is only used
to synchronise the clock every 44 hours and following
synchronisation, the clock will either skip seconds
or double-step to reach the correct time. After
synchronisation the microcontroller is also able to
calculate the inherent inaccuracy of its crystal oscillator
and will compensate by occasionally skipping or
double-stepping a second. This process can also compensate
for aging of the crystal and will keep the clock accurate
between synchronisations.
<http://geoffg.net/GPS_Synchronised_Clock.html>
<http://www.siliconchip.com.au/cms/gallery/article.html?a=111709>


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
On Sun, 20 May 2012 00:32:56 -0700, isw <isw@witzend.com> wrote:

Incidentally, I'm building my own 10MHz GPSDO for running my test
eqipment and ham junk. It's NOT a trivial or inexpensive exercise:
http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm

You can get short-term performance equal to a good crystal and long-term
accuracy pert' near equal to the cesium beam clocks on the satellites
for a whole lot less than that.

Start with a 10 MHz rock in a home-brew oven, and divide it down to make
a "regular clock". Compare the time given by that clock to GPS time, and
twiddle the crystal oscillator to make it track, using a very long time
constant filter in the loop. The longer it runs, the more accurate it
gets.
Good idea. However, if I'm going to go through the trouble of
dividing down to 1pps (1Hz), I might as well add a FLL (frequency lock
loop) and sync the oscillator a 1pps GPS reference.

The regular clock idea also has a potential problem. One missed pulse
and the clock is off by 1 full second. Since there's no way to detect
a missing pulse, or recover gracefully from such a clock slip, it has
the potential for going awry.

After it's run for a while, you can do without GPS updates for fairly
long periods (depending on how good your oscillator/oven is) with very
little loss in accuracy.
Maybe true, but I would have no easy way to determine if it needed to
be resynced with the GPS clock. If it drifted off frequency for some
reason, all my measurements would be off. Setting the frequency to
1*10^-11 takes days to set, and the same time to reset. Might as well
leave it on all the time.

In order to produce usable synchronization of simulcast data
transmitters, the frequency accuracy and stability has to be <0.05ppm
or (<5*10^-8) for wide area coverage. That's not going to require a
GPSDO but it would be nice.

I've considered building that sort of rig where the oscillator's
frequency is controlled by altering the temperature of the oven.
Groan. Low noise oscillators use SC cut quartz crystals. That
produces a rather steep frequency/temp curve at room temperature, but
flattens out at about 80-100C. Vary the temperature around this flat
part of the curve, and you'll see almost no change in frequency.
Worse, it might go the wrong direction if it shows a peak or dip in
the curve.
<http://www.conwin.com/pdfs/at_or_sc_for_ocxo.pdf>
Nice idea, but it will only work if you use an AT cut crystal and then
only if you can deal with the dip in the curve.

The big
advantage of doing things that way is that there are no extra paths for
noise to get into the oscillator, as there would be if you used some
sort of variable capacitor.
Good idea, but glass piston trimmers and varactor electronic tuning
are usually good enough. Microphonics are the major danger of using
trimmers. If the trimmer is solidly built, and the components are
buried in RTV for shock proofing, it should be adequate.

The 1/f phase noise of an oscillator is directly related to the power
density of the device. That means using a moderately high current
device at about 10% of its rated current. That's also why nobody uses
low noise RF front end transistor for low noise oscillators. That
leaves flicker noise, which reduced with lots of negative feedback.

This should give you an idea of what commercial GPSDO oscillators are
doing:
<http://www.leapsecond.com/pages/fury/phase.htm>

Check the Flexradio forum for what the SDR users are running. A GPSDO
is preferred, but there are plenty of less accurate devices being
used. The stock radio has a TCXO. The GPSDO upgrade costs $700.
Little wonder users are looking at alternatives.
<http://cart.flexradio.com/FLEX-6000-GPSDO-Upgrade_p_899.html>

Incidentally, some data on the accuracy of the 60Hz power line
frequency:
<http://www.leapsecond.com/pages/mains/>




--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
On Sun, 20 May 2012 00:13:49 -0700, isw <isw@witzend.com> wrote:

