How trustworthy is the accuracy of a surplus GPSDO? - Page 1
Forum section: electronics / general

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2020, 08:33:39 pm
content: I recently picked up one of those Trimble GPSDO modules that are available inexpensively from China, presumably from decommissioned mobile phone sites. My assumption is that since the oscillator is locked to a trustworthy external reference that I can assume it to be very accurate so long as it locks and does not drift around, but I'm curious how confident in that I can be.I have now three reasonably accurate 10MHz reference sources, there's a FE-5680a rubidum oscillator I picked up several years ago when they were cheap, the high precision OCXO reference in my HP 8594E and now this GPSDO. I connected all three to my scope with the trigger set to one channel and all of them drift slightly relative to each other although the rate of drift does not seem to change much once things settle in. Now I'm faced with the classic problem "a man with one clock always knows what time it is, a man with several clocks never knows what time it is" and I'm trying to work out a source I can trust from which to calibrate my others.The GPSDO and 8594E reference agree most closely, if I trigger on one of them the other will rotate through a complete 360 degree phase difference in about 7 seconds. The rubidium source is about every 2 seconds for the same thing. I'm wondering if it makes sense to pick up a second GPSDO and compare the two, surely if those both agree exactly then I can assume that they're accurate? Of course any one of these frequency sources is likely accurate enough for any real need I have but that's not really the point.

author: jpb
author_rank: Super Contributor
timestamp: February 19, 2020, 09:31:00 pm
content: In absolute terms the GPSDO should be most accurate but only if averaged over a day or so - it should approach the accuracy of the GPS system with ADEV of around 10^-14. In the shorter term, depending on the time constant of the PLL, it may be relatively inaccurate (the jitter on a GPS 1PPS is around 10 nsecs, if the time constant is around 100 seconds or so then it is being steered with an accuracy of around 10^-10 in the short term as a back-of the envelope sort of calculation). The Rubidium is probably the most stable over time periods of a few hours but it sounds like it has a small offset. The OCXO is probably got the best short term stability (over a few seconds).I have a couple of GPSDOs (actually I have 4 or 5 but 2 ones which I use regularly) - I did some measurements of them against each other with different time constants set :https://www.eevblog.com/forum/metrology/some-adev-measurements-for-star4-gpsdo-at-different-time-constants/msg2854130/

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2020, 10:37:00 pm
content: Interesting, thanks. So it sounds like I could count the pulses over a long period and use the result to tweak the rubidium standard until that's bang on. It turns out it's the sort with an internal DDS driven from a ~56MHz signal locked to the physics package so I may not be able to get it to precisely 10MHz, I don't know off hand how close it can get. Both the GPSDO and the rubidium standard are currently bare units, I need to box them both up and let them settle in for a few days to stabilize.My frequency counter is only 8 digits however I have FPGA boards so it would be trivial to make an extremely precise counter that can be gated to some ridiculously long interval. In the short term watching the phase drift on a scope seems to be quite precise.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 19, 2020, 11:21:15 pm
content: As mentioned, the strength of the GPSDO is the long term stability. Telecom GPSDOs, which are most of the cheap surplus units, still have decent oscillators and general performance, but the real benefit to be had is the self-correction to essentially zero out long term drift. A rubidium oscillator on its own (some are basically the output straight from it, some are buffered by disciplining an OCXO) will have excellent long term stability (though not as good as a GPSDO in the scale beyond a day or two), but can suffer from some shorter term stability issues or phase noise variation - something that is generally worked around with the rubidium source disciplining an OCXO which is better for low phase noise and short term stability.A useful thing to do with the GPSDO is actually to make comparative measurements over a long period - taking your best counter and using the GPSDO as its reference, then measuring the output of another source. If you measure the rubidium with this, for example, after a couple days of measurement you can get a pretty good idea of what frequency the rubidium source has aged to, and trim it accordingly. While it may take a week or two of adjustments and measurement, you can probably recalibrate your rubidium reference's frequency, then when it's at the right place and stabilized, you can use that source as a reference to evaluate your OCXO/TCXO based oscillators for short term drift, since the rubidium reference should be an order of magnitude or two more stable than the output of the GPSDO in the short term.Still better would be to make a disciplining signal (PPS or otherwise) from the GPSDO's oscillator output (often the broken out PPS is directly from the GPS module and doesn't factor in the smoothing of the OCXO's control loop), and use it to discipline the rubidium source. Then the rubidium's stability reduces the noise on the GPSDO's output signal with its long time constant control loop, while also locking it to effectively the most stable reference available outside of a laboratory.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 19, 2020, 11:53:06 pm
content: Without knowing a reason why not to, I'd trust the GPSDO. I have a Trimble Thunderbolt that I removed the OCXO from because the OCXO was really bad. I brought the EFC and 10Mhz leads out on SMA connectors and use that Thunderbolt as a test GPSDO for any 10Mhz oscillator I want to test. If I don't connect the EFC from the Thunderbolt to the oscillator under test I will see the oscillator drift without the corrections that would normally come from the Thunderbolt. Using Lady Heather gives me a good graphic display to see how the oscillator under test is doing. I trust the information I get from the Thunderbolt as displayed by Lady Heather.I usually don't use a counter for testing drift but my digital scope triggered from one of my known very good GPSDOs that is on 24/7. I feed the test 10Mhz into a vertical channel of the scope and set the sweep to either 5ns or 2ns and watch for drift. Here are 2 photos of the scope displays. the top one shows what you would normally see if you set the scope up to view the waveform. The lower one has the vertical and sweep set to about 10x what they are in the top one. I have the 'Color-Grade On' and the persistence set to 'Persist Infinite'. This allows the screen to display the drift on either side of center. In the lower photo you can see that the total drift between these 2 GPSDOs over the time period I left this running is about 2ns which isn't bad. If the trace drifts to the left, the test oscillator is higher in frequency than the trigger GPSDO and if it drifts to the right, the test oscillator is lower in frequency than the trigger GPSDO. Because it is recording and displaying what the input is doing I don't have to sit there and watch it.

