Author Topic: Economical option for precision frequency reference?  (Read 119423 times)

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Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #50 on: July 25, 2015, 05:44:20 am »
I have been trying to get the Z38XX to run but having no luck and looks like my REF1 unit took a dump today, doesnt seem to be receiving anything anymore, yellow GPS light just stays on now. It was working yesterday (excepts the com prog.)

Keep working on Z38xx.  Until you can talk to the box, there's very little you can troubleshoot.  Are you using an RS-422 converter or the hack?

Ed
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #51 on: July 25, 2015, 06:16:32 am »
I have been trying to get the Z38XX to run but having no luck and looks like my REF1 unit took a dump today, doesnt seem to be receiving anything anymore, yellow GPS light just stays on now. It was working yesterday (excepts the com prog.)

Keep working on Z38xx.  Until you can talk to the box, there's very little you can troubleshoot.  Are you using an RS-422 converter or the hack?

Ed

I am using an RS422 converter from Gearmo, and two DB9 breakout connectors and some wires to correct the cable pinout. It's a bit of a hack, but I've got that working now. The main issue I was having was that I had it connected to J6 instead of J8   :palm:

So, good news is, I can run the :SYST:STAT? command you gave me. Bad news is, it doesn't appear to be tracking any satellites at all. The antenna is indoors, but it's right next to a window. The window faces East.
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #52 on: July 25, 2015, 06:51:31 am »
Is :SYST:STAT? reporting a time of day or a location, in particular, wrong ones?  I don't know how much you know about GPS, but the receiver has to find at least one bird to get the date and time and then download the almanac & ephemeris data before it can start to find the other birds.  It's amazing that it can do it at all.  If it already has a location stored, iti'll be looking for the wrong birds in the wrong directions and it'll take a very long time to sort itself out.  You might be able to help it out by giving it a date, time, and location.  There are commands for that in the manual for the Z3801A or 58503A (aka Z3816A - yes more alternate names).  Remember that all these boxes have the same basic command structure.

Or, just let it run and see if magic happens.  It sometimes looks hopeless, but then you turn away and when you look back, it's done!

Ed
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #53 on: July 25, 2015, 06:59:22 am »
Is :SYST:STAT? reporting a time of day or a location, in particular, wrong ones?  I don't know how much you know about GPS, but the receiver has to find at least one bird to get the date and time and then download the almanac & ephemeris data before it can start to find the other birds.  It's amazing that it can do it at all.  If it already has a location stored, iti'll be looking for the wrong birds in the wrong directions and it'll take a very long time to sort itself out.  You might be able to help it out by giving it a date, time, and location.  There are commands for that in the manual for the Z3801A or 58503A (aka Z3816A - yes more alternate names).  Remember that all these boxes have the same basic command structure.

Or, just let it run and see if magic happens.  It sometimes looks hopeless, but then you turn away and when you look back, it's done!

Ed

This is what :SYST:STAT? is reporting:

-------------------------- Secondary Receiver Status --------------------------
SYNCHRONIZATION ............................ [ Outputs Valid/Reduced Accuracy ]
SmartClock Mode ___________________________   Reference Outputs _______________
>> Locked to Ext: stabilizing frequency       TFOM     3             FFOM     1
   Recovery                                   1PPS TI -50.0 ns relative to Ext
   Holdover                                   HOLD THR 1.000 us
   Power-up                                   Holdover Uncertainty ____________
                                              Predict  432.0 us/initial 24 hrs

ACQUISITION ................................................ [ Ext 1PPS Valid ]
Tracking: 0 ____   Not Tracking: 8 ________   Time ____________________________
                   PRN  El  Az                GPS      06:56:19 (?) 25 Jul 2015
                   * 1  -- ---                GPS 1PPS Invalid: not tracking
                   * 3  -- ---                ANT DLY  60 ns
                   *11  -- ---                Position ________________________
                   *12  -- ---                MODE     Survey:      0% complete
                   *13  -- ---                         Suspended: track <4 sats
                   *18  -- ---                INIT LAT     0:00:00.000
                   *26  -- ---                INIT LON     0:00:00.000
                   *29  -- ---                INIT HGT           +0.00 m  (GPS)
ELEV MASK 10 deg   *attempting to track
HEALTH MONITOR ......................................................... [ OK ]
Self Test: OK    Int Pwr: OK   Oven Pwr: OK   OCXO: OK   EFC: OK   GPS Rcv: OK
E-113>


Also, while trying to copy that to the clipboard, I inadvertently pasted it back in the terminal window. Now the prompt I am receiving says "E-101>" instead of "E-113>". Any idea what that means? I'm worried that I changed some setting...