ALmost certainly a crystal; ceramic resonators have way too much drift
with temperature. 32 KHz crystals are very difficult to trim because
they don't like to have any extraneous capacitance hung on them.
32.768KHz crystals are somewhat of a special case when it comes to
trimming. The problem is that the crystals are tuning forks, not bulk
quartz devices like higher frequency devices. What's inside:
<http://en.wikipedia.org/wiki/File:Inside_QuartzCrystal-Tuningfork.jpg>
Typical specs are something like 7-12pF shunt capacitance. It is
possible to trim the frequency with a variable capacitor that is
centered around this capacitance. It's done all the time in
electronic wrist watches.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
isw <isw@witzend.com> wrote in
news:isw-C3E189.00325620052012@[216.168.3.50]:

In article <gkqfr7tifijlt1n4l7frdrb5pumccv7tad@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> wrote:

On Sat, 19 May 2012 18:24:04 +0000 (UTC), "Geoffrey S. Mendelson"
gsm@mendelson.com> wrote:

It's because the US is trying to get rid of those broadcasts. With
GPS they are obsolete.

I beg to differ. The cost of a GPS disciplined oscillator or clock
in a consumer product is prohibitive. Running continuously, GPS is a
major power drain. GPS doesn't work well indoors. There are huge
number of products that currently use 60KHz time sync that will go
dark if the US pulls the plug on WWVH and WWVB. That's not going to
happen. Quite the contrary, there are plans to add a US east coast
transmitter. See:
http://en.wikipedia.org/wiki/WWVB
under "Service Improvement Plans".

Incidentally, I'm building my own 10MHz GPSDO for running my test
eqipment and ham junk. It's NOT a trivial or inexpensive exercise:
http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm

You can get short-term performance equal to a good crystal and
long-term accuracy pert' near equal to the cesium beam clocks on the
satellites for a whole lot less than that.

Start with a 10 MHz rock in a home-brew oven, and divide it down to
make a "regular clock". Compare the time given by that clock to GPS
time, and twiddle the crystal oscillator to make it track, using a
very long time constant filter in the loop. The longer it runs, the
more accurate it gets.

After it's run for a while, you can do without GPS updates for fairly
long periods (depending on how good your oscillator/oven is) with very
little loss in accuracy.

I've considered building that sort of rig where the oscillator's
frequency is controlled by altering the temperature of the oven. The
big advantage of doing things that way is that there are no extra
paths for noise to get into the oscillator, as there would be if you
used some sort of variable capacitor.

Isaac
the time hysteresis would be too great for temp control of the oven.

--
Jim Yanik
jyanik
at
localnet
dot com
 
Jim Yanik wrote:
isw <isw@witzend.com> wrote in
news:isw-C3E189.00325620052012@[216.168.3.50]:

In article <gkqfr7tifijlt1n4l7frdrb5pumccv7tad@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> wrote:

On Sat, 19 May 2012 18:24:04 +0000 (UTC), "Geoffrey S. Mendelson"
gsm@mendelson.com> wrote:

It's because the US is trying to get rid of those broadcasts. With
GPS they are obsolete.

I beg to differ. The cost of a GPS disciplined oscillator or clock
in a consumer product is prohibitive. Running continuously, GPS is
a major power drain. GPS doesn't work well indoors. There are huge
number of products that currently use 60KHz time sync that will go
dark if the US pulls the plug on WWVH and WWVB. That's not going to
happen. Quite the contrary, there are plans to add a US east coast
transmitter. See:
http://en.wikipedia.org/wiki/WWVB
under "Service Improvement Plans".
According to the "Service Improvement Plans" , the east coast project has
been abandoned, at least for the forseeable future. Funding for it was part
of the 2009 stimulus bill, which, as much of the money in that bill, was
never spent. Marshall Space Flight Center objected to the high power
transmitter being built so close to its operation in Huntsville, AL.
--
Dave M
A woman has the last word in any argument. Anything a man says after
that is the beginning of a new argument.
 