author: james_s
author_rank: Super Contributor
timestamp: February 20, 2020, 01:49:31 am
content: Hm that's an interesting idea, I'll try out the infinite persistence once I have two signals that are close enough to need it.I'm curious how you could tell that the OCXO was bad? My issue is that I don't have a known good setup to compare it to. It's the classic issue of having multiple sources that are all slightly different and not knowing which one to trust. I suppose if I disconnect the feedback or at least monitor the voltage I could get an idea how much the oscillator drifts on its own. I assume that low short term drift is the important factor here since the long term will be smoothed out by the GPS.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 20, 2020, 01:56:20 am
content: I'd do an ADEV run between the Trimble and the FE-5680A. The shape of the resulting graph is well known. If the GPSDO says it's locked and the various DOP values look good (assuming it reports DOP values) and the ADEV graph looks good and the Rb says it's locked, then everything is good. It doesn't matter if the Rb is off-frequency. As long as it's stable, the ADEV graph is valid. Then, if necessary, you can tweak the Rb to be on frequency.In any case, it won't be very long before you're knee-deep in GPSDOs and Rb standards so you'll have lots of things to compare. Don't ask how I know this!Ed

author: james_s
author_rank: Super Contributor
timestamp: February 20, 2020, 02:04:24 am
content: Is there a document somewhere that explains more or less how to do this? I don't need it all handed to me on a platter but an overview of what is needed and how to set it up would be great.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 20, 2020, 04:04:31 am
content: Quote from: james_s on February 20, 2020, 01:49:31 amIf you run Lady Heather on the output of a thunderbolt and use the commands G O <ENTER> then G O S 200 <ENTER>, the graph will display the 10Mhz signal. If the OCXO is o.k. you will see a fairly smooth white line at the horizontal center of the graph. In the case of the bad OCXO, the oscillator was wildly unstable and the line was bouncing all over the place. Replacing the oscillator with a good external OCXO cured the problem. I have had over 200 Trimble Thunderbolts so I got to recognize problems pretty quickly. Here's an old graph of a good unit from turn on to 10 hours.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 20, 2020, 04:07:48 am
content: Quote from: james_s on February 20, 2020, 02:04:24 amI can't think of a document as such, but if you search here for 'Allan Deviation' you'll pick up a lot of chatter about making the measurements. Basically, you need either a Frequency or Time Interval counter and some way to get the data from the counter to a computer. Connect the Rb and GPSDO to the counter and collect the data. Run the data through a program like Timelab or Stable32 (both free) and the graph pops out.What could be simpler?Ed

author: james_s
author_rank: Super Contributor
timestamp: February 20, 2020, 08:00:02 am
content: Well it sounds like I'd need some more gear to do that, which is not the end of the world because it's always fun to get toys but I need to mind the hobby budget. My only frequency counter is a cheap Protek I bought when I was a teen probably 25 years ago. It works fine but is nothing fancy and has no provision to get the data out. I've never upgraded since once I had a decent scope I've rarely used the counter.I'll set up Lady Heather and have a go with that. I hadn't realized it could get so much info off these GPSDO modules. I'm still waiting for BNC pigtails that fit the connectors on the module to arrive but that won't block me from wiring up the serial port.

author: jpb
author_rank: Super Contributor
timestamp: February 20, 2020, 08:42:09 pm
content: For measuring 10MHz these counters seem to be good (I don't have one but lots of others on the forum do) and are cheap (around $100/£100)https://www.eevblog.com/forum/metrology/bg7tbl-fa1-frequency-analyzer/

author: james_s
author_rank: Super Contributor
timestamp: February 21, 2020, 10:11:03 pm
content: Well I soldered up a serial adapter and got Lady Heather going, I left the GPSDO powered up overnight and took a look at it this morning before work. I'm a total noob with this software and this is the first GPSDO that I've owned but initial results give me some concern. Pictured is I believe a 3 hour window. It is toggling occasionally between LOCKED and Settling and that very sharp downward spike is when I held my hand on top of the oscillator for about a minute. The Trimble OCXO is quite low profile compared to other OCXOs I've encountered so I'm wondering if it is not well insulated. There are a few other spikes of unknown origin but in a scale of ppt I'm not even sure what sort of deviation would be reason to suspect problems. I'm not sure how good the GPS reception is as the antenna is just sitting inside on the windowsill currently but it does look like it sees a handful of satellites.So now I'm hoping someone with more expertise with these can give me some pointers to assess the overall health of this thing, I will follow up with additional data as I gather it and hopefully I can determine whether or not there are any obvious faults that would give me reason to try to get the seller to replace it as it was sold as a working unit.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 22, 2020, 04:33:01 pm
content: I just happened to have the same Trimble 57964 GPSDO that I had put together running for some time and there are a few things I noticed on mine that are different than yours. I’ve posted your graph and mine below so you can see the differences although I only ran Lady Heather on mine for a few minutes this morning. Note that my laptop screen is set to a different aspect ratio than yours but that doesn’t affect any of the data. You are running the old version 5 of LH and I have the newer V6.14 of Lady Heather so there are a number of things that are displayed differently.First note that you do not have good signal strength. I have a very good view of the sky and the satellites display green on the list except for one that has a poor signal because of its position in the sky and that displays yellow. All yours are yellow which isn’t good so you need to position your antenna to get a better view of the sky. The blue trace at the bottom of your graph shows weak reception and sometimes there are zero useable satellites, which is not good. I have also noticed on the 10Mhz (white) trace on my uccm GPSDO unit that, like yours, there are strange jumps about every hours but I never tried to see what was causing them because I have 3 GPSDOs running 24/7 that I trust to give good data. I did change some of the ref positions and gain of the various displays on the graph to give a clearer display and some times that can be a little tricky. You can also make the graph lines smoother by typing ‘F’ ‘D’ ‘10’ or F D 100 or F D 0 to turn it off.