 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #54 on: July 25, 2015, 07:21:04 am »
Ignore my last question about the prompt. I found the manual for the Z3801A, and based on that got the commands to set the time (which I assume is in UTC), and the Lat/Long for my location. So let's see how things go now.
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #55 on: July 25, 2015, 07:42:00 am »
This doesn't look too bad.

Notice that it's got the date and time right, even though it's not quite sure it believes it.  That means that it did receive at least one satellite.  As Dave would say:  "There's a trap for young players."  The time isn't UTC.  It's GPS time.  GPS Time doesn't pay any attention to leap seconds so it's currently 17 seconds ahead of UTC.  There is a command to change from GPS Time to UTC Time but it's not in the manual.  Google for it.

It may or may not have been able to download all the data it wants.  It obviously hasn't tracked enough birds simultaneously to calculate its location.  Since you've now given it your approximate location, it should speed up the process.  Until it's finished the initial acquisition, you can't even tell if your signal strength is going to be acceptable or not.

See where it talks about 'Mode: Survey' ?  After it finds the birds, it goes into a survey mode where it figures out its exact location.  I forget how long that takes.  Then it changes to 'Mode: Hold' .  That's when it can really start to lock down the OCXO.  Of course, at that point, the OCXO is still probably staggering around like a drunken sailor so the control is not entirely effective.  ;)  The HUP will stay at 432 us (its default value) for some hours, at least, and then start to come down.  Mine is still staggering around.

By the way, don't forget that 'staggering around' means that over the last three days my unit has deviated from perfection by less than 300 parts per billion so for most normal purposes, it's perfectly usable.  The error of the 10 MHz signal would be less than 1 Hz.  Yes, I'm a Time-Nut.  :)

Ed
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #56 on: July 25, 2015, 08:08:49 am »
This doesn't look too bad.

Notice that it's got the date and time right, even though it's not quite sure it believes it.  That means that it did receive at least one satellite.  As Dave would say:  "There's a trap for young players."  The time isn't UTC.  It's GPS time.  GPS Time doesn't pay any attention to leap seconds so it's currently 17 seconds ahead of UTC.  There is a command to change from GPS Time to UTC Time but it's not in the manual.  Google for it.

It may or may not have been able to download all the data it wants.  It obviously hasn't tracked enough birds simultaneously to calculate its location.  Since you've now given it your approximate location, it should speed up the process.  Until it's finished the initial acquisition, you can't even tell if your signal strength is going to be acceptable or not.

See where it talks about 'Mode: Survey' ?  After it finds the birds, it goes into a survey mode where it figures out its exact location.  I forget how long that takes.  Then it changes to 'Mode: Hold' .  That's when it can really start to lock down the OCXO.  Of course, at that point, the OCXO is still probably staggering around like a drunken sailor so the control is not entirely effective.  ;)  The HUP will stay at 432 us (its default value) for some hours, at least, and then start to come down.  Mine is still staggering around.

By the way, don't forget that 'staggering around' means that over the last three days my unit has deviated from perfection by less than 300 parts per billion so for most normal purposes, it's perfectly usable.  The error of the 10 MHz signal would be less than 1 Hz.  Yes, I'm a Time-Nut.  :)

Ed

I diddled around with the antenna location, and disconnected it several times. I wonder if I should restart the survey.

BTW - I noticed when I gave it the command to set the time that it set the time ahead of the value I gave it, so I suspect it is set to take the time in UTC and it converted it to GPS time.
« Last Edit: July 25, 2015, 08:10:47 am by motocoder »
 

Offline Shock

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Re: Economical option for precision frequency reference?
« Reply #57 on: July 25, 2015, 10:14:24 am »
Regarding not using an antenna, can GPSDOs or specifically the Z3811A, RFTG-U REF-1 units be run without the antenna while testing it and setting up the RS422? or can damage occur running it this way?
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Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #58 on: July 25, 2015, 02:04:25 pm »
It ran all night, checked this morning - still not tracking any satellites. So either that eBay antenna is broken or  its just not sensitive enough to pick up any satellites indoors.