"Cydrome Leader" <presence@MUNGEpanix.com> wrote in message
news:jpb9f2$rrc$5@reader1.panix.com...
gregz <zekor@comcast.net> wrote:
"William Sommerwerck" <grizzledgeezer@comcast.net> wrote:
Many years ago I reviewed Heath's "Most-Accurate Clock" for one of Ed
Dell's
magazines. It used the Bureau of Standards' shortwave time signals. Sync
was
a bit touchy (I eventually replaced the carbon calibration pots with
ceramic), but it otherwise worked very well. It even had an interface
that
allowed your computer to reset its clock each time the machine
restarted.

When I needed money a few years back, I sold it on eBay for something
like
$400, without anyone questioning the price.

If Heath wants to come back as a kit company, it needs to design
products
that have no commercial equivalents.

Heath had an fm tuner with direct frequency entry push button. I never
saw
another for home use, but there may have been another or commercial use.

I think it was GE that had an alarm clock like that in the late 70s. It
was actually pretty cool. It had a little keypad on it.

About that era, Henry's radio had a batch of project pages in their
catalogue - one of which was a digital clock.

I started building it but never got around to finishing it. Usual excuses;
can't afford the next lot of parts, by the time I got around to it the parts
had become obsolete etc.

The project was ongoing for many years and the parts added to it depended
where I was working at the time!

Pretty sure I still have the board in a tea chest at the back of the garage
somewhere.
 
gregz <zekor@comcast.net> wrote:
"William Sommerwerck" <grizzledgeezer@comcast.net> wrote:
Many years ago I reviewed Heath's "Most-Accurate Clock" for one of Ed Dell's
magazines. It used the Bureau of Standards' shortwave time signals. Sync was
a bit touchy (I eventually replaced the carbon calibration pots with
ceramic), but it otherwise worked very well. It even had an interface that
allowed your computer to reset its clock each time the machine restarted.

When I needed money a few years back, I sold it on eBay for something like
$400, without anyone questioning the price.

If Heath wants to come back as a kit company, it needs to design products
that have no commercial equivalents.

Heath had an fm tuner with direct frequency entry push button. I never saw
another for home use, but there may have been another or commercial use.
I think it was GE that had an alarm clock like that in the late 70s. It
was actually pretty cool. It had a little keypad on it.
 
I think it was GE that had an alarm clock like that
in the late 70s. It was actually pretty cool. It had a
little keypad on it.
I mentioned that in a preceding post. It was the 7-4760, I believe. I still
have it. The keypad needed cleaning every couple of years.
 
On Sun, 20 May 2012 18:34:03 +0000 (UTC), "Geoffrey S. Mendelson"
<gsm@mendelson.com> wrote:

Jeff Liebermann wrote:
dark if the US pulls the plug on WWVH and WWVB. That's not going to
happen. Quite the contrary, there are plans to add a US east coast
transmitter. See:
http://en.wikipedia.org/wiki/WWVB
under "Service Improvement Plans".

That's dead. What's left as of March 2012 is a plan to move from AM to BPSK,
which is supposed to make it possible to receive the signal farther away
and in areas with more noise and multipath distortion.

There was a trial in early March, any see any results yet?

How will this affect any old devices that use the AM signal method?
No. AM transmissions were to continue, alternating with phase shift
modulation.
<http://www.jks.com/wwvb.pdf>
In an attempt to cost-effectively address the reception
challenges, NIST is introducing a new protocol for
the WWVB broadcast, which will preserve its amplitude modulation
properties, in order to maintain backwards compatibility and not
impact existing devices, while adding phase-modulation that would
allow for the greatly improved performance.

Some details on the test. I couldn't find any conclusions or official
results:
<http://www.jks.com/wwvb/wwvb.html>

Will someone be selling, as I proposed (and shouldda patented) devices that
get the correct time via NTP and broadcast microwatt signals for local area
time sync?
Good idea. I haven't seen any such device. Instead of broadcasting,
this might be a good use for power line communications (HomePlug, X10,
etc.) or RF (Z-wave).

Incidentally, while broadcasters seem to be killing off their OTA time
signals (used to set VCR clocks), the cable providers are using OOB
(out of band) signaling to set the clock in their cable set top boxes.
You could probably retransmit that data to a wall clock.