author: james_s
author_rank: Super Contributor
timestamp: February 22, 2020, 06:13:59 pm
content: Those spikes are strange, I don't really know what to make of those. I'm starting to get the hang of the software but it's not really very intuitive, can't really complain given that it's free though. I'll see if I can get the latest version going, I just grabbed the binary I found and assumed it was the latest but obviously it's not. I didn't realize yellow indicated low signal strength, looks like I need to get a better antenna and put it up outside. Right now I'm just using some random little Chinese active antenna that I got years ago for a different GPS receiver. It seems the receiver in this thing is not really very good by modern standards, I have some little Ublox modules that easily maintain lock on a bunch of satellites using their internal antenna inside the house. My house is surrounded by tall trees on all sides and a large hill on one side with not very much unobstructed sky visible anywhere but hopefully a decent antenna outside will be adequate.Any thoughts on affordable surplus commercial antennas? Are some better than others, some to avoid?

author: edpalmer42
author_rank: Super Contributor
timestamp: February 22, 2020, 07:18:33 pm
content: If you compare the position of spikes on the white trace to the bottom trace that shows the number of satellites, you'll notice that an upward spike occurs when a satellite is added and a downward spike correlates with a satellite being removed. That means there's an error in the GPS location. Depending on your model and what LH can do with it, there might be the possibility of a precise survey to determine a better average location that will minimize these disruptions. Check the LH documents for more info.The most important thing with the antenna is visibility toward the equator (south in your case). The farther you are from the equator, the more important this becomes. If you expect to have a very long cable run you might want to look for a particularly high gain antenna, otherwise it doesn't much matter. Obviously a brand name would be preferable to a no-name. I've seen names like PC-Tel, Ace, Lucent, Panasonic, Andrew, etc. You might want to find a web page or datasheet for a model you're interested in and compare the specs to one of the higher price units. They're not very expensive so you're not risking much. My antenna system is a Panasonic VIC-100 with a 38db amp running through 100m of RG-59 coax to a 4-port HP splitter and then to my Tbolt. I have clear visibility east, west, and south. LH reports C/N levels of over 50db on some birds.Ed

author: notfaded1
author_rank: Frequent Contributor
timestamp: February 23, 2020, 02:20:12 am
content: Quote from: edpalmer42 on February 22, 2020, 07:18:33 pmIs the C/N level like the signal to noise ratio or SNR on the LH screen Ed? I was looking for it.Bill

author: edpalmer42
author_rank: Super Contributor
timestamp: February 23, 2020, 03:19:28 am
content: Quote from: notfaded1 on February 23, 2020, 02:20:12 amYes, C/N stands for Carrier/Noise ratio. AFAIK, that term is used with RF measurements rather than SNR because at the RF level there is no 'signal' i.e. it hasn't been decoded or demodulated yet. Trimble uses AMU which stands for Amplitude Measurement Units. Since nobody knows how it's measured or calculated, I've also seen it described as Arbitrary Mystery Units.Ed

