A look at my Symmetricom GPSDO / 10MHz reference (OCXO + Furuno receiver)

[NivagSwerdna]

https://www.ebay.com/itm/312248369350

Looks like a couple more in circulation... not a bad price either

[texaspyro]

I connected a Samsung GPSDO (-L8 version) to a TAPR TICC clocked by an HP-5071A cesium oscillator and measured the ADEVs of the PPS and 10 MHz outputs.  Note that at tau values less than around 40 seconds or so the measurements are dominated by the TICCs 60 ps resolution.

All in all, it seems tp be a rather nice GPSDO.  My only real complaint is that it does a 40 minute self-survey every time you power it up...  but Lady Heather now lets you create a file with your position in it and can set the location into the receiver from the file.

I'm going to try mounting it (or a Trimble / Symmetricom one) in a Smartipi Touch case with the deep back cover,  a Ras Pi and the 800x480 touchscreen.

[Bryan]

I believe the Symmetricom comes with a GT-8031 receiver and the Samsung a LEA-6t. Is one receiver better than the other?

[texaspyro]

I believe the Symmetricom comes with a GT-8031 receiver and the Samsung a LEA-6t. Is one receiver better than the other?

The Ublox is a better receiver, but that does not necessarily mean the Samsung is a better GPSDO.

[jpb]

I connected a Samsung GPSDO (-L8 version) to a TAPR TICC clocked by an HP-5071A cesium oscillator and measured the ADEVs of the PPS and 10 MHz outputs.  Note that at tau values less than around 40 seconds or so the measurements are dominated by the TICCs 60 ps resolution.

All in all, it seems tp be a rather nice GPSDO.  My only real complaint is that it does a 40 minute self-survey every time you power it up...  but Lady Heather now lets you create a file with your position in it and can set the location into the receiver from the file.

I'm going to try mounting it (or a Trimble / Symmetricom one) in a Smartipi Touch case with the deep back cover,  a Ras Pi and the 800x480 touchscreen.

Hi Texaspyro - thanks for posting this and other ADEV measurements (such as the Star 4 one on another thread). It would be interesting if you posted a collection of them for comparison on the same thread (I mean a separate new thread)
 to see how these different GPSDOs compare.

[Bryan]

The Ublox is a better receiver, but that does not necessarily mean the Samsung is a better GPSDO.

That is true, one version could have a better OCXO. -L8 or -LPK does anyone know the difference?

[Macbeth]

https://www.ebay.com/itm/312248369350

Looks like a couple more in circulation... not a bad price either

I do like he includes an FPC50 to IDC/DIP breakout board and flex cable. It's this attention to detail that probably only costs $1 in China but is worth its weight in gold over here!

[jpb]

I connected a Samsung GPSDO (-L8 version) to a TAPR TICC clocked by an HP-5071A cesium oscillator and measured the ADEVs of the PPS and 10 MHz outputs.  Note that at tau values less than around 40 seconds or so the measurements are dominated by the TICCs 60 ps resolution.

All in all, it seems tp be a rather nice GPSDO.  My only real complaint is that it does a 40 minute self-survey every time you power it up...  but Lady Heather now lets you create a file with your position in it and can set the location into the receiver from the file.

I'm going to try mounting it (or a Trimble / Symmetricom one) in a Smartipi Touch case with the deep back cover,  a Ras Pi and the 800x480 touchscreen.

I've now got myself one of these from ebay:
https://www.ebay.co.uk/itm/312279920851?ViewItem=&item=312279920851
I was a bit nervous because the seller only has about 5 people in his feedback but it arrived well packed and seems to work well. I've not yet connected up the RS232 but I did the power and compared it to my Star+ GPSDO and at 500 seconds gate there is no offset as shown in the picture I have it running on an antenna taped to my window (so indoors) while the Star+ is running on my external GPS antenna. The ADEV is a bit more than might be surmised from the combined measurements by TexasPyro but this may be down to an indoor aerial.

One thing that strikes me though is how much the current draw bounces around. It started at around 1.7A and then after warming up dropped to 870 mA approximately but it is bouncing between around 856 mA and 878 mA changing every second which implies that the OCXO is being corrected frequently or pehaps it sleeps in between 1 second adjustments which seems a bit strange. I'd expect the current drawer to be very steady once it was in a warmed up state.