If all else fails, you can call the NIST at 303-499-7111 for WWV and
just play the time over a speaker. Yep, it works.

Web clock:
<http://nist.time.gov>
It says accurate to 0.3 seconds.

More on such clocks:
<http://www.precisionclock.com>


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
Jeff Liebermann wrote:
dark if the US pulls the plug on WWVH and WWVB. That's not going to
happen. Quite the contrary, there are plans to add a US east coast
transmitter. See:
http://en.wikipedia.org/wiki/WWVB
under "Service Improvement Plans".
That's dead. What's left as of March 2012 is a plan to move from AM to BPSK,
which is supposed to make it possible to receive the signal farther away
and in areas with more noise and multipath distortion.

There was a trial in early March, any see any results yet?

How will this affect any old devices that use the AM signal method?

Will someone be selling, as I proposed (and shouldda patented) devices that
get the correct time via NTP and broadcast microwatt signals for local area
time sync?

Geoff.


--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379
In 1969 the US could put a man on the moon, now teenagers just howl at it. :-(
 
On 5/20/2012 9:45 AM, William Sommerwerck wrote:
Heath had an FM tuner with direct frequency entry.
I never saw another for home use, but there may
have been another for commercial use.
Not quite correct.

AFAIK, Heath made the world's first digitally tuned stereo FM tuner. The kit
was appallingly expensive -- around $550 35+ years ago. It used little cards
you notched for a particular station. I saw it at a hi-fi show, with the
Heath rep explaining how you could use it to monitor the station's broadcast
frequency. (In my best Menckenesque manner, I set him straight.)

Tuners with direct frequency entry were and are uncommon, because it
requires a keypad, plus a decoder to output the digital value needed to set
the local oscillator. As the tuner would have a station memory anyway, which
most people would use to store their favorite stations, direct entry has
little advantage (except during initial setup).

GE made at least two clock radios with direct entry. (Yes, I have one.)
Because there's no overlap between AM and FM frequencies (in kHz and MHz),
you didn't need to specify the band.

Component tuners with direct entry are virtually unheard-of. Toshiba had
one, I believe, and my Parasound T3 permits direct entry from the remote
control.

Does anyone know of any others?

Dick Sequerra made one as I recall, and the RACAL, Rhode and Schwartz,
and other surveillance receiver makers also had direct synthesis /
tuning as well.
 
William Sommerwerck <grizzledgeezer@comcast.net> wrote:
I think it was GE that had an alarm clock like that
in the late 70s. It was actually pretty cool. It had a
little keypad on it.

I mentioned that in a preceding post. It was the 7-4760, I believe. I still
have it. The keypad needed cleaning every couple of years.
I once had the older vacuum flourescent version. They changed to LEDs at
one some point. I forgot how/why mine broke.
 
In article <9d2ir7prbb5eghsnr549v9meagqorkklav@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> wrote:

On Sun, 20 May 2012 00:41:44 -0700, isw <isw@witzend.com> wrote:

The 1pps output can be used to run a digital clock, but I would hate
to see the final cost.

Why use the 1 pps? Any cheap GPS you get on eBay will output NMEA
"sentences" in ASCII that tell you the precise time. Just use those.

The NEMA sentences are not synchronized to GPS time and add delays in
the decoding process. The time might be off a fraction of a secod.
However, using the NEMA sentence is probably adequate for a consumer
alarm clock.
Much better, in the long run -- the time they report has a bit of
sample-to-sample jitter, true, but the long-term drift is close enough
to zero to not matter.

You don't correct your local oscillator on the basis of its time
difference compared to a particular sample -- you correct it to force
the long-term drift to zero. The longer it runs, the more accurate it
becomes.

The notion of using irregular or not-continuous data samples to create
accurate clocks even over jittery channels has been around for a while.
Both MPEG 2 and Cable Modems use versions of it.

Isaac
 
In article <XnsA05987787147Cjyaniklocalnetcom@216.168.3.44>,
Jim Yanik <jyanik@abuse.gov> wrote:

isw <isw@witzend.com> wrote in
news:isw-C3E189.00325620052012@[216.168.3.50]:

In article <gkqfr7tifijlt1n4l7frdrb5pumccv7tad@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> wrote:

On Sat, 19 May 2012 18:24:04 +0000 (UTC), "Geoffrey S. Mendelson"
gsm@mendelson.com> wrote:

It's because the US is trying to get rid of those broadcasts. With
GPS they are obsolete.