author: james_s
author_rank: Super Contributor
timestamp: February 23, 2020, 08:28:59 pm
content: Well I've let this continue to run while keeping an eye on things. This is the last 12 hours, the reception is mediocre but it is at least locked a significant portion of the time. The frequency plot seems awfully wiggly to me, not sure if that's indicative of a fault or just expected behavior with poor reception.I found a used Trimble bulkhead antenna for 10 bucks that I figure will work as a proof of concept and then I can look for something a bit nicer if needed once I determine an optimal location and get a cable run. I have a mast on one end of the house that has my scanner discone and weather station on it, I can probably stick an arm on that to hold a GPS antenna. It'll be a few days before that gets here and then I also need to get a cable to connect it. Until then I'm stuck with the little magnetic puck which I suspect contains one of those square ceramic antennas.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: April 28, 2020, 04:58:00 am
content: Quote from: james_s on February 23, 2020, 08:28:59 pmI wouldn't knock that puck (patch) antenna. From what I've been reading of u-blox's antenna application note (UBX-15030289) and other sources, on the merits or otherwise of all the different antenna types used with GPS modules, the patch antenna is, by a small margin over the helical, the best type for this application. Monopole antennas such as the Chip antenna, the Fractal antenna (FEA), the dipole antenna, the loop antenna and the Planar Inverted F Antenna (PIFA) types aren't recommended when the navigation/timing function is of vital importance.The application note mentions the usefulness of mounting the patch antenna onto a modest sized groundplane (70mm square being recommended). I'm sure this true for the non-mag mount types but I have good reason to believe that in the case of a mag mount type which has been tuned for use on a vehicle roof, it should be mounted onto a much larger groundplane than the 70mm square recommendation.In my case, a 19cm diameter biscuit tin lid seems to have rather surprisingly provided a 3 dB boost over the 70 by 100mm groundplane raised by some 11 feet above the original ballasted biscuit tin location on a flat bay window roof to provide a 360 degree view of the far horizon that had otherwise been blocked on the eastward side.I came across this topic thread during my search for confirmation that mag mount patch antennas actuallyrequirea much larger groundplane than that suggested by the perceived wisdumb that a 70mm square groundplane is the optimum size for patch antennas in general. You might want to try mounting yours onto a dinner plate sized groundplane once you've relocated it to somewhere with a better view of the whole sky.When I first started experimenting with a u-blox M8N based GPS module early last year, my first obstacle to receiving any satellites was due to circuit traces in the the groundplane upon which the 25mm square ceramic patch had been stuck onto, coupling self inflicted interference that completely overloaded the LNA to the point where I was only able to catch a brief glimpse of the odd satellite once every few hours in the U-Centre's history chart. I didn't discover this disgraceful board layout design error until I finally decided to remove this "ornamental patch antenna" several weeks later.In the meantime, I'd experimented with a quarter wave (45mm) wire antenna plugged into the SMA-F socket which failed to produce the desired result until I tried a 3/4 wavelength wire as a substitute for the mag mount patch antenna with 5 metre cable I'd ordered. I was receiving more satellite signals (though some 10 dB lower) than your attached LH screenshot shows just from placing the module and attached 3/4 wavelength wire antenna on my bay window window ledge.When the three quid mag mount patch antenna with 5 metre cable (bargain of the century it turned out when I later started looking for more of the same) finally turned up, I was able to receive even stronger signals from the window ledge location. However, since I had rather a restricted view of the sky, I decided to place that ballasted 19cm diameter biscuit tin onto the bay window roof (accessible from the second floor bedroom window) to stick the antenna onto and drop its five meter cable down to my office/workshop below via a window left cracked open just enough for it to pass through with only some modest level of crushing (it was only intended to be a temporary test setup - it remained like this for nearly a year).This provided yet another increase in signal strength but little extra in the number of satellites - the eastward side was still blocked by the brickwork of the gable ended second floor bedroom window. What was needed was a short mast bolted to this gabled ended wall above the flat roof of the bay window upon which to raise the antenna above the roof ridge. However, I was a little reluctant to risk damaging the brickwork by drilling so made do with this arrangement.Eventually, I was inspired to build on my idea of using a ballasted biscuit tin to secure the antenna by using an old metal drawer ballasted with four car batteries with a 9 foot aluminium pole bolted into a corner upon which to raise the mag mount antenna almost clear of the roof line (I used a 60mm diameter pickle jar lid screwed onto a wooden plug in the end of the pole to stick the antenna onto). I'd have used a longer pole except that the 5 metres of RG174 wasn't long enough for the task.This served me quite well for the next six months or so. Whilst not quite perfect, it had provided a less obstructed sky view but I wasn't happy with the cable routing and it being some 30 inches shy of a full 360 degree sky coverage. Last week with the advent of warm dry weather, I finally decided to sort this out once and for all. I'd been considering this upgrade for some time and had decided to use RG58 with a BNC to SMA adapter to extend the cable run so I could route it via a hole that had been occupied with a TV co-ax that had been made redundant some three decades earlier.I extended the existing pole with a 33 inch length of 10mm diameter aluminium tubing glued into a hole drilled into the wooden plug and mounted a 70 by 100 mm copper clad board onto the end onto which I'd glued a non-mag mount patch antenna I'd subsequently acquired which seemed to offer excellent receive performance when tested indoors by the bay window.Unfortunately, after re-rigging my improved GPS antenna, I discovered that this antenna was inflicting some 12 dB of desensing from the local cell tower just 103 metres or so from my location every time if fired up (which was most of the time). I had seen some desensing (typically 2 to 4dB) with that cheap 3 quid mag mount antenna but nowhere near that of this other puck antenna. Since I wasn't sure whether it was the antenna or the use of some 11 metres of RG58 which was responsible for this increased level of desensing, I connected the original mag mount antenna which I'd parked back on its ballasted biscuit tin, to the BNC to SMA-F adapter to make further observations, noting that I could see C/NO signal strength reports peaking as high as 51 dB for GPS SVs passing close to directly overhead with no more than 2 to 4dB desensing effect from the cell tower transmissions.Assured that the RG58 co-ax wasn't the culprit, I reassembled the antenna with the original mag mount puck antenna, noting the lack of any 50 and 51 dB C/NO reports. In this case, I was only seeing signals peaking at 47 dB C/NO but at least the massive 12dB desensing effect had been dialled back to just 2 to 4dB as it had always been prior to this latest antenna upgrade attempt.However, this did leave me wondering what had happened to those 50 and 51 dB signal strength reports I'd previously noted. The cabling make up hadn't changed, only the biscuit tin versus the 100 by 70mm copper clad groundplane being the difference in this case. From what I've read on the subject of GPS patch antennas, it would seem that my mag mount antenna has (possibly all properly designed mag mount patch antennas have) been optimised for a much larger groundplane area than the 70 by 70 mm typically recommended in the literature.I'm currently at the stage where I feel I should pull the existing setup apart yet again to 'upgrade' the existing copper clad board to a 19cm diameter biscuit tin lid in the hope that it will not only boost signals by some 3 or 4 dB but that it doesn't present excessive wind loading on its supporting 'mast'.I've attached some images relating to the above.The large black puck antenna in the third image is the one that causes a 12dB desense in response to the cell tower transmissions (andisn'tmag mount). It's also a little larger than the mag mount antenna and of lower profile and a little weird in having an additional SMA-F terminated fly-lead the function of which I've never been able to ascertain.The Vbox screen captures show the effect of LH on the u-blox M8N's antenna configuration settings and the subsequent recovery after they've been restored. The presence of 50+dB readings suggests this predates the upgrade from biscuit tin to pickle jar lid screwed onto the wooden plug at the top of a 9 foot pole.The final four images show the results of the recent antenna upgrade work in the past 3 or 4 days. The sky survey plots are pretty close to the ideal suggested by u-blox themselves. The first two are the result of using only the American GPS constellation with the last two using both the American GPS and the Russian GLONASS constellations combined. I'd normally restrict myself to just the GPS SVs for best timing accuracy but I was curious to see how the GLONASS SVs compared (they seem to peak another 2 dB or so higher than the GPS SVs, BTW).JBG

