Author Topic: Economical option for precision frequency reference?  (Read 117727 times)

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Offline awallin

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Re: Economical option for precision frequency reference?
« Reply #350 on: January 04, 2016, 01:24:34 pm »
Taking this into consideration what is the best way to "adjust" a RB to a GPSDO or is it really feasible to do so considering the GPSDO is better long term, but suffers from short term stability.

PI-controller that looks at the difference between the GPSDO and Rb phase (either at 10MHz or 1PPS) and steers the Rb with a DAC. Time constant for a bad Rb-clock maybe hours, for a good one 1-2 days.
http://www.thinksrs.com/assets/instr/PRS10/PRS10diag2LG.gif

The measurement of GPSDO vs Rb needs to be quite good. 9-digit/s frequency counter at least.
You'd want the GPSDO vs Rb measurement noise to reach an ADEV of 1e-12 (the limit for a typical Rb clock) a bit before the PI-loop time-constant, maybe at 1000s, so a 1PPS measurement good to 1ns that averages down as about 3e-9/tau should work.
 

Offline TheSteve

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Re: Economical option for precision frequency reference?
« Reply #351 on: January 04, 2016, 04:56:49 pm »
Now do the same test with 3 of your Rb oscillators.  You'll be shocked by the difference!  :o  Over a period of a few hours, the Rb oscillators are more stable than the GPSDOs.  It's only over longer time frames that the long-term frequency stability of the GPSDO starts to show its advantage over the Rb standard.

Of course, we have to remember that these GPSDOs are not optomized to give the lowest possible jitter.  You could make the time constants longer, but there are tradeoffs there too.  The same applies to the Rb units.  Low jitter just wasn't one of the design requirements.

Ed


Taking this into consideration what is the best way to "adjust" a RB to a GPSDO or is it really feasible to do so considering the GPSDO is better long term, but suffers from short term stability.

In the Temex LPFRS user manual it suggests using a GPSDO as a reference for an Agilent 53131A frequency counter. Being they have an analog voltage adjust this might be as stable as you can reasonably expect.
VE7FM
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #352 on: January 04, 2016, 10:04:56 pm »
Now do the same test with 3 of your Rb oscillators.  You'll be shocked by the difference!  :o  Over a period of a few hours, the Rb oscillators are more stable than the GPSDOs.  It's only over longer time frames that the long-term frequency stability of the GPSDO starts to show its advantage over the Rb standard.

Of course, we have to remember that these GPSDOs are not optomized to give the lowest possible jitter.  You could make the time constants longer, but there are tradeoffs there too.  The same applies to the Rb units.  Low jitter just wasn't one of the design requirements.

Ed


Taking this into consideration what is the best way to "adjust" a RB to a GPSDO or is it really feasible to do so considering the GPSDO is better long term, but suffers from short term stability.

To do a one-time adjustment, the best way that I know is to use a time interval counter or DSO to measure the time from the rising edge of the GPSDO to the rising edge of the Rb.  It doesn't matter if it's 1 PPS or 10 MHz, but a square wave signal is best.  Note the time interval and then watch it over many minutes.  If the time interval increases, the Rb is falling behind the GPSDO, so increase its frequency a bit.  If the time interval decreases, the Rb is overtaking the GPSDO so slow it down.  If you log the time interval and the length of time between measurements, you can calculate the frequency difference.  The resolution of your measurement will be determined by the resolution of your time interval counter and the time between measurements.  A one nanosecond counter and measurements one hour apart gives you a resolution of < 3e-13.  The long interval between measurements helps average out the GPSDO's jitter.  This type of measurement can be made manually or you can collect the data electronically.

If you want to discipline the Rb to the GPS, that's a totally different situation.  I've never really understood the advantages to that (other than the technical challenge).  The only situation where I can see an advantage is if you have really poor GPS reception and you want your standard to maintain service through the outages.  Other than that, you can build an OCXO-equipped GPSDO that provides short-term performance that's limited only by the OCXO itself and long-term performance that's limited by GPS itself.  Both are better than the Rb's performance.

