Author Topic: bg7tbl gpsdo master reference  (Read 249097 times)

0 Members and 2 Guests are viewing this topic.

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #850 on: December 04, 2020, 11:00:58 pm »
My E-GPSDO arrived today.
It is working but i still have to check how accurate.
I will try to measure it within the next days.
Attached some pictures from inside. PCB is marked with 10.1.2020 BG7TBL

GPS fix is only possible if i put the antenna outside (then within <30s (inside, behind my window there is no fix).
 

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #851 on: December 04, 2020, 11:55:19 pm »
Hm wonder why the E-GPSDO should be more accurate than the PLL-GPSDO ?
Looks the same (even same PCB marking) except the ocxo number.
But its 7€ more expensive..
 

Online MegaVolt

  • Frequent Contributor
  • **
  • Posts: 420
  • Country: by
Re: bg7tbl gpsdo master reference
« Reply #852 on: December 05, 2020, 09:17:21 am »
What does the letter "E" stand for?
 

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #853 on: December 05, 2020, 09:43:33 am »
I don't know.
Maybe "Expensive"  :horse:
Seller has 2 different ones E- GPSDO:

https://a.aliexpress.com/_BU3HTD

PLL-GPSDO:

https://a.aliexpress.com/_BTItxD

With different accuracy. I also asked him what is the difference and he said "accuracy" no further explanations.
 
The following users thanked this post: MegaVolt

Offline 0xFFF0

  • Contributor
  • Posts: 44
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #854 on: December 05, 2020, 10:13:54 am »
Hm wonder why the E-GPSDO should be more accurate than the PLL-GPSDO ?
Looks the same (even same PCB marking) except the ocxo number.
But its 7€ more expensive..

Isn't it a PLL-gpsdo? I see a 74HC86 xor gate which are used in PPL style gpsdos.
« Last Edit: December 05, 2020, 10:16:59 am by 0xFFF0 »
 

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #855 on: December 10, 2020, 07:39:34 am »
I asked the seller, he says slightly different hardware and another software. But i don't know where, probably the Atmel.
I will look if i can read out the thing.
Another observation: sometimes it's loosing his fix. I put the antenna outside of my window but even then it's loosing the fix.
Does somebody know if there is something programmed within the flash of the GPS module? I assume it's a NEO-M7N maybe 6M or chinese clone.
Thinking about switching it to a NEO-M9N to get 4 GNSS concurrent. Hope that heps..

I measured it with my DG992 and SVA1032X i got 10.000.012 MHz but it was unlocked....again... grr
Hope this is fixed with a lock..Or my measurement equipment is more off than the GPSDO i don't know doensn't have something better to measure the GPSDO.
I purchased it to get a reference for these 2 devices..so..
 

Offline 0xFFF0

  • Contributor
  • Posts: 44
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #856 on: December 10, 2020, 08:32:23 am »
I have a DYI frequency standard with NEO7M and indoor antenna. Typical deviation 10.000.000,001 MHz (<+-1mHz, 1E-10..-11). The accuracy does not necessarily depend on the GPS receiver.
 

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #857 on: December 10, 2020, 08:39:09 am »
Ok but whats wrong then?
The Isotemp OCXO looks ok its from 1212 so pretty "young" . But 12Hz off? Or is it my measurment equipment?
I don't know what they have inside but i assume "only" a TCXO.

The Atmel has a "uncommon" crystal frequency (uncommon for my i would expect something like 8MHz or so.) but i don't think this is a problem or?

Indoor antenna? Can you give me a link?

Maybe someone can help. I will also buy another antenna. "Cheap but good" (i would assume) ones are:
https://www.mouser.de/datasheet/2/398/AA.166.301111-1508818.pdf
https://www.mouser.de/datasheet/2/398/AA.162.301111-1508634.pdf

But which one is better? Couldn't find a real difference.
« Last Edit: December 10, 2020, 09:51:35 am by Noy »
 

Offline 0xFFF0

  • Contributor
  • Posts: 44
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #858 on: December 10, 2020, 10:04:14 am »
A $4 antenna from ebay is sufficient. However, I believe that the problem is not the antenna, but the device. It is either defective or the quality is poor. The Chinese very often use very old oscillators.
 