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2020, 05:15:52 am
content: Quote from: Johnny B Good on April 28, 2020, 04:58:00 amThere's more to the antenna than just the form factor. I got the Trimble antenna some time ago, I suspect it's a ceramic patch antenna too inside the plastic housing but it performs dramatically better than the cheap random Chinese one I was using. I still haven't had a chance to mount it outside but I did try setting it out there and I was consistently receiving twice as many satellites. I need to make a bracket and then I'll install it on the roof.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 13, 2020, 01:00:20 am
content: Quote from: james_s on April 28, 2020, 05:15:52 amHi James,Apologies for the rather delayed reply but I wanted to try the 'biscuit tin lid upgrade' and get some more data before responding and I've been grabbing my 'screenshots' the easy way once I finally managed tounforgethow I'd done this with the VBox winXP VM nearly a year back. No more messing about with photos and processing them in GIMP to compress them into jpgs before adding them as attachments - it's just a matter of using the 'host key' (RH CTRL key) with the E key followed by pressing the ENTER to save each screenshot in Full HD.This convenience means I've now grabbed some 324 screenshots over the last 12 days from which I've gathered 93 png files into a short list folder. They're a mere 150K each in size so I'll be hitting the 25(?) file limit before hitting the 5MB one for each post. I've altered the windows layout in u-centre to allow both the deviation map and sky view windows to be shown in each screenshot.Unfortunately, I couldn't find any screenshots showing elevations of -1 or -2 degrees just prior to the final departure of the exiting SV from the sky map which makes me suspect the ENTER is being used as the "shutter button" rather than to confirm a (n already) captured image saving operation.Regarding your comment about antenna form factor, I heartily agree (that funny non-mag mount with two RG316 flyleads that I'd tried out is testament to that). That cheap random chinese antenna may simply have been zapped somewhere along the line, killing the 'half a grain of rice' sized LNA chip.From what you say, that Trimble unit seems to be working normally - probably about the same as my 3 quid mag mount patch antenna with 5 metre RG174 'fly lead' that I'd purchase over a year ago from a UK based car accessory supplier on Ebay (seemingly an exact match to a 53 quid version being sold by some comms company that I happened to come across on the interweb just a week or two ago). That Trimble antenna probably already contains its own 70 or 80mm diameter ground plane (assuming it's of large enough diameter).Upgrading from the 70 by 100mm ground plane to that 190mm diameter biscuit tin lid doesn't seem to have made any discernible difference to the C/N reports. I now rather suspect that the 3dB boost was simply the result of constructive interference with reflections from the gable ended wall for high elevation angle passes of the stronger satellites whilst the antenna was mounted on the ballasted biscuit tin sat on the bay window roof.Relocating the antenna to get a clear 360 deg sky view eliminates any such 'assist'. Since the increased wind loading from the biscuit tin lid doesn't seem excessive as witnessed during the recent spell of strong wind conditions, I'll leave it as it is. In any case, despite the failure to improve signal strengths, it's almost certainly reducing any ground reflections interference effects so it might as well stay put for the foreseeable future.Locating your GPS antenna in the clear for a full unobstructed view of the whole sky makes a considerable difference to the timing/frequency accuracy of any GPSDO, especially in the case of the more basic hardware PLL designs such as my own homebrewed M8N based unit.The biggest problem for such basic hardware only PLL based GPSDO designs lies with the effect of the ionosphere on the total GPS error budget. It is far and away, the biggest contributor, the effect of which is to impose a subsonic (typically 2 or 3 mHz) phase modulation of 10 to 12ns peak to peak amplitude on the 10MHz output during settled conditions.As far as I can make out, ionospheric correction data is transmitted alongside of the ephemeris data in the GPS signalswhich most, if not all, recent GPS receiver modules can process as part of the navigation computation. Even so, this can't completely compensate the effect. Indeed I rather suspect the sudden movements that show up in the deviation map plot from time to time are due to such updates trying to catch up with a rapid change of 'reality'.Although the u-blox M8N module I'm using is only a humble consumer grade navigation unit with seemingly no 'timing optimisation' features, I did spot an interesting feature in the deviation map display. If you set it to display the tiny yellow marker dot centred in the 'cross hairs' after a cold start, the reported position alters, eventually to remain fixed at the average of all the positional fixes, typically barely changing after a 12 hour run and coming to a standstill after 24 hours where further plotting data makes no further change.Inspired by the fact that it seemed to be performing a 'survey in' operation, I decided to use this data source to manually enter a static location in the Time Mode 2 configuration menu. Normally, when you try to access unsupported features, the only hint that such features aren't actually supported is the lack of response to such changes. In this case, it actually accepted and stored the parameters I'd entered.They even survive a power off/on cycle (in this case, it's stored in the BBRAM - it's one of the innumerable fakes with neither flash nor I2C - EEPROM nor SPI-FLASH so I had to fit a CR2032 coin cell to supplement the 80mF supercap which only provides 45 minutes or so of backup time versus the 300 days of a CR2032). In any case, this seems to have tamed the wilder excursions of position I'd previously experienced prior to this and the recent antenna upgrade work.Of course, the apparent improvement in positional stability may have nothing to do with my programming a fixed location in the Time Mode 2 config and everything to do with the improved antenna siting alone but I thought I'd mention it here in case anyone using an M8N module in a homebrewed GPSDO project of their own would like to experiment with the TM2 settings after it had collected enough navigation data to produce an averaged position fix to a resolution better than +/-10cms.Anyway, I've attached the first twenty or so images to give you some idea of what to expect from a well sited active GPS antenna on the end of an 11 metre run of RG58 attached to the 5 metres of RG174 antenna flylead (a 16 metre run in all).The first screen capture shows a happier time, almost a whole year ago, when I was regularly seeing 54 and 55 dB C/N figures out of the mag mount antenna on its ballasted biscuit tin ground plane on the bay window roof with a 5 metre RG174 extension cable (which mysteriously only reduced the C/N reports by a mere two dB at most compared to the expected 6 to 7 dB loss at 1.6GHz - the use of SDR technology and signal strengths based on Carrier to Noise ratio figures rather than to some fixed voltage reference such as dBμV might have something to do with this).The second image file is a photo showing the typical wanderings in the deviation map when it was set for an 1800 seconds (half hour) trail. I'd reduced this to a 600 seconds trail some months ago (1800 points was making for a rather scrambled and confusing display).The rest of the images are recent screen captures starting from about a fortnight ago. It seems my recollection of the 25 files / 5MB limit per post was correct. I'll hold off posting the other 68 image files for now but I'll happily oblige anyone interested in seeing more of these images.You can see the effect of including the GLONASS and GPS constellations on the deviation map display, confirming why it's not a good idea to use both together. I normally stick with the GPS constellation to achieve the most stable timing data possible, after all, itisaGPSDO when all is said and done.JBG

author: cdev
author_rank: Super Contributor
timestamp: May 13, 2020, 01:29:36 am
content: Just wanted to warn you about waterproofing those kinds of antennas. if water gets into them, they sometimes oscillate and turn into a GPS jammer by mistake. Or so I have read. Also, you can improve the accuracy by putting one or two ferrite split beads on the antenna lead It makes a noticable improvement in the pattern. Otherwise there are big irregularities in the direction the cable exits in.Also, you should set at least a 10 degree elevation mask setting because those patch antennas don't work well with low angle satellites. As far as ground plane a slightly larger GP is better but a CD/DVD sized GP works okay. Anything flat and conductive works the bigger ones work better up to around the size of an old 33 1/3 RPM record. (12 inches)

