Time and Frequency References
 

On 4/18/2016 5:43 PM, David Woolley wrote:
On 18/04/16 15:39, rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?


Anyone who wants high accuracy off air time and frequency standards
would use GPS these days. Even that is almost two decade old technology
in amateur radio usage: http://www.tapr.org/kits_tac2.html

Is there something about GPS that is inherently superior for a frequency
reference? For setting a time, GPS can provide a smaller offset, but I
don't see where it has any advantages over WWVB or similar station
broadcasts where you can receive them.

The main limitation of a GPS receiver is the need for an outside antenna
for many installations. A WWVB receiver is self contained and much
lower cost.

--

Rick

Time and Frequency References
 

On 4/19/2016 7:34 AM, Rob wrote:
rickman wrote:
What happens now is that the DAC in the GPSDO steps up and down every
couple of seconds, and the oscillators wobble around the correct frequency
in the 1E-9 area (with of course an average that is very close, more like
1E-11), and this results in funny interference patterns. What is
happening is clear when you put the 10 MHz outputs of two independent
boxes on the scope in X-Y mode.

I don't know what they are using for an oscillator, but if you have
control over it, can you reduce the corner frequency of the LPF on the
control loop? It sounds like the control loop is hunting to me. But at
10-9 I suppose it could be ambient thermal drift too. Yes, I think a
rubidium GPSDO would do the job.

I think the problem is that our GPSDO has a 16-bit DAC and it is
dithering the digital value to obtain the correct frequency. So when
the correct DAC value would be 32000.2 it will do 32000 for 8 seconds
then 32001 for 2 seconds, obtaining a long-term average that is quite
good, but a wobble with 10-second period as well.

Are you referring to the voltage used to control the VCXO? Not sure
what parts you can tweak, but I don't see why the dithering can't be
sped up (say 1 kHz) and low pass filtered to produce an actual value of
32000.2.


These are old Datum 9390 units, we also have some Trimble Thunderbolts
that should be better.

I remember a couple/three years ago Symmetricom came out with a chip
scale atomic clock that can sync to a 1 pps. "Two orders of magnitude
better accuracy than oven-controlled crystal oscillators". Only $1,000.
Might do the job. They likely package this in a box level product
that will do what you want. Check out this one...

I would prefer a box that works from GPS and outputs the 1PPS.
The Datum and Trimble are in that category.

http://www.microsemi.com/products/ti...-2750#overview

They were bought by Microsemi it seems.

I Have seen and considered these before. That is indeed a GPS referenced
rubidium standard. Of course we always prefer stuff that is either
cheap or available as surplus (like the Datum and Trimble) :-)

One man's "cheap" is another man's "precious". You'd have to use a
number for me to know what you consider "cheap".

You seem to be buying boxes rather than building stuff, but if you have
access to the innards of the system above and can up the sample rate, $1
worth of components can low pass filter the output to give better
accuracy of the control signal.

--

Rick

Time and Frequency References
 

rickman wrote:
I think the problem is that our GPSDO has a 16-bit DAC and it is
dithering the digital value to obtain the correct frequency. So when
the correct DAC value would be 32000.2 it will do 32000 for 8 seconds
then 32001 for 2 seconds, obtaining a long-term average that is quite
good, but a wobble with 10-second period as well.

Are you referring to the voltage used to control the VCXO? Not sure
what parts you can tweak, but I don't see why the dithering can't be
sped up (say 1 kHz) and low pass filtered to produce an actual value of
32000.2.

Probably what the box does is check every second what the frequency
error is and change the DAC output. It works out as I described.
But indeed, it would be better to add some extra precision that way.
Those Datum units are over 20 years old...

I Have seen and considered these before. That is indeed a GPS referenced
rubidium standard. Of course we always prefer stuff that is either
cheap or available as surplus (like the Datum and Trimble) :-)

One man's "cheap" is another man's "precious". You'd have to use a
number for me to know what you consider "cheap".

We run a co-channel diversity repeater with 5 transmitter sites (and three
times as much receiver sites), from donations made by individual hams.
We get lots of surplus equipment donated, that is how we got a large
pile of Datum 9390 units from a decommissioned pager network.

Spending $5000 on new GPSDOs is a bit difficult for us, we also have other
expenses. Fortunately Trimble Thunderbolts are cheaper and look a lot
better on the scope. I need to finish a bit of monitoring code (that we
use together with "Lady Heather's Disciplined Oscillator Control Program")
and we can start testing how much difference there is in practice.

We also want to try software-defined FM modulation of the signal instead
of the analog modulator that of course also introduces awful frequency
differences because of imperfect alignment.

Time and Frequency References
 

In article rickman writes:
On 4/18/2016 5:43 PM, David Woolley wrote:
On 18/04/16 15:39, rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?