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #859 on: December 10, 2020, 10:34:09 am »
So its getting closer... :-D
After several minutes its really close to 10.000.000,xx and somteimes even 9.999.995,2 (statistics on my "counter") but still no fix.

I think if its cold it gets a fix "fast" but its loosing his fix if its getting warm. Antenna is still on the same position but fix is gone.

Or I'm wrong.

Green is constantly blinking, red is permanantly on if its fixed or?
After a few minutes both leds (green and red) are blinking..

Does somebody know if its really rs232 or "only" UART? Will not destory it if i hook it up to my USB-RS232 adaptor?


Hmm don't know whats the issue here.
Now it got a fix even behind my window. Or the GPS Antenna isn't useable below +5°C?
And my Counter now says ist 10.000.000,31 / 9.999.999,23 (mean value).

Maybe the OCXO / GPS Module needed a "burn in"...
But changing the GPS Module / Antenna is maybe still a good idea to get more fix points. Don't know, will take a breath and decide than if i will spent additional 30€...
But my GPSDO was "only" 71€ in sale so still close to 100€ even with part changes...

« Last Edit: December 10, 2020, 03:03:25 pm by Noy »
 

Offline Hans_18T

  • Contributor
  • Posts: 10
  • Country: nl
Re: bg7tbl gpsdo master reference
« Reply #860 on: December 11, 2020, 11:25:26 pm »
Maybe have a look at this thread https://www.eevblog.com/forum/testgear/new-gpsdo-bg7tbl/
There are two versions, lets call them the old and the enhanced version. I have two old versions (one I got by "accident"), comparing the frequency they agree with each other in phase/frequency for at least something like 15 minutes. I have also the the updated (enhanced) version, this one slightly differs with respect to the frequency, not sure how much as I was not able to verify this (yet). In fact I bought the 3rd one, hoping that the known error of the first version had been fixed. Once I will be allowed to go to the office again I can perform further tests.
 

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #861 on: December 30, 2020, 12:06:11 am »
I want to drive my 2 devices with a 10MHz reference input from my GPSDO.
1. DG922 Rigol: It says 10MHz ref in 1kOhm AC
2. Siglent SVA1032X: It says 10 MHz ref in 50 Ohm. Probably AC? But not sure..

Can i drive both inputs directly from the GPSDO with 1 BNC T Connector + 2x BNC RG316 1m cables? Or do in need a distribution amplifier board?
1k||50Ohm are ~47 Ohm should be ok?

How should i connect the units? Special chain?:

GPSDO -> DG922 + T Connector -> SVA   <-Best one?
GPSDO -> SVA + T Connector -> DG922
SVA<-GPSDO + T Connector -> DG922
 

Offline rfclown

  • Regular Contributor
  • *
  • Posts: 147
  • Country: us
Re: bg7tbl gpsdo master reference
« Reply #862 on: December 30, 2020, 12:28:14 am »
> GPSDO -> DG922 + T Connector -> SVA   <-Best one?

I'd do that one.

> Does somebody know if its really rs232 or "only" UART? Will not destory it if i hook it up to my USB-RS232 adaptor?

It's not RS232. It's just UART. I don't have a unit labeled GB7TBL, but one that seems to be a copy. Mine is 0V to 5V output on the "RS232", but it is inverted. They take the UART output of the ublox module, and drive the base of a NPN which inverts the signal. To interface with a UART to USB converter I connected the Tx output to the base of another NPN (through a series 47k) and the collector to my UART input with a 4.7k pullup to the USB 5v. Resistors values not critical, I used ones that were lying on the bench.
 

Offline Electro Fan

  • Super Contributor
  • ***
  • Posts: 2847
Re: bg7tbl gpsdo master reference
« Reply #863 on: January 03, 2021, 12:14:04 am »
Happy New Year EEVblogers!

1. Just checking to see if a BG7TBL GPSDO should have a PPS pulse that:
a. occurs every second for a duration of 200ms?
b. has an amplitude of 3 volts?

2. (not exactly on topic), same questions for a Star 4+, ie should a Star 4+ have a PPS pulse that:
a. occurs every second for a duration of 50ms?
b. has an amplitude of ~2.5 volts?
 