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 13, 2020, 01:24:27 pm
content: Quote from: cdev on May 13, 2020, 01:29:36 amThanks for those timely reminders about the potential snags regarding damp ingress into enclosed active antennas and the issue of the feeder cable possibly distorting the polar response. The effect of the feeder did cross my mind but there doesn't appear to be any obvious indication of this in the sky plots. All the imperfections appear to be accounted for by local clutter (chimney stacks and TV antennas).I haven't cracked this one open to see exactly what lies inside. It might already be blessed with some form of co-ax balun to mitigate this. At circa 1.6GHz, I suspect any ferrite tubes will act more like a lossy highish impedance in this case, no bad thing of course.As for the issue of damp ingress, there's not too much that can be done about that since any attempt to 'hermetically seal' against this risk is almost certainly doomed to failure. It only needs the slightest pin prick hole to let the water in as a vapour (any seals would need to be gastight) and condense out every time the temperature drops. It's usually better to allow some ventilation so that subsequent heating cycles will vaporise any water allowing the ventilation to carry it away as a gas.The housing being made from black plastic improves the heating by solar insolation to help effect this drying out process so I figured it was best to 'leave well alone'. If damp ingress does become an issue, it will become evident soon enough.Mind you, in view of the apparent "6dB drop" from just over a year ago with a total of ten metres of RG174 feeder cable and the original genuine M8N module, this issue is playing a little on my mind (the current feeder setup 11 metres of RG58 plus the 5 metres of RG174 antenna cable should be within a dB of the original feeder losses).I've retested using the original module (it's only the PPS line that I'd fubared) which surprisingly, now gives almost identical C/N figures to the 2nd of the 'fake' modules I'm currently using in the GPSDO (the 1st and most expensive of these 'fakes' was most definitely a good (bad?) 6 to 10 dB down on the original (although this didn't seem to impact its navigation performance)) which points to some performance drop off in the antenna system.I now know that some 3dB of this came from the serendipitous effect of constructive interference from wall reflections in the original antenna setup, leaving some 2 to 3 dB currently unaccounted for which might possibly simply be down to my bodged feeder cabling setup. In any case, this is something I'm mindful of.As for the possibility of the LNA becoming a GPS jammer, I imagine that I will be the first to know anyway (total loss of all SV signals) so it's extremely unlikely to become a long term problem for any other nearby GPS users. An obvious test for such a fault would be to disable the antenna power feed option and see whether this restores some SV reception at very low C/N values (around 10 to 15 dB the last time LH did this to my M8N module) as opposed to a total blackout. IOW, stay mindful of the possibility that any sudden complete blackout of SV signals could be the result of your active GPS antenna turning itself into a GPS Jammer rather than the result of squirrels making off with the feeder.JBGPS. I almost forgot to comment on your minimum elevation angle filter recommendation and ground plane sizes. I've set the elevation filter to 9 deg rather than the recommended 10 deg (What's one degree between friends?) since they can usually be detected joining and leaving the plot at -1 (and on rare occasions, -2) degrees where the antenna has a reasonably clear view of the horizon as it now does in my case. The problem of using extremely low elevation SVs is the higher risk of multipath rather than a lack of signal strength.I did some experimentation with elevation filtering afteralmostgetting the antenna into its final position just to see whether there was any benefit to increasing the 5 deg default to 10, 30 and 45 deg test angles. To be honest, the higher, 30 and 45 deg settings seemed to make things worse and the 10 deg didn't seem to offer anything different from the 5 deg default so I set it to zero degrees just to see whether this would actually make things worse compared to the 5 and 10 deg settings by way of verifying whether this received wisdumb of "10 degrees is better than the 5 deg default." had any basis in fact.It might have something to offer when the antenna is in a bit of topological hole where the SVs fall off the edge of the sky view map long before their computed elevation angles can even drop to +2 or +3 degrees but in my case (departing computed elevation angles of -1 and the odd -2 deg report are relatively common), setting the filter to zero degrees showed little to no impediment to positional accuracy leaving me somewhatThe upshot of all this is that, having raised the antenna clear of the roof ridge tiles, I felt I could afford the luxury of discarding the final 10 degree's worth of usable SV signal to reduce the multi-path error risk. Initially setting it to 10 deg before realising that the final 10 degrees' worth is actually in the range -1 to +9, hence the 9 degree filter setting I'm now using (which is still 4 deg more than the default setting).You can take it from my experience that there's no 'magic bullet' setting for the elevation angle mask setting. Whilst it's true that restricting yourself to high elevation angle SV passes reduces the ionospheric induced error in the transit times for the GPS signals, the restricted triangulation angles available will counter this benefit. You need good positional data to get good timing data if you can't do a "Survey in" or manually configure a static fixed location. The larger number of and the wider triangulation angle reference points you have, the better the positional accuracy you can achieve, given the same accuracy of distances (transit times - it's all about accurate timings and ephemeris data). Unfortunately, this is definitely a case of of being "Damned if you do and damned if you don't."Since my 'manual' configuration of the Time Mode 2 settings may have actually had the desired effect, I'll retest the 45 deg filtering option to see whether this can now provide any benefit. It never occurred to me to retest this until just now.I noticed that whilst trying to knock the SV signals right down to test the lower limits of C/N usability by disabling the antenna power, it failed to disable the antenna power feed despite claiming to have accepted the changes. No wonder LH failed to have her wicked way with these settings as she had with the original and the other fake module.Since confused antenna power settings in the past have required a reloading of the defaults with a power cycle to activate them, I shut my GPSDO down for the first time in over a fortnight to do the opposite of what LH had obliged me to do to restore 'normality' with the other modules. I'm rather loath to needlessly keep power cycling the GPSDO but in this case it was needed.This power cycling exercise did reveal some interesting things about which settings could survive a power outage event and which didn't. Interestingly, restoring the deviation map showed it had retained the position settings whilst the Time Mode 2 settings appeared to have lost them. This could simply have been the consequence of loading the system defaults before cycling the power - I'll have to repeat the power cycling test sans any attempt to get the antenna power settings unlocked by means of forcing it back to defaults.However, it was surprising that the deviation map had retained its hard won "survey in" settings (but I've read where similar behaviour with location settings on 2nd hand acquired GPSDOs have caused problems for their new owners so this might be a deliberate behaviour that's generic to all GPS modules - I think in this case, the answer is as simple as just pressing the C centering button at the bottom the deviation map display window to centre it onto the current plotted position).Obviously, I'll now be busy running more tests to check what settings are saved and which are lost during a power outage.PPS. I've taken this opportunity to attach another 25 screenshots