This may be a deal killer for me. I'm not sure I want to go through all the hassle of setting up an outdoor antenna.
« Last Edit: July 25, 2015, 02:24:02 pm by motocoder »
 

Offline SeanB

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Re: Economical option for precision frequency reference?
« Reply #59 on: July 25, 2015, 02:22:46 pm »
tried by a south facing window? my cheapie gps works with it sitting on the sill inside, facing north west.
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #60 on: July 25, 2015, 02:25:30 pm »
tried by a south facing window? my cheapie gps works with it sitting on the sill inside, facing north west.

I have seen several GPS, including the one in my phone, work indoors here. However, those all use newer, more sensitive GPS chips.

The houses are very close together here. There is a house blocking the view of the sky on the south-facing window.
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #61 on: July 25, 2015, 02:28:49 pm »
I diddled around with the antenna location, and disconnected it several times. I wonder if I should restart the survey.

BTW - I noticed when I gave it the command to set the time that it set the time ahead of the value I gave it, so I suspect it is set to take the time in UTC and it converted it to GPS time.

I don't think it's necessary to restart the survey.  If it loses satellites, it just suspends the survey until it sees more and then it resumes where it left off.  Is the survey still at 0%?  If it's > 0%, that means that it did get a reasonable fix sometime through the night and started the survey.  If so, is the location still exactly what you entered?

It didn't 'convert' the time to GPS.  It assumed that the time you gave it was GPS, but then it managed to receive a satellite signal and updated its time to the time that it got from the bird.

Quote
It ran all night, checked this morning - still not tracking any satellites. So either that eBay antenna is broken or  its just not sensitive to pick up any satellites indoors.

This may be a deal killer for me. I'm not sure I want to go through all the hassle of setting up an outdoor antenna.

Before you give up, try to get your antenna temporarily to a better location.  Tie it to a broom and stick it out the window so that it can see towards the equator if you have to.  Don't forget that you want to point the antenna up.   ;)  What antenna have you got?

A GPSDO does require a reasonable antenna.  I'm on the top floor of an apartment building.  I was able to get an adequate signal with a timing-grade antenna indoors but since I was able to, I moved the antenna outdoors and now I've got a great signal.

Ed
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #62 on: July 25, 2015, 02:37:00 pm »
I diddled around with the antenna location, and disconnected it several times. I wonder if I should restart the survey.

BTW - I noticed when I gave it the command to set the time that it set the time ahead of the value I gave it, so I suspect it is set to take the time in UTC and it converted it to GPS time.

I don't think it's necessary to restart the survey.  If it loses satellites, it just suspends the survey until it sees more and then it resumes where it left off.  Is the survey still at 0%?  If it's > 0%, that means that it did get a reasonable fix sometime through the night and started the survey.  If so, is the location still exactly what you entered?

It didn't 'convert' the time to GPS.  It assumed that the time you gave it was GPS, but then it managed to receive a satellite signal and updated its time to the time that it got from the bird.

Quote
It ran all night, checked this morning - still not tracking any satellites. So either that eBay antenna is broken or  its just not sensitive to pick up any satellites indoors.

This may be a deal killer for me. I'm not sure I want to go through all the hassle of setting up an outdoor antenna.

Before you give up, try to get your antenna temporarily to a better location.  Tie it to a broom and stick it out the window so that it can see towards the equator if you have to.  Don't forget that you want to point the antenna up.   ;)  What antenna have you got?

A GPSDO does require a reasonable antenna.  I'm on the top floor of an apartment building.  I was able to get an adequate signal with a timing-grade antenna indoors but since I was able to, I moved the antenna outdoors and now I've got a great signal.

Ed

Survey still at 0%, location has not changed.

 

Offline G0HZU

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Re: Economical option for precision frequency reference?
« Reply #63 on: July 25, 2015, 02:54:15 pm »
Meanwhile, in much less time than it takes to type this post I connected a counter to a ferrite loop antenna + amplifier/filter + hard limiter circuit and was able to see 198.000000kHz +/- 0.000001kHz from BBC Radio 4 on an old Philips reciprocal counter on a 10 second gate time. The counter is connected to a decent OCXO that should be within about 0.02Hz at 10MHz.

i.e it takes maybe 30 seconds to get this resolution/agreement of 1mHz in 198kHz and confirm that the counter is seeing a stable signal on a 10 second gate time.