[cdev]

When you say you have your antenna taped to your window, is it taped so the top is facing sideways (out the window) ? If so, I would move it so its on a flat piece of metal facing up - on the windowsill or what would be best is right outside the window. Top facing up and flat metal beneath is the important thing. ('ve had the best luck with a pot lid that actually has a perfectly patch antenna sized concavity at the top)

But just a flat piece of metal is fine-

 That might be substantially better than facing to the side. Reason is that with patch antennas the antenna polarization (and so also the multipath rejection) gets screwed up if the top of the antenna is not facing up.

(If you are using a quadrifilar helix timing antenna disregard the above, it doesnt apply)

I connected a Samsung GPSDO (-L8 version) to a TAPR TICC clocked by an HP-5071A cesium oscillator and measured the ADEVs of the PPS and 10 MHz outputs.  Note that at tau values less than around 40 seconds or so the measurements are dominated by the TICCs 60 ps resolution.

All in all, it seems tp be a rather nice GPSDO.  My only real complaint is that it does a 40 minute self-survey every time you power it up...  but Lady Heather now lets you create a file with your position in it and can set the location into the receiver from the file.

I'm going to try mounting it (or a Trimble / Symmetricom one) in a Smartipi Touch case with the deep back cover,  a Ras Pi and the 800x480 touchscreen.

I've now got myself one of these from ebay:
https://www.ebay.co.uk/itm/312279920851?ViewItem=&item=312279920851
I was a bit nervous because the seller only has about 5 people in his feedback but it arrived well packed and seems to work well. I've not yet connected up the RS232 but I did the power and compared it to my Star+ GPSDO and at 500 seconds gate there is no offset as shown in the picture I have it running on an antenna taped to my window (so indoors) while the Star+ is running on my external GPS antenna. The ADEV is a bit more than might be surmised from the combined measurements by TexasPyro but this may be down to an indoor aerial.

One thing that strikes me though is how much the current draw bounces around. It started at around 1.7A and then after warming up dropped to 870 mA approximately but it is bouncing between around 856 mA and 878 mA changing every second which implies that the OCXO is being corrected frequently or pehaps it sleeps in between 1 second adjustments which seems a bit strange. I'd expect the current drawer to be very steady once it was in a warmed up state.

[Bryan]

When you say you have your antenna taped to your window, is it taped so the top is facing sideways (out the window) ? If so, I would move it so its on a flat piece of metal facing up - on the windowsill or what would be best is right outside the window. Top facing up and flat metal beneath is the important thing. ('ve had the best luck with a pot lid that actually has a perfectly patch antenna sized concavity at the top)

But just a flat piece of metal is fine-

 That might be substantially better than facing to the side. Reason is that with patch antennas the antenna polarization (and so also the multipath rejection) gets screwed up if the top of the antenna is not facing up.

(If you are using a quadrifilar helix timing antenna disregard the above, it doesnt apply)

Interesting tip on improving an antenna. Would the metal plate improve upon the usual cone shaped antennas often seen surplus for GPS receivers.

[glarsson]

(If you are using a quadrifilar helix timing antenna disregard the above, it doesnt apply)

Interesting tip on improving an antenna. Would the metal plate improve upon the usual cone shaped antennas often seen surplus for GPS receivers.

Cone shaped antenna = quadrifilar helix timing antenna

[metrologist]

I can't help it. I've built several of these types.

http://orbanmicrowave.com/wp-content/uploads/2015/03/TheBasicsofQuadrifilarHelixAntennas.pdf

[texaspyro]

Cone shaped antenna = quadrifilar helix timing antenna

Not always (actually not usually).  They are almost always patch antennas inside.   The cone shape is to shed water/snow/birds.

[Bryan]

Cone shaped antenna = quadrifilar helix timing antenna

Not always (actually not usually).  They are almost always patch antennas inside.   The cone shape is to shed water/snow/birds.

Googled quadrifilar helix timing antenna images and it certainly doesn't look like what I have, unless it is covered in the protective plastic cone, which I doubt. It's probably only about 6 inches tall.

[metrologist]

I have one of those common white pointy cone antennas. Opened it up and this is inside (stock photo).

I was going to guess something like added multi-path with a groundplane. I think it depends.