author: tkamiya
author_rank: Super Contributor
timestamp: May 22, 2020, 05:46:18 am
content: Getting back to OP's original question....I am right in the middle of process of looking at and measuring surplus purchased standards. All I can say is, it's ALL OVER THE PLACE. I have about 10 currently running, and only few agree closely to each other. Interestingly, I have factory made (meaning not DIY) frequency standard that has 60Hz ripple on 1 pps. Also the shape of leading edge isn't straight. It really affects adev measurement. I have two of them and they do the same thing. I have another "just the module" type, and again, they are all over the place. Fortunately, some are showing expected results.They all are fine for casual use. But for more detailed time-nuts type work, some are clearly not acceptable.

author: FransW
author_rank: Frequent Contributor
timestamp: May 22, 2020, 09:03:52 am
content: Quote from: tkamiya on May 22, 2020, 05:46:18 amPlease explain. Especially the "detail".Are you dissatisfied with the NIST performance or with the quality of (ex-) commercial GPSDO's and/orrubidium - cesium reference equipment?The contributions above give a clear sight on these matters, although sometimes somewhat personal.Thanks, Frans

author: notfaded1
author_rank: Frequent Contributor
timestamp: May 22, 2020, 02:38:37 pm
content: Quote from: tkamiya on May 22, 2020, 05:46:18 amI can totally relate to this... once you start measuring ADEV and want to get into -11, -12, -13 it's whole different ball game. In addition to the GPSDO, GNSSDO's I've gotten into GPS disciplined Rb's. I have two of them now in addition to some stand alone Rb. I had a GPS antenna splitter, which grew to a x4 splitter and now I'm moving up to x8 splitter for the antenna so I can run more of these at the same time. My newest adventure is comparing older Thunderbolt to my Thunderbolt E I started running a few months ago. I also just took delivery of a TURN Dynamic Systems FSA3011 OCXO GPSDO Atom Clock Frequency Stability Analyzer 2020 and hope to finally start getting some real phase data. Up to this point I couldn't really measure phase at all because traditionally this has been an expensive endeavor.Bill

author: cdev
author_rank: Super Contributor
timestamp: May 22, 2020, 03:16:27 pm
content: I'm in the same boat as you are, James -where I have nothing to compare it to thats even remotely as accurate let alone more accurate.LOL, so I guess thats what makes it fun.Some time ago tvb who has the means of testing them properly, posted a comparison of a bunch of new high end, and surplus GPSDOs here. (No DIY units though) Indeed, there was a lot of variation between them.Its worth looking for.Quote from: james_s on February 20, 2020, 01:49:31 am

author: jpb
author_rank: Super Contributor
timestamp: May 22, 2020, 07:00:04 pm
content: It is difficult/expensive to have much better reference oscillators to measure against.A compromise (the approach I take) is to have two of everything and measure against each other. For noise this works quite well but it is not ideal where there is any correlation. With GPSDOs I try to avoid excessive correlation by using different antennas on different edges of the house rather than a split feed from the same one but with influences such as temperature there must be correlated changes that are lost.Stable32 has the option of measuring as 1 of 2 that does the scaling for you (I think it just divides by sqrt(2)).The other method is three cornered hat but in theory this requires enough expensive measurement kit to do 3 measurements in parallel. Again, I cheat a bit by doing them sequentially but then repeating to check results are repeatable.

author: tkamiya
author_rank: Super Contributor
timestamp: May 23, 2020, 12:07:41 am
content: I'd be glad to.... except for the fact the testing is still on going. I'll tell you what I know so far. I, too, LACK the precise house standard. Everything is surplus so, every one of the equipment is a suspect. So I decided (with help of an expert) do comparison of my key pieces. Test equipment used was TICC and some software. Oh, and a Siglent 200MHz 2 trace scope.I did basically a loop back test. DUT (the device under the test) supplies both the 10MHz clock and 1 pps. If the DUT didn't have 1 pps, I used TADD2 to generate 1 pps off the same 10MHz signal from the same equipment.You'd expect they will all be straight line with slope of -1 and right on top of each other. Nope! Out of 8 DUT, 3 was above very tight cluster of 4 together, and one was slightly below. (see upload) Hum.... what is this? I've consulted the expert. He told me to look at the signal both 1 pps and 10MHz with a scope. I did both.... connecting 1 pps to ch 1, connecting 10Mhz to ch 2. First, trigger off of 1 pps. I see 10MHz wobble.... I zoomed into the 1 pps. Huh? I saw 60Hz overlayed on the signal. About 50mV p-p. Clearly, 1 pps is dirty and trigger point is shifting at 60Hz. Then, I triggered off of 60Hz and looked at 1 pps. Rise time is awfully slow. Actually it follows the 10MHz sine wave, then slope changes and rounds to level. (see upload) No wonder TICC thinks it's seeing a jitter. See 3 graphs to see what I am talking about. Then I hooked up TADD2 to 10MHz and created both pps and 10Mhz. Yup. Looks good.I won't go into the second piece because principle is the same. DUT was a commercially made and nicely cased 10MHz/1pps GPSDO.I need to look deeper into this. The slow rise could be caused by coax, and 60Hz noise could be common mode. But these were the reason why Adev was worse of the bunch. I would have never known if I wasn't told to measure both signal and check them out.That was just one example. I will need to look at all of them and see what else is going on.On first pic, yellow line is the leading edge of 1 pps.On second pic, purple line is the falling edge of 1 pps magnified to show 60Hz humOn third pic, almost representation of my clock collection