Ed
 

Offline TheSteve

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Re: Economical option for precision frequency reference?
« Reply #353 on: January 04, 2016, 10:23:27 pm »
I did the quick tuning using my frequency counter and then for fine tuning I compared the GPSDO output to the Rb output on the scope using infinite persistence. Once they are very close it will take a few minutes to see it start drifting either direction. Obviously you can't do this really fast or jitter in the GPSDO itself would mess you up. No matter how much adjustment you do it will always eventually drift with respect to the GPSDO. Ideally I'd like it so that a full cycle of drift took an hour +. Not sure it would last day to day though, in my Temex Rb's you can adjust frequency electronically but also via an analog voltage set by a pot - which will drift some on its own.
VE7FM
 

Offline Vgkid

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Re: Economical option for precision frequency reference?
« Reply #354 on: January 04, 2016, 10:38:22 pm »
I bought a gpsdo in addition to the symmetricom. From what i can see: Furuno receiver + 8663-xs receiver. Hopefully it artives intact. Update in a few months.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline Bryan

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Re: Economical option for precision frequency reference?
« Reply #355 on: January 05, 2016, 09:47:41 am »
I did the quick tuning using my frequency counter and then for fine tuning I compared the GPSDO output to the Rb output on the scope using infinite persistence. Once they are very close it will take a few minutes to see it start drifting either direction. Obviously you can't do this really fast or jitter in the GPSDO itself would mess you up. No matter how much adjustment you do it will always eventually drift with respect to the GPSDO. Ideally I'd like it so that a full cycle of drift took an hour +. Not sure it would last day to day though, in my Temex Rb's you can adjust frequency electronically but also via an analog voltage set by a pot - which will drift some on its own.

That is pretty much the procedure that I have done, but with some OCXO's. For the home lab I don't think one can do much better, nor need to. What I do is watch the output of the GPSDO on Lady heather and when I see it at the lowest oscillator fluctuations is when I will do an adjustment. We are talking >+/- 50PPT fluctuations in Lady Heather so think that is about as good as I will get it. If I had a better view of the sky I might be able to get better ADEV readings.



« Last Edit: January 05, 2016, 09:54:11 am by Bryan »
-=Bryan=-
 

Offline FlyingHacker

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Re: Economical option for precision frequency reference?
« Reply #356 on: March 01, 2016, 07:08:31 pm »
How are you using these GPSDOs? Do you leave them up all the time as your lab reference?

Could you simply turn it on, let it warm up and lock, and then calibrate a regular OCXO to it? Then use that regular OCXO as your lab reference frequency?

How accurate a 10MHz do you get with one of these if you only turn it on for a few hours?

My issue is that an OCXO is plenty accurate for a few months, but I need some way to calibrate it periodically. I doubt I can get a GPS signal on a regular basis in my basement hobby lab.

Thoughts?
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Offline Macbeth

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Re: Economical option for precision frequency reference?
« Reply #357 on: March 01, 2016, 09:19:16 pm »
How are you using these GPSDOs? Do you leave them up all the time as your lab reference?

Could you simply turn it on, let it warm up and lock, and then calibrate a regular OCXO to it? Then use that regular OCXO as your lab reference frequency?

How accurate a 10MHz do you get with one of these if you only turn it on for a few hours?

My issue is that an OCXO is plenty accurate for a few months, but I need some way to calibrate it periodically. I doubt I can get a GPS signal on a regular basis in my basement hobby lab.

Thoughts?
The GPSDO has an OCXO onboard (that it disciplines, hence "Lady Heather" the dominatrix  :-DD ). I don't have any other reference because I am not a time-nut (yet).

Yes, if you turn it off then it will need to re-establish itself, but the 10MHz OCXO will still be available with a good degree of confidence for quite a while during GPS blackouts. That Figure of Merit is available via the serial terminal.

Also, your basement lab shouldn't be a problem as an external active "GPS mouse" is all you need - I have mine attached to a drain pipe outside my window. If you need more than a couple of meters of cable then there are trim factors you can specify to the GPSDO to let it correct for that.
 

Offline FlyingHacker

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Re: Economical option for precision frequency reference?
« Reply #358 on: March 01, 2016, 10:19:27 pm »
Thanks for the reply.

So whenever you turn off the GPSDO do you have to re-establish a GPS lock for it to be accurate again, or is the control voltage stored in some non-volatile medium, allowing it to boot up to the same accuracy as when it shut down?

I guess I am trying to figure out if you need a GPS signal every time you power cycle the unit.

Is this comparable:
http://www.ebay.com/itm/121530825744?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I mainly want something to calibrate to.

Thanks.
--73
 

Offline TheSteve

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Re: Economical option for precision frequency reference?
« Reply #359 on: March 01, 2016, 10:39:50 pm »
I have found my Trimble units to be very good at power on even without a GPS lock. However if you're storing them for several months unpowered I think you'd want to power them on for several hours with a solid GPS lock before using them to calibrate other equipment.
VE7FM
 

Offline FlyingHacker

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Re: Economical option for precision frequency reference?
« Reply #360 on: March 01, 2016, 11:11:32 pm »
I have found my Trimble units to be very good at power on even without a GPS lock. However if you're storing them for several months unpowered I think you'd want to power them on for several hours with a solid GPS lock before using them to calibrate other equipment.

I assume they feed a control voltage to the OCXO PLL based on the GPS info. Do you know if that control voltage is stored between boots? Or is power on today the exact same as power on last year? (Rather than the same as the last power off?)
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Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #361 on: March 01, 2016, 11:35:18 pm »
I have found my Trimble units to be very good at power on even without a GPS lock. However if you're storing them for several months unpowered I think you'd want to power them on for several hours with a solid GPS lock before using them to calibrate other equipment.

I assume they feed a control voltage to the OCXO PLL based on the GPS info. Do you know if that control voltage is stored between boots? Or is power on today the exact same as power on last year? (Rather than the same as the last power off?)

It's not just a matter of preserving the control voltage in non-volatile memory, it's physical changes to the crystal that happen when it sits powered off - the longer it sits that way, the worse. There's some good info on that on the thread (may be the other thread I started on this topic).  That said, I think all those issues have to do with phase noise, not the overall usefulness of the frequency standard for 99.999% of the use cases.

FWIW, I installed an OCXO in my HP 53131A, but I've since switched over to using the GPSDO. I got a splitter and use it as the external ref for my Siglent function generator as well.


 

Offline FlyingHacker

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Re: Economical option for precision frequency reference?
« Reply #362 on: March 01, 2016, 11:52:54 pm »
I assume they feed a control voltage to the OCXO PLL based on the GPS info. Do you know if that control voltage is stored between boots? Or is power on today the exact same as power on last year? (Rather than the same as the last power off?)

It's not just a matter of preserving the control voltage in non-volatile memory, it's physical changes to the crystal that happen when it sits powered off - the longer it sits that way, the worse. There's some good info on that on the thread (may be the other thread I started on this topic).  That said, I think all those issues have to do with phase noise, not the overall usefulness of the frequency standard for 99.999% of the use cases.

I see. So with the GPS whipping the OCXO into shape it can account for phase noise as well?

I have a 5 port video amp on order. I was going to convert that to 50ohm.

Do you think this eBay item is comparable to the ones you guys use:

http://www.ebay.com/itm/121530825744?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

EDIT: This seller looks better:
http://www.ebay.com/itm/10MHZ-Output-Sine-Wave-GPS-Disciplined-Clock-GPSDO-Antenna-Power-Supply-USA/281802105734?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D35851%26meid%3Dbd3ea62677a64cec9b16b74b0a4ecc33%26pid%3D100011%26rk%3D1%26rkt%3D7%26sd%3D281701995263

I see a few out there, and want to make sure I get the "good one."

Thanks!
« Last Edit: March 01, 2016, 11:56:38 pm by FlyingHacker »
--73
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #363 on: March 02, 2016, 12:10:58 am »
I see. So with the GPS whipping the OCXO into shape it can account for phase noise as well?

No, the GPS is very good for long-term stability, but bad for short-term stability. The OCXO is exactly he opposite - very good for short-term stability, but not good for long-term stability. By using the GPS signal to slowly condition the OCXO, we get the best of both worlds.

Also, FWIW, my GPSDO takes about 24 hours to pronounce itself at full quality after being off for a significant amount of time.

I have a 5 port video amp on order. I was going to convert that to 50ohm.

I think a lot of people go that route. I ended up getting the TAPR TADD-1, but only because at the time I was having trouble finding a good used dist. amp on eBay, with the exception of some giant units that were not practical for me.

Do you think this eBay item is comparable to the ones you guys use:

http://www.ebay.com/itm/121530825744?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

EDIT: This seller looks better:
http://www.ebay.com/itm/10MHZ-Output-Sine-Wave-GPS-Disciplined-Clock-GPSDO-Antenna-Power-Supply-USA/281802105734?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D35851%26meid%3Dbd3ea62677a64cec9b16b74b0a4ecc33%26pid%3D100011%26rk%3D1%26rkt%3D7%26sd%3D281701995263

I see a few out there, and want to make sure I get the "good one."

There is this guy in China who has made himself a little PCB to which contains the LEDs, a TTL to RS232 converter chip, mount points for the BNC connectors, and a linear regulator. To this board, he attaches various surplus GPSDO boards from various manufacturers, all of which were manufactured for the cell industry. He also has one GPSDO of his own design, which is the one you list. It is actually pretty decent, except that the frequency is very slightly off due to some error in his calculations. I think the RS-232 interface is also a little whacky (or maybe that unit doesn't even have one, I can't recall). Check out this thread for details on all the BG6TBL units:

https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/

If you can find the BG7 unit with the STAR4 board inside, which is made by and uses an Oscilloquartz OCXO, that is the one IMHO to get. It's got a sensitive GPS chip, very good OCXO, and with a small mod to swap the RS232 connection inside the BG7TBL box, you can send commands and receive info to/from the internal processor.
https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/msg722795/#msg722795
« Last Edit: March 02, 2016, 12:37:00 am by motocoder »
 

Offline FlyingHacker

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Re: Economical option for precision frequency reference?
« Reply #364 on: March 02, 2016, 12:56:25 am »
Yeah, I found the other thread. I don't seem to be able to find one of the STAR4 models. Grrr.... Anyone?
--73
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #365 on: March 02, 2016, 01:44:54 am »
Yeah, I found the other thread. I don't seem to be able to find one of the STAR4 models. Grrr.... Anyone?

AFAIK, this guy at 168electronics is hording the actual GPSDO inside the BG7TBL wrapper. He does not want to sell them in small quantities.

http://www.168electronics.com/oscilloquartz-star-4-osa-oem-10mhz-gps-clock-p-122.html
 

Offline usagi

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Re: Economical option for precision frequency reference?
« Reply #366 on: March 19, 2016, 03:37:07 pm »
some of the 2015-09-17 models are apparently being manufactured with oscilloquartz ocxo's. you should ping the seller to verify if it's a morion or oscilloquartz (if you actually care).

in any case the most important detail is you get a bg7tbl with a ublox gps receiver.

Offline dagg

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Re: Economical option for precision frequency reference?
« Reply #367 on: August 14, 2016, 02:16:43 pm »
I have been running my BG7TBL / 2014-12-09 for quite a while now and it is working fine. I have been trying to get it working as a source for an NTP server using the instructions at: http://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html

I cannot get the receiver t change its settings to match the requirements to work at 4800 baud and use only $GPRMC sentences. I am using the u-center software but none of the settings I send take hold. With the default settings the NTP server software does not use the GPS as the clock.

Any suggestions?

You don't need the (coarse gps) time of the gpsdo, only the pps time sync from it if you manage to see that signal on the windows serial port. In linux (raspberry pi 3) that is no problem.

The clue is:

server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 refid PPS

and get your coarse time in the normal way through an internet time server.

Jan.

Ham Operator PH3J
 

Offline Lee

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Re: Economical option for precision frequency reference?
« Reply #368 on: November 01, 2016, 05:04:54 am »
I know these units might be a few dollars more than the BG7 units but how come no one has experience with units such as :

Leo Bodnar Unit
http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=234

Nick Sayer
https://www.tindie.com/products/nsayer/gps-disciplined-xcxo/

And I recently discovered a GPSOD by Bob Stewart that looks very promising.
http://ae6rv.com/

I am very interested in the BG7 GPSDO's but I keep wondering if spending a few dollars for a unit that is a more modern design and has support might be a better investment in the long run. The BG7's seem to be a little bit of crap shoot as to what surplus gps and oscillator are available.

Anyone using any of these units ?

Lee
 

Offline TheSteve

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Re: Economical option for precision frequency reference?
« Reply #369 on: November 01, 2016, 05:16:55 am »
You can buy surplus Trimble units for less then 60 USD shipped on ebay, tough to beat the performance per dollar.
VE7FM
 

Offline Vgkid

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Re: Economical option for precision frequency reference?
« Reply #370 on: November 01, 2016, 06:32:13 am »
From the Time nuts mailing list, the AE6RV version will be interesting.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline Lee

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Re: Economical option for precision frequency reference?
« Reply #371 on: November 02, 2016, 02:29:16 am »
You can buy surplus Trimble units for less then 60 USD shipped on ebay, tough to beat the performance per dollar.

Yes but that is not exactly plug n play GPSDO. You are buying a pig in a poke with no idea of the condition or accuracy.
If someone is trying to get a dead-nuts frequency standard and you don't have $1000 of dollars of calibrated test gear and Rubidium standard then that $60 might sounds like a deal but in the long run is it really worth it ?

I love finding a great hamfest bargain as much as the next guy but I wonder if an aging and deaf gps is really a smart move.
This new generation of GPSDO's are small, accurate, have super sensitive receivers and offer firmware updates and human being on available to help with questions.

I guess what I am suggesting is it might be fun to experiment with a bargain gpsdo board but in the long run does it make good sense financially and in regards to accuracy?

For example: You can get a BG7 setup for around $160. From what I gather from the forum they have had a wide range of every changing surplus OCXO's.
No specs, no measurement, no idea of what you will end up with.

But for $200 someone could purchase something like a Nick Sayers GPSDO with documentation, actual measurements/specs and support.

I think what I am trying to get across is to remind people there Price and there is Cost. And the lowest price might not actually be the lowest in cost in the long run.

 

Offline TheSteve

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Re: Economical option for precision frequency reference?
« Reply #372 on: November 02, 2016, 03:27:16 am »
You have valid points, but I don't think it is that much of a crap shoot buying one unless it is physically damaged - and if it is you can get a refund or replacement. The thread title is economical - pretty tough to beat 60 bucks for a Trimble. Of course you can get better/newer but if someone needs one for the lowest cost possible the surplus Trimble wins. They weren't scrapped because of the GPSDO performance, it was because the cellular protocol in the base station they are pulled from is obsolete.

VE7FM
 

Offline rickells

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Re: HP 70K / HP 70000 MMS Microwave system Thread
« Reply #373 on: February 11, 2018, 10:14:47 pm »
hi,

Please note the start of a Thread for the HP 70000 or HP 70K MMS system & modules:
Future discussion hopes to include: Configuration, Use, repair, software, etc... to further understanding.
HP 70004a, HP 70205a & HP 206a Display units + the HP 70001 Mainframe & a host of HP 70xxx modules.
See:
        https://www.eevblog.com/forum/testgear/hp-70khp-70000-modular-measurement-system-thread/msg1420425/#msg1420425

thank you
 


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