[jpb]

When you say you have your antenna taped to your window, is it taped so the top is facing sideways (out the window) ? If so, I would move it so its on a flat piece of metal facing up - on the windowsill or what would be best is right outside the window. Top facing up and flat metal beneath is the important thing. ('ve had the best luck with a pot lid that actually has a perfectly patch antenna sized concavity at the top)

But just a flat piece of metal is fine-

 That might be substantially better than facing to the side. Reason is that with patch antennas the antenna polarization (and so also the multipath rejection) gets screwed up if the top of the antenna is not facing up.

(If you are using a quadrifilar helix timing antenna disregard the above, it doesnt apply)

I connected a Samsung GPSDO (-L8 version) to a TAPR TICC clocked by an HP-5071A cesium oscillator and measured the ADEVs of the PPS and 10 MHz outputs.  Note that at tau values less than around 40 seconds or so the measurements are dominated by the TICCs 60 ps resolution.

All in all, it seems tp be a rather nice GPSDO.  My only real complaint is that it does a 40 minute self-survey every time you power it up...  but Lady Heather now lets you create a file with your position in it and can set the location into the receiver from the file.

I'm going to try mounting it (or a Trimble / Symmetricom one) in a Smartipi Touch case with the deep back cover,  a Ras Pi and the 800x480 touchscreen.

I've now got myself one of these from ebay:
https://www.ebay.co.uk/itm/312279920851?ViewItem=&item=312279920851
I was a bit nervous because the seller only has about 5 people in his feedback but it arrived well packed and seems to work well. I've not yet connected up the RS232 but I did the power and compared it to my Star+ GPSDO and at 500 seconds gate there is no offset as shown in the picture I have it running on an antenna taped to my window (so indoors) while the Star+ is running on my external GPS antenna. The ADEV is a bit more than might be surmised from the combined measurements by TexasPyro but this may be down to an indoor aerial.

One thing that strikes me though is how much the current draw bounces around. It started at around 1.7A and then after warming up dropped to 870 mA approximately but it is bouncing between around 856 mA and 878 mA changing every second which implies that the OCXO is being corrected frequently or pehaps it sleeps in between 1 second adjustments which seems a bit strange. I'd expect the current drawer to be very steady once it was in a warmed up state.

Thanks - useful tips.

It is only a temporary arrangement at present. My external antenna has a (laser cut) aluminium disk as a ground plane (I got it off ebay).

I might try moving my taped to the window antenna though if I continue my experiment (I was only making sure that the new GPSDO worked and I didn't want to connect it to the same antenna as that would be too correlated  - I do have a 4 way divider for my external antenna I've used in the past but haven't set it up yet in my present lab.)

[cdev]

Metrologist, that is a QFH. The reason why they can be oriented in any direction is because the direct signal from the sky is RHCP and a QFH antenna naturally selects for RHCP signals and has a substantial rejection of LHCP nomatter what direction it is coming from. (The pattern of QFH antennas can be tweaked quite a bit, a tall narrow one will have more gain towards the horizon, good for amateur and especially APT satellite reception, a squatter QFH will have a rounder pattern and more gain vertically).

Reflections because they are a mirror image reverse polarization. This is a very useful property and can be used for interesting things like determining the water content of the ground by ratio of reflected vs direct GPS signals over it.

Patch antennas ability to reject LHCP signals is dependent on the angle of incidence to the patch, at low angles (off to the side) the rejection is very poor. They basically become useless as far as rejection at low angles. This is why its smart to set your GPS software to reject low angle sats for highest accuracy in urban canyons (where the reflections are severe) , but the need to do that varies depending on what kind of antenna you use.

Odd reflections are canceled while even reflections add.

I have a PCTel (originally made by Lucent) timing antenna that looks like a soft serve ice cream cone or missile nose cone in shape and it has a QFH inside it that looks much like yours. (Have not opened it, just seen pictures)

[jackenhack]

I hooked up a raspberry pi to it so I could have a local ntp server running.

Did you get the PPS signal working with the server? I thought it would be too narrow so built a pulse stretcher. 
(Previously I did have a similar NTP server working using an old VP Oncore receiver for which I followed the instructions at http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html)

Thanks for the diff. 