author: tkamiya
author_rank: Super Contributor
timestamp: May 23, 2020, 12:45:18 am
content: To op:As far as I know, FE-5680a has a design limitation that makes it a bit wild on phase shift. It synchronizes an OCXO with rubidium source that is not interger multiple of 10MHz. Then use DDS to generate the frequency application requires. That makes it frequency agile at the expense of added jitter. Good thing is, it never drifts.On the other hand, something like PRS-10 synchronizes THE 10MHz OCXO, the same one used to generate output. So it's stable at an expense of not being frequency agile.The "precision" OCXO in HP products are quite good. I'm testing one, too. I'm seeing very good stability to 100 second. Then it starts to fall apart. It drifts. Also, setting it to precise frequency is very difficult. The adjustment trimmer doesn't allow for fine FINE tuning. GPSDO is good for that.One thing I've learned is, all OCXO, GPSDO, Rb, whatever are not created equal. They are almost tailor made for certain applications. So one aspect maybe great while others, not so. Difficulty is to choosing the right one for your particular needs. That's why I have so many. Either way, though, in practical sense, 10E-10 is plenty good for most of the stuff. Little jitter here and there doesn't matter unless we are doing things like compare each other. We do it for fun for the ultimate precision and accuracy. We are basically chasing if the signal is coming out of socket earlier or later at a pace light travels about one inch. It's quite fun to chase though....tkamiyaTimeNuts Jr.

author: texaspyro
author_rank: Super Contributor
timestamp: May 23, 2020, 04:56:48 am
content: Quote from: cdev on May 22, 2020, 03:16:27 pmI made the measurements on those GPSDOs:https://www.eevblog.com/forum/metrology/comparing-the-performance-of-17-different-gpsdos/I then sent the raw data to TVB and he ran it through TIMELAB to get much prettier plots. Also, he sorted/grouped/compared them... many thanks, Tom:http://leapsecond.com/u/sims/gpsdo17/I seem to remember that Tom's plots are missing a couple of devices... and the Brandywine GPSDO is labeled "brady".

author: FransW
author_rank: Frequent Contributor
timestamp: May 23, 2020, 10:44:17 am
content: I am happy with my Star-4 equipment (2).Does not seem a wrong decision, ref. TvB's comparisons.The only non=commercial references are the NIST & BIPM's equipment.Trying to improve and/or "confirm - deny" their performance is already paid for.

author: texaspyro
author_rank: Super Contributor
timestamp: May 24, 2020, 02:32:02 am
content: The Samsung "UCCM" style GPSDOs also give very good performance for the dollar. The Trimble and Symmetricom units are also pretty good, but the Samsung has a better GPS receiver.

author: tkamiya
author_rank: Super Contributor
timestamp: May 24, 2020, 08:07:37 pm
content: I'm starting to think, "accurate", "precise", and "stable" must be judged in context of what particular use case we are talking about.We all want ultimately accurate, absolutely precise, and rock solid stable, but if the test equipment it will be hooked up to it were to be far worse (most cases) and have enough variance, no precise source alone will necessary improve it to the precision of the time source. I'm running into this now.It's fun to play with these. I'm not sure if OP just wants a nice system, or wants to be a time nut. I enjoy this aspect of the hobby but in practical sense, I'm fully aware I'm chasing a ghost.

author: FransW
author_rank: Frequent Contributor
timestamp: May 25, 2020, 12:12:01 am
content: Absolute measurements do not exist (yet).What else does not exist:1 ultimate accuracy2 absolute precision3 rock solid stabilityWhat does exist:a. Common sense w.r.t. physicsb. getting / acquiring equipment that fulfils your requirementsc. understanding the basic principles of your measurements and the equipment restrictionsd. understanding the environmental and physical parameters that influence your measurementse. understanding the difference between systematic and random errors.f. enjoying what you are doing and avoid becoming too nutty or myopicTo complete the above:When you wish to compare the instrument with a reference, this reference has to be at least 10 times "better".Otherwise, your readings-observations will be "all over the place".That is how I use my GPSDO's. I so use NIST reference by using GPS.

author: cdev
author_rank: Super Contributor
timestamp: May 27, 2020, 09:55:30 pm
content: Did @tvb characterize your TruePosition data eventually?Quote from: texaspyro on May 24, 2020, 02:32:02 amIt seems to me to perform well compared to WWV, but because the ionosphere moves a lot I have no idea what say, beating it against WWV is measuring because it does change phase. I assume that phase change is similar to what happens with a GPS position over time, its a lot like waves in the ocean, which you have to average out to get the sea level at that time.

author: wkb
author_rank: Super Contributor
timestamp: December 25, 2020, 03:22:34 pm
content: Quote from: texaspyro on May 24, 2020, 02:32:02 amI happen to have tested all three vendors on a single GPS roof antenna and a signal splitter.In order from best rx sensitivity to worst:1. Samsung (uses an uBlox)2. Trimble3. Symmetricom (uses a Furuno)#1 and #2 are reasonably close, #3 is completely deaf in comparison.YMMV,Wilko

author: notfaded1
author_rank: Frequent Contributor
timestamp: December 25, 2020, 03:42:32 pm
content: Quote from: texaspyro on May 23, 2020, 04:56:48 amI love the comparison you did and Tom worked the data on. We need more of this kind of comparisons. I know it takes a LOT of work because I've been doing it myself. Even figuring out a creative and good way to organize all the files alone is a task and an art in a way!Bill