Nothing needs to 'lock' nothing needs to stagger about and the above lashup will work anywhere in the house. OK, maybe if I sat it next to a wallwart battery charger it might not work due to the RFI from a charger but it will realistically give a decent result in way less than 1 minute from cold anywhere in the house with no need for an external antenna.

How much more accuracy do you need?
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #64 on: July 25, 2015, 03:14:48 pm »
Meanwhile, in much less time than it takes to type this post I connected a counter to a ferrite loop antenna + amplifier/filter + hard limiter circuit and was able to see 198.000000kHz +/- 0.000001kHz from BBC Radio 4 on an old Philips reciprocal counter on a 10 second gate time. The counter is connected to a decent OCXO that should be within about 0.02Hz at 10MHz.

i.e it takes maybe 30 seconds to get this resolution/agreement of 1mHz in 198kHz and confirm that the counter is seeing a stable signal on a 10 second gate time.

Nothing needs to 'lock' nothing needs to stagger about and the above lashup will work anywhere in the house. OK, maybe if I sat it next to a wallwart battery charger it might not work due to the RFI from a charger but it will realistically give a decent result in way less than 1 minute from cold anywhere in the house with no need for an external antenna.

How much more accuracy do you need?

Thanks for the comment. We've gone well beyond the bounds of "need" here.

Back to the topic at hand, I think the pole I had the antenna on, which was sitting in teh window sill, may have been putting the antenna in a really bad spot (blocked by the window frame). At least that's my theory. I fashioned a crude mount for the antenna, so that it can rest stably at the *bottom* of the window, which is where I was able to get a GPS lock on other devices.

 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #65 on: July 25, 2015, 03:16:31 pm »
And, that made a big difference. Tracking 2 satellites now.
 

Offline G0HZU

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Re: Economical option for precision frequency reference?
« Reply #66 on: July 25, 2015, 03:34:48 pm »
Realistically, you need to deply the GPS antenna outside with a decent view of the sky. A few years ago I borrowed a Quartzlock E8-Y GPSDO from work to review it.

http://quartzlock.com/product/frequency-reference/gps/E8-Y

I found it would declare it was locked after about 45minutes even with an indoor antenna sat by a window (like you have done).
But the performance was nowhere near as good as it should be. It flashes LEDs to say how many satellites it is tracking and even with several satellites it delivered relatively poor frequency stability with the window antenna. Much worse than I can get with the above 198kHz offair lashup.

But once I put the antenna outside and it could see a fair bit of sky it performed a lot better. However, I could still get it to misbehave just by moving the unit or rotating it.

Basically, it just isn't worth the hassle. I'm not alone in this view at my place of work. I'm not aware of ANY engineer who actually uses this E8-Y as a 10MHz reference in the labs at work. The old school Quartzlock 198kHz offair standards are much preferred because they are portable anywhere within the building and work within about 1 minute and deliver enough performance to calibrate a decent OCXO in our designs or in a piece of test gear.
« Last Edit: July 25, 2015, 03:57:16 pm by G0HZU »
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #67 on: July 25, 2015, 03:46:28 pm »
Meanwhile, in much less time than it takes to type this post I connected a counter to a ferrite loop antenna + amplifier/filter + hard limiter circuit and was able to see 198.000000kHz +/- 0.000001kHz from BBC Radio 4 on an old Philips reciprocal counter on a 10 second gate time. The counter is connected to a decent OCXO that should be within about 0.02Hz at 10MHz.

i.e it takes maybe 30 seconds to get this resolution/agreement of 1mHz in 198kHz and confirm that the counter is seeing a stable signal on a 10 second gate time.

Nothing needs to 'lock' nothing needs to stagger about and the above lashup will work anywhere in the house. OK, maybe if I sat it next to a wallwart battery charger it might not work due to the RFI from a charger but it will realistically give a decent result in way less than 1 minute from cold anywhere in the house with no need for an external antenna.

How much more accuracy do you need?

Excellent question!  And one that each user needs to answer.  At the beginning of this thread I gave a list of options.  Decide where you need or want to be on that list and go from there.

For 'normal' users, zero-beating against an HF station or using one of the LF stations as you've described might be the perfect solution.