I've made a pulse streatcher that I use with my Trimble GPSDO. works perfectly with both Raspberry Pi and Odroid. The board is open source hardware.

https://www.jackenhack.com/wider-pulse-width-1-pps-signal-gpsdo/


Sent from my iPad using Tapatalk

[cdev]

When its being used I never use my PCTel QFH with any kind of reflector but all my other GPS antennas I always use with one.

Here is an interesting web page. (The author's software, RTKlib is a real game changer for low cost high accuracy GPS, its also one of the best teaching aids Ive ever seen for understanding how GPS - and GPS antennas work.. ).

This shows RTKlib's performance with various hardware and antenna combinations which were available in 2006.

http://gpspp.sakura.ne.jp/anteva/anteva.htm  (even the Japanese papers which have not been translated are worth using Google Translate to read. )

http://gpspp.sakura.ne.jp/anteva/antpcv.htm#Fig06

http://gpspp.sakura.ne.jp/index.shtml


You'll notice with the hockey puck patch antennas that there is a significant loss of accuracy caused by the feedline. That can be eliminated by using ferrite beads at both ends, especially close to the antenna..

Below is the picture of his test ground plane for evaluating patch antennas, with the one QFH he tested.

Definitely a non-optimal setup. In the case of the QFH he really didn't need to use the ground plane but since it was intended for inclusion in a scientific paper I think he did what is shown because he had to take the same approach for all the antennas for the data to be comparable.

I have the 26 db version of the same antenna.

[metrologist]

Metrologist, that is a QFH. The reason why they can be oriented in any direction is because the direct signal from the sky is RHCP and a QFH antenna naturally selects for RHCP signals and has a substantial rejection of LHCP nomatter what direction it is coming from.

This is the type of antenna I think Bryan has. I had not considered it would be of much benefit to add a ground plane. You might remember the QFH antenna I made using 12ga copper wire and semi-rigid cable feed. No ground plane there either, but I just copied an author's design. And that reminds me to 3D print a similar housing as in your picture above. I did not know they would perform while being oriented in any direction.

I generally set a patch antenna, at least the small ones I have, on a metal plate as they are magnetic.

[cdev]

Definitely. A good GP improves the signal a lot.. with a patch antenna- 

I think a metal plate wouldn't be good on a QFH, or might not make much difference either way, but basically having at least a small ground plane is almost essential for decent performance with a patch antenna.

The ground plane should be oriented to the Earth's horizon, so that a line drawn through its center perpendicular to the plane it describes is as close to vertical as possible. So an ideal location for a GPS patch antenna might be the center of the roof of a car, or similar.

http://gpsworld.com/wirelesspatch-antennas-new-gnss-12552/

https://abracon.com/uploads/resources/PatchAntenna-Application-Note.pdf

http://www.taoglas.com/images/product_images/original_images/TAOGLAS%20-%20GPS%20Patch%20Antenna%20Integration%20Application%20Note.pdf

[Bryan]

There are now some Samsung versions of this GPSDO showing up on Ebay... here's one:


One thing about them is their rather picky power requirement... it needs to around 5.5V.  They throw a power alarm if too far off of that (but mine does work from 5 .. 6V).   I used a 6V power supply dropped down with a Schottky diode.

They do a self-survey (around 40 minutes) every time you power up or after an extended holdover event.   If no antenna is connected they report the antenna current as 200 mA... WTF?

Texaspyro, where are you applying the +5.5 to the board, by the connector pins 44-50. I would have thought the heavy initial current draw would be too much for the ribbon cable?. Mine just arrived yesterday.

[texaspyro]

On the Samsung I power it through the ribbon cable / breakout board.   I use several power/ground pins.

With the Symmetricom and Trimble units I have a 2.1mm barrel jack soldered to ground and the input fuse (or is it a diode?).

[jpb]

Yes, they use no less than 7 connections for the VCC and 3 power ground connections (I guess less ground are needed because there are a total of 19 ground connections with all the signal grounds).

I soldered my power leads to connect pins 43-50 for VCC and 38-42 for ground and it seems happy.

I hope to get time this weekend to connect up the RS232 connections so I can get LH to talk to it.

The heavy current draw doesn't last long and it soon drops to around 850 mA.

[texaspyro]

I hope to get time this weekend to connect up the RS232 connections so I can get LH to talk to it.

The Samsung devices require a tweaked version of Lady Heather.   I hope to get this posted this weekend (also the source code for Linux users).