I beg to differ. The cost of a GPS disciplined oscillator or clock
in a consumer product is prohibitive. Running continuously, GPS is a
major power drain. GPS doesn't work well indoors. There are huge
number of products that currently use 60KHz time sync that will go
dark if the US pulls the plug on WWVH and WWVB. That's not going to
happen. Quite the contrary, there are plans to add a US east coast
transmitter. See:
http://en.wikipedia.org/wiki/WWVB
under "Service Improvement Plans".

Incidentally, I'm building my own 10MHz GPSDO for running my test
eqipment and ham junk. It's NOT a trivial or inexpensive exercise:
http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm

You can get short-term performance equal to a good crystal and
long-term accuracy pert' near equal to the cesium beam clocks on the
satellites for a whole lot less than that.

Start with a 10 MHz rock in a home-brew oven, and divide it down to
make a "regular clock". Compare the time given by that clock to GPS
time, and twiddle the crystal oscillator to make it track, using a
very long time constant filter in the loop. The longer it runs, the
more accurate it gets.

After it's run for a while, you can do without GPS updates for fairly
long periods (depending on how good your oscillator/oven is) with very
little loss in accuracy.

I've considered building that sort of rig where the oscillator's
frequency is controlled by altering the temperature of the oven. The
big advantage of doing things that way is that there are no extra
paths for noise to get into the oscillator, as there would be if you
used some sort of variable capacitor.

Isaac


the time hysteresis would be too great for temp control of the oven.
Then you need a slower filter. This sort of loop does not make
sample-by-sample corrections; it zeroes long-term drift. Tweaking the
frequency by altering the temperature will work fine for that. If your
clock is drifting slow, tweak the temperature slightly in the proper
direction and wait a few hours -- or a day or two. Check again, tweak
again. Pretty soon, the corrections will become less and less frequent,
and also smaller.

Think about it; it's exactly the same way you regulate a balance-wheel
clock.

Isaac
 
In article <kd4ir7132smh8biksjl55mhhaileaauo3m@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> wrote:

On Sun, 20 May 2012 00:32:56 -0700, isw <isw@witzend.com> wrote:

Incidentally, I'm building my own 10MHz GPSDO for running my test
eqipment and ham junk. It's NOT a trivial or inexpensive exercise:
http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm

You can get short-term performance equal to a good crystal and long-term
accuracy pert' near equal to the cesium beam clocks on the satellites
for a whole lot less than that.

Start with a 10 MHz rock in a home-brew oven, and divide it down to make
a "regular clock". Compare the time given by that clock to GPS time, and
twiddle the crystal oscillator to make it track, using a very long time
constant filter in the loop. The longer it runs, the more accurate it
gets.

Good idea. However, if I'm going to go through the trouble of
dividing down to 1pps (1Hz), I might as well add a FLL (frequency lock
loop) and sync the oscillator a 1pps GPS reference.

The regular clock idea also has a potential problem. One missed pulse
and the clock is off by 1 full second. Since there's no way to detect
a missing pulse, or recover gracefully from such a clock slip, it has
the potential for going awry.
By "regular clock" I meant anything you like that reads out in "time of
day"; not necessarily something you bought.

I don't understand why there would ever be a missed once-per-second
pulse (assuming you know how to design digital stuff), but even so, the
next time a NMEA sentence comes along (or whenever you decide to read
the next one), you'd know you were off, and which way.

After it's run for a while, you can do without GPS updates for fairly
long periods (depending on how good your oscillator/oven is) with very
little loss in accuracy.

Maybe true, but I would have no easy way to determine if it needed to
be resynced with the GPS clock. If it drifted off frequency for some
reason, all my measurements would be off.
But if it ran at the same rate as the GPS clock, then it wouldn't be
drifting ...