There are ham operators who operate at 10 GHz.  They need to be within a few Hertz of the correct frequency to contact the other end so they probably need accuracy of 1e-9 or even better.  If you're a space hobbyist who's trying to pick up signals from deep space, 1e-9 might be so bad that your equipment is broken!

If you're a time-nut like me, need has very little to do with it.  You want to see how far you can push things.  A while ago, I bought a dead Cesium Frequency Standard.  I tested the tube to see if it had any life left in it.  I was able to measure the frequency of the response peak as 9.192 631 790 GHz.  Curses!  My calibration was 20 Hz off!  I should have measured .... 770, not ....790.  ;D

By the way, when I said that my 'staggering around' system was within 300 parts per billion, I was wrong.  I made a silly mistake.  Now that I've had some sleep I looked at the numbers again and found that the frequency error of the 10 MHz is around .001 Hz or 0.1 parts per billion.  In fact, that's likely the limit of my measurement system.  Making measurements at these levels is always an issue!

Ed
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #68 on: July 25, 2015, 03:55:17 pm »
Realistically, you need to deply the GPS antenna outside with a decent view of the sky. A few years ago I borrowed a Quartzlock E8-Y GPSDO from work to review it.

http://quartzlock.com/product/frequency-reference/gps/E8-Y

I found it would declare it was locked after about 45minutes even with an indoor antenna sat by a window (like you have done).
But the performance was nowhere near as good as it should be. It flashes LEDs to say how many satellites it is tracking and even with several satellites it delivered relatively poor frequency stability with the window antenna. Much worse than I can get with the above 198kHz offair lashup.

But once I put the antenna outside and it could see a fair bit of sky it performed a lot better. However, I could still get it to misbehave just by moving the unit or rotating it.

Basically, it just isn't worth the hassle. I'm not alone in this view at my place of work. I'm not aware of ANY engineer who actually uses this E8-Y as a 10MHz reference in the labs at work. The old school Quartzlock 198kHz offair standards are much preferred because they work within about 1 minute and deliver enough performance to calibrate a decent OCXO in our designs or in a piece of test gear.

I'm not familiar with the Quartzlock products, but I've often thought of GPSDOs and precision quartz oscillators as 'Prima Donnas'.  They can be incredible performers, but any little thing can throw them off.  If you need or want this level of performance, you accomodate their eccentricities.

Ed
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #69 on: July 25, 2015, 09:26:10 pm »
It will occasionally list a satellite as tracking, but it quickly drops it, and now it's been running for almost 24 hours with zero progress on teh survey. Looks like this thing is a nonstarter unless I'm willing to drill holes in the wall and run a cable to an outside antenna. I wish I had realized that, as I'm sure that other unit on eBay, using a modern GPS chip, would have no issues pulling in the sats.

Anyone interested in buying this, PM me.
 

Online Vgkid

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Re: Economical option for precision frequency reference?
« Reply #70 on: July 25, 2015, 09:36:45 pm »
Pm sent, i might give it a try.
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Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #71 on: July 25, 2015, 10:27:59 pm »
What puzzles me is that if you read the original Time Nuts mailing list post on these things, the author describes using a "poorly-sited indoor GPS antenna", and yet his system apparently completed the survey in a few hours. Something is wrong here. I am wondering if the antenna I got off of eBay is broken.

Can someone recommend an antenna that is known to work well with this unit?
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #72 on: July 26, 2015, 12:11:35 am »
Sorry for the thread spam, just updating all the details in case anyone else gets one of these. I am not a big fan of mailing lists because the archives are really hard to search for useful info. I like this site, and searching past threads has helped me numerous times. So I think it's good to capture some of this data here.

I got the wife to help me set up the antenna outside. It's still on that wood block I mounted it on earlier this morning; the whole thing is just resting on the ground about 10 feet from my window. But it immediately, upon power-up, started tracking 4 satellites, and now it's up to 6. So a huge improvement.
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #73 on: July 26, 2015, 12:17:31 am »
Woo hoo! Status now says:

"Locked to GPS: stabilizing frequency"

And the NO GPS light is no longer lit.
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #74 on: July 26, 2015, 12:55:45 am »
Wow, even located outside it is having some issues. The survey has now been suspended because it no longer has 4 satellites tracking. Really piss-poor GPS in this thing, I guess. I guess I'll  have to get the antenna up on the roof to use it.
 


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