Anyone who wants high accuracy off air time and frequency standards
would use GPS these days. Even that is almost two decade old technology
in amateur radio usage: http://www.tapr.org/kits_tac2.html

Is there something about GPS that is inherently superior for a frequency
reference? For setting a time, GPS can provide a smaller offset, but I
don't see where it has any advantages over WWVB or similar station
broadcasts where you can receive them.

The main limitation of a GPS receiver is the need for an outside antenna
for many installations. A WWVB receiver is self contained and much
lower cost.

GPS allows accurate locking to frequency, WWVB no longer does,
since they use phase modulation on the WWVB signal. The "low
cost" WWVB receivers never could do that, they are only able to
be used for clock setting, not accurate frequency determination.

The WWVB signals are much more affected by the ionosphere,
as daytime absorbtion can make the signal unusable to small
receve antennas.

Also, WWVB does need an antenna for good performance, especially
compared to small indoor antennas.

WWVB suffers from occasional interference on the east coast from MSF.

Alan

Time and Frequency References
 

rickman wrote:
On 4/18/2016 5:43 PM, David Woolley wrote:
On 18/04/16 15:39, rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?


Anyone who wants high accuracy off air time and frequency standards
would use GPS these days. Even that is almost two decade old technology
in amateur radio usage: http://www.tapr.org/kits_tac2.html

Is there something about GPS that is inherently superior for a frequency
reference? For setting a time, GPS can provide a smaller offset, but I
don't see where it has any advantages over WWVB or similar station
broadcasts where you can receive them.

The direct-sight UHF radio link provides less jitter and uncertainty
than the VLF signal that suffers from propagation effects.

The main limitation of a GPS receiver is the need for an outside antenna
for many installations. A WWVB receiver is self contained and much
lower cost.

Sure it can be easier to place an antenna for a VLF station, but on
the other hand there is much more interference, mainly from switchmode
powersupplies these days (in the old days it was from CRT computer
monitors), but also from lightning.

Time and Frequency References
 

In article ,
Rob wrote:

I think the problem is that our GPSDO has a 16-bit DAC and it is
dithering the digital value to obtain the correct frequency. So when
the correct DAC value would be 32000.2 it will do 32000 for 8 seconds
then 32001 for 2 seconds, obtaining a long-term average that is quite
good, but a wobble with 10-second period as well.

Could you modify it, to "scale down" the analog control voltage coming
out of the DAC to a smaller range which "trims" around a center
voltage that you set manually?

I did something of that sort when building a VE2ZAZ GPSDO. This has
only a 10-bit DAC (or 12, I can't remember for sure) which is dithered
fairly rapidly to add a couple of additional bits of precision. I
low-pass-filtered the DAC output pretty heavily and then used some
resistor scaling to reduce its effect to a few percent of the total
5-volt control range that my OCVCXO allows. The "coarse" setting is
via an external potentiometer, driven from a good 5-volt precision
reference. In addition to the oven for the crystal, I added a
separate thermistor/heater board which levels out the temperature in
the whole enclosure, which I then insulated pretty heavily.

This approach wouldn't change the period of the wobble but would
reduce its magnitude by whatever divide-down ratio you chose to
implement.

Time and Frequency References
 

Dave Platt wrote:
In article ,
Rob wrote:

I think the problem is that our GPSDO has a 16-bit DAC and it is
dithering the digital value to obtain the correct frequency. So when
the correct DAC value would be 32000.2 it will do 32000 for 8 seconds
then 32001 for 2 seconds, obtaining a long-term average that is quite
good, but a wobble with 10-second period as well.

Could you modify it, to "scale down" the analog control voltage coming
out of the DAC to a smaller range which "trims" around a center
voltage that you set manually?

This is kind of risky. It may work for some of the units and not for
others. They tend to have a systematic drift, the alignment procedure
is to turn a trimmer in the oscillator so the DAC outputs a value close
to 32768 and then it tends to walk one way. When it approaches the
end of the range (say 5000 or 65000) the center has te be re-aligned.

As original, they drift about 10000 clicks in a year. Narrowing the
tuning range could make it drift too quickly to be practical.

As I said before, the first thing to do is to check the Thunderbolt
in practice. It should be much better than the aging Datums.
(they are so old that they do not even handle GPS weeknumber rollover
which first occurred in august 1999)

Time and Frequency References
 

On 4/20/2016 12:01 PM, wrote:
In article rickman writes:
On 4/18/2016 5:43 PM, David Woolley wrote:
On 18/04/16 15:39, rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?


Anyone who wants high accuracy off air time and frequency standards
would use GPS these days. Even that is almost two decade old technology
in amateur radio usage: http://www.tapr.org/kits_tac2.html

Is there something about GPS that is inherently superior for a frequency
reference? For setting a time, GPS can provide a smaller offset, but I
don't see where it has any advantages over WWVB or similar station
broadcasts where you can receive them.

The main limitation of a GPS receiver is the need for an outside antenna
for many installations. A WWVB receiver is self contained and much
lower cost.