Setting the frequency to
1*10^-11 takes days to set, and the same time to reset. Might as well
leave it on all the time.
Well, of course it's on all the time; there's no other way to keep the
oscillator stable. But if your local notion of time (what the NTP folks
call "epoch") tracks the data coming from the GPS, then it will
eventually be within that error band. And if it stays on longer, it'll
become ever more accurate (long-term).

In order to produce usable synchronization of simulcast data
transmitters, the frequency accuracy and stability has to be <0.05ppm
or (<5*10^-8) for wide area coverage. That's not going to require a
GPSDO but it would be nice.

I've considered building that sort of rig where the oscillator's
frequency is controlled by altering the temperature of the oven.

Groan. Low noise oscillators use SC cut quartz crystals. That
produces a rather steep frequency/temp curve at room temperature, but
flattens out at about 80-100C. Vary the temperature around this flat
part of the curve, and you'll see almost no change in frequency.
Worse, it might go the wrong direction if it shows a peak or dip in
the curve.
http://www.conwin.com/pdfs/at_or_sc_for_ocxo.pdf
Nice idea, but it will only work if you use an AT cut crystal and then
only if you can deal with the dip in the curve.
Use whatever crystal you want/need to get the temperature/frequency
curve you need. Pick an oven temperature range that gives the slope you
need. I don't see what's so hard about it. It's not even clear to me why
you even need a low-noise oscillator for a clock, assuming it's being
disciplined by a more accurate "master" (the GPS time system).

The big
advantage of doing things that way is that there are no extra paths for
noise to get into the oscillator, as there would be if you used some
sort of variable capacitor.

Good idea, but glass piston trimmers and varactor electronic tuning
are usually good enough. Microphonics are the major danger of using
trimmers. If the trimmer is solidly built, and the components are
buried in RTV for shock proofing, it should be adequate.
Yes, but if they're not even there to begin with, then they're pretty
noiseless even without special measures ...

The 1/f phase noise of an oscillator is directly related to the power
density of the device. That means using a moderately high current
device at about 10% of its rated current. That's also why nobody uses
low noise RF front end transistor for low noise oscillators. That
leaves flicker noise, which reduced with lots of negative feedback.
The short-term noise and the long-term stability are different. That's
why super-precision sources use rubidium oscillators (low noise, poor
drift), stabilized by cesium devices (higher noise, essentially zero
long-term drift).

If you're building a clock, the noise and jitter aren't very important;
long-term drift is.

Isaac
 
In article <jpa8gt$jne$1@dont-email.me>, "N_Cook" <diverse@tcp.co.uk>
wrote:

Klaatu <whichway@today.org> wrote in message
news:Gs-dnZcc7uhR0SXSnZ2dnUVZ_hmdnZ2d@giganews.com...
"Bill Proms" <none@anywhere.us> wrote in message
news:jp850p$fk2$1@dont-email.me...
I have 3 Intelli-Time LED alarm clocks around the house, just like the
one
here:


http://www.acurite.com/clock/alarm-clock/intelli-time-digital-alarm-clock-13
027a2.html

I initially bought these due to them keeping time when the power goes
off
and auto resetting for DST. There is a problem, however. Each of the
clocks becomes inaccurate over time. If I set them all manually to the
same time, within a few months, each one will be off by 3-5 minutes.

So I ask, what is the problem and is there any way to repair it?

Thanks in advance,
Bill



http://www.ebay.com/itm/Westclox-70026A-LCD-Atomic-Digital-Alarm-Clock-/2610
19821915?pt=LH_DefaultDomain_0&hash=item3cc5fe675b

or, if you want to see the display at night


http://www.ebay.com/itm/Atomic-Projection-Alarm-Clock-/310396561330?pt=US_Cl
ocks&hash=item484513a7b2

I've used one of the projection alarm clocks for 4 years now, with the
projector light on 24/7.
No problems. Its has battery back-up, but no projection if the power
fails.
All the alarm and wall clocks in the house are now atomic. So is my
wristwatch.
IMHO, If a clock doesn't show the correct time, its not a clock...its a
timer.



Confucius , he say, but even a stopped clock tells the right time twice a
day
And his smarter cousin says "Quite true, but can you tell me when I
should look at it to set my watch?"

Isaac
 

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