Offline DaveR

  • Regular Contributor
  • *
  • Posts: 161
  • Country: gb
Re: bg7tbl gpsdo master reference
« Reply #864 on: January 03, 2021, 06:09:37 am »
It appears to depend on the model you've got.  My LCD display BG7TBL gives a 50mS pulse every second at 3.4v, but the multi-GNSS BG7TBL gives a 100mS pulse at 3.4v.  By comparison, my TBolt-E gives a 200mS pulse at 5.2v.  No doubt other makes will have different variations on the theme.

I can't say what the Star 4+ outputs, as I haven't got one.
 
The following users thanked this post: Electro Fan

Offline Noy

  • Frequent Contributor
  • **
  • Posts: 322
  • Country: de
Re: bg7tbl gpsdo master reference
« Reply #865 on: January 03, 2021, 08:09:40 am »
The pulse with is propably chaneagble.
You have to reconfigure the "fake neo-7" module.
 

Offline DaveR

  • Regular Contributor
  • *
  • Posts: 161
  • Country: gb
Re: bg7tbl gpsdo master reference
« Reply #866 on: January 03, 2021, 12:18:36 pm »
Correct - I'd forgotten about that.  You can change the pulse width using the TP configuration option in U-Center.
 

Offline Electro Fan

  • Super Contributor
  • ***
  • Posts: 2847
Re: bg7tbl gpsdo master reference
« Reply #867 on: January 03, 2021, 07:29:10 pm »
Ok, thanks for the feedback.  Looks like the answer is a) it varies by model, and b) it might be / probably is configurable.

When I connect both the BG7TBL and the Star 4+ to a scope and trigger one channel off the other they clearly align the beginning of the pulse.  After that of course they are not synchronized due to the differing pulse widths.

fwiw, each GPSDO is attached to it's own patch antenna sitting next to one another with the same view of the sky.  The two antennas are similar but not identical.

So on to new questions.

What would you expect would happen when instead of the using the pulse outputs, the two GPSDOs have their 10 MHz ref outputs inputted to the oscilloscope?  With one channel triggered off the other, would you expect the beginning of the two sine waves to be synchronized, or not?
 

Offline DaveR

  • Regular Contributor
  • *
  • Posts: 161
  • Country: gb
Re: bg7tbl gpsdo master reference
« Reply #868 on: January 03, 2021, 11:41:58 pm »
Very unlikely, as different makes / models will probably have different processing circuitry which will introduce slight time differences into the output paths.  I have the three different GPSDOs mentioned above connected to a single external antenna via a splitter and identical cables, and they all differ marginally when compared against each other.  With different GPSDOs you have to bear in mind that the signal received by any antenna is constantly changing due to atmospheric/tropospheric influences, and the firmware in each unit will have its own way of correcting the errors, so the outputs from each will always be slightly different by default.

Two nominally identical GPSDOs might produce synchronised 10MHz sine waves, but even then I wouldn't count on it - although the difference should be very slight - because of component manufacturing tolerances. 
 
The following users thanked this post: Electro Fan

Offline Electro Fan

  • Super Contributor
  • ***
  • Posts: 2847
Re: bg7tbl gpsdo master reference
« Reply #869 on: January 03, 2021, 11:50:00 pm »
Very unlikely, as different makes / models will probably have different processing circuitry which will introduce slight time differences into the output paths.  I have the three different GPSDOs mentioned above connected to a single external antenna via a splitter and identical cables, and they all differ marginally when compared against each other.  With different GPSDOs you have to bear in mind that the signal received by any antenna is constantly changing due to atmospheric/tropospheric influences, and the firmware in each unit will have its own way of correcting the errors, so the outputs from each will always be slightly different by default.

Two nominally identical GPSDOs might produce synchronised 10MHz sine waves, but even then I wouldn't count on it - although the difference should be very slight - because of component manufacturing tolerances.

Thanks.  But we should expect that after each GPSDO settles in, while they will not start their 10 MHz sine waves at the same point they should remain consistent in their relative position to one another (minus any small drift, which should be very little)?
 

Offline DaveR

  • Regular Contributor
  • *
  • Posts: 161
  • Country: gb
Re: bg7tbl gpsdo master reference
« Reply #870 on: January 04, 2021, 01:49:38 am »
Therein lies the fundamental problem: except for a few random moments, different GPS units will tend to show different positions all the time, as each will apply compensations to the antenna signals in its own way.  The differences between them may be only a few cm or mm, but there will always be relative movement between their outputs.  Connect them both to an oscilloscope and view the Lissajou figure they create: no matter how long you leave it, the circle or ellipse you see will always be changing over time, sometimes barely perceptibly, other times relatively quickly.  There will never be absolute stability from a single GPS unit, let alone two different ones, but the differences (you hope) will be small enough for your required degree of accuracy.
 