GPS allows accurate locking to frequency, WWVB no longer does,
since they use phase modulation on the WWVB signal. The "low
cost" WWVB receivers never could do that, they are only able to
be used for clock setting, not accurate frequency determination.

The present WWVB signal still allows phase locking, just not by a
simplistic algorithm.


The WWVB signals are much more affected by the ionosphere,
as daytime absorbtion can make the signal unusable to small
receve antennas.

Also, WWVB does need an antenna for good performance, especially
compared to small indoor antennas.

Every receiver needs an antenna. But the LF time signals are easily
received inside buildings without an external antenna while the GPS
signals are readily blocked by walls and roofs. There are even problems
receiving the GPS signals accurately based on geography and local terrain.


WWVB suffers from occasional interference on the east coast from MSF.

I think "occasional" is the right word, if that. From here it is 1460
miles to WWVB transmitting at 50 kW ERP (I'm near Washington, DC) and
3,462 to MSF in Anthorn transmitting 15 kW. The directions are not far
from orthogonal (120°). A loop stick antenna at my location would
receive more noise than signal from one when receiving the signal from
the other if the signal from Anthorn can be received at all.

--

Rick

Time and Frequency References
 

On 4/20/2016 12:01 PM, Rob wrote:
rickman wrote:
On 4/18/2016 5:43 PM, David Woolley wrote:
On 18/04/16 15:39, rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?


Anyone who wants high accuracy off air time and frequency standards
would use GPS these days. Even that is almost two decade old technology
in amateur radio usage: http://www.tapr.org/kits_tac2.html

Is there something about GPS that is inherently superior for a frequency
reference? For setting a time, GPS can provide a smaller offset, but I
don't see where it has any advantages over WWVB or similar station
broadcasts where you can receive them.

The direct-sight UHF radio link provides less jitter and uncertainty
than the VLF signal that suffers from propagation effects.

That is important if you are looking for microsecond timing. But it has
very little impact on use as a frequency reference.


The main limitation of a GPS receiver is the need for an outside antenna
for many installations. A WWVB receiver is self contained and much
lower cost.

Sure it can be easier to place an antenna for a VLF station, but on
the other hand there is much more interference, mainly from switchmode
powersupplies these days (in the old days it was from CRT computer
monitors), but also from lightning.

I guess you aren't familiar with the extremely narrow band timing
signals, 1 bps. I'm working on a receiver with a 30 Hz bandwidth to
exclude environmental noise.

--

Rick

Time and Frequency References
 

rickman wrote:
On 4/20/2016 12:01 PM, Rob wrote:
rickman wrote:
On 4/18/2016 5:43 PM, David Woolley wrote:
On 18/04/16 15:39, rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?


Anyone who wants high accuracy off air time and frequency standards
would use GPS these days. Even that is almost two decade old technology
in amateur radio usage: http://www.tapr.org/kits_tac2.html

Is there something about GPS that is inherently superior for a frequency
reference? For setting a time, GPS can provide a smaller offset, but I
don't see where it has any advantages over WWVB or similar station
broadcasts where you can receive them.

The direct-sight UHF radio link provides less jitter and uncertainty
than the VLF signal that suffers from propagation effects.

That is important if you are looking for microsecond timing. But it has
very little impact on use as a frequency reference.

It depends on the stability of your oscillator and the integration
time that you can use as a result of that. Short-term frequency accuracy
is not much different from accurate timing.

The main limitation of a GPS receiver is the need for an outside antenna
for many installations. A WWVB receiver is self contained and much
lower cost.

Sure it can be easier to place an antenna for a VLF station, but on
the other hand there is much more interference, mainly from switchmode
powersupplies these days (in the old days it was from CRT computer
monitors), but also from lightning.

I guess you aren't familiar with the extremely narrow band timing
signals, 1 bps. I'm working on a receiver with a 30 Hz bandwidth to
exclude environmental noise.

I have experience with receivers for DCF77, which is a similar station
to MSF and WWVB. The frequency is 77.5 kHz.
Of course the results depend on the quality of the receiver.
I use some receiver modules from "atomic clocks" but I also have a
somewhat better receiver which has a crystal lattice filter.
I need to find a good spot for the antenna, away from certain equipment,
for it to work well. E.g. I had a problem with a switchmode powersupply
I used for the station in the past, which is switching at around 25 kHz.
The third harmonic (which of course drifts around depending on load and
temperature) interfered with the DCF77 receiver when it is within about
3 meters. I now have a different supply and this problem is gone, but
of course that is just coincidence.
Older CRT monitors for computers also emit quite a strong field in this
frequency range. Fortunately the frequency is quite stable, so it is
either OK or it is a problem, and it could be solved by changing the
display parameters.
Finally, when there are thunderstorms around, the signal often becomes
undetectable due to the many interference bursts.