The following users thanked this post: Electro Fan

Online MegaVolt

  • Frequent Contributor
  • **
  • Posts: 420
  • Country: by
Re: bg7tbl gpsdo master reference
« Reply #871 on: January 04, 2021, 09:45:11 am »
"2020 Symmetricom LCD GPSDO" has appeared on Ebay. Does anyone have additional information?

https://www.ebay.com/itm/2020-Symmetricom-LCD-GPSDO-10MHz-1PPS-OCXO-GPS-Disciplined-Oscillator/233279321675
 

Online EV

  • Frequent Contributor
  • **
  • Posts: 524
  • Country: fi
  • Aficionado
Re: bg7tbl gpsdo master reference
« Reply #872 on: January 04, 2021, 11:18:37 am »
Pictures are however from 2019 model!

Maybe "Fixed known errors". The 2019 model restarts when getting wrong data from gps. Don't know if this is fixed. I don't have this new model.

"2020 Symmetricom LCD GPSDO" has appeared on Ebay. Does anyone have additional information?

https://www.ebay.com/itm/2020-Symmetricom-LCD-GPSDO-10MHz-1PPS-OCXO-GPS-Disciplined-Oscillator/233279321675
 
The following users thanked this post: MegaVolt

Offline Electro Fan

  • Super Contributor
  • ***
  • Posts: 2847
Re: bg7tbl gpsdo master reference
« Reply #873 on: January 05, 2021, 01:51:25 am »
Therein lies the fundamental problem: except for a few random moments, different GPS units will tend to show different positions all the time, as each will apply compensations to the antenna signals in its own way.  The differences between them may be only a few cm or mm, but there will always be relative movement between their outputs.  Connect them both to an oscilloscope and view the Lissajou figure they create: no matter how long you leave it, the circle or ellipse you see will always be changing over time, sometimes barely perceptibly, other times relatively quickly.  There will never be absolute stability from a single GPS unit, let alone two different ones, but the differences (you hope) will be small enough for your required degree of accuracy.

I replaced a Star 4+ with a second BG7TBL using the same model antennas for both side by side - but I have an antenna cable issue that will take me a few days to iron out before I can make the cables same length. 

In the meantime I'm watching the sine waves do their dance relative to one another on an oscilloscope and thinking about what you said.

If two GPSDOs don't stay aligned in phase, but they both keep track of 10 MHz with a fair amount of accuracy, to a pretty small level of resolution, is it possible that the accuracy of the 10 MHz signal for both GPSDOs is more or less independent of the particular state of the phase at their given level of frequency accuracy?  In other words, for either (and both GPSDOs) once the phase slips somewhere between 0 and 360 degrees the GPSDO is letting it slip such that it goes back to it's given frequency?  Or said differently, the phase slippage for a single GPSDO is just jitter within one 1 Hz (or perhaps jitter within 1 milliHz or 1 microHz or 1 nanoHz or whatever the level of accuracy is that that GPSDO produces)? 

And while this jitter bracketing is occurring with one GPSDO, the second GPSDO is doing it's own jitter bracketing within it's own level of accuracy - which could (if the two GPSDOs were identical with identical antenna feeds) be the same level of accuracy but nonetheless be out of phase with one another. 

This last part doesn't seem so likely if the two GPSDO's were really identical including the antenna feed.  Then they should have the same accuracy and same phase jitter, and the only question is did they startup/lock at the same time (perhaps unlikely) - but at least two identical GPSDOs with the same antenna signals should at least have their 10 MHz sine waves stay relative to one another.  I think... maybe?

Trying to get my head around this.  Thx
 

Online MegaVolt

  • Frequent Contributor
  • **
  • Posts: 420
  • Country: by
Re: bg7tbl gpsdo master reference
« Reply #874 on: January 05, 2021, 08:32:56 am »
Trying to get my head around this.  Thx
Each source has its own noise. Parallel connection of several noise sources results in noise reduction by the root of N times.

It's simple for voltage sources. I don’t know how to parallelize the frequency sources.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf