Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic. Ignoring the above notice (It's
my thread after all
), this is intended as a final follow up to provide closure for the sixty or so intererested enough to view the attached image of my delidded M8N module since that last posting way back in May.
First off, I have to say that that "Ublox-NEO-M8N GPS navigation signal amplify module for arduino Rasppery PI" I had been seeking opinions on from anyone who may have bought one, turns out to have been that rarest of beasts, an actual genuine U-blox module, unlike the three other U-blox based navigation modules I've subsequently purchased off Ebay since then. In hindsight, it had proved to be the best of the lot (although the final unit I bought earlier this month just might have the edge in terms of marginally improved position fixes as plotted by U-centre's deviation map).
The first replacement module was a NEO-6M at just 4 quid delivered. Surprisingly, the flash ram worked (no need to add a CR2032 for long term (circa 18 months) backup of settings) and it came supplied with the tiniest of active ceramic patch antennas I've ever seen (pictures attached). It had only an IPEX connector with not even the solder lands for an SMA socket and only a serial interface which at least, did include a PPS pin.
Once I'd gotten it interfaced to my winXP VM via an FTDI usb2serial adapter, I discovered, via U-blox's rather klunky u-centre application program, that the PPS option was limited to a minimum time period of just one millisecond (effectively 1KHz maximum pulse frequency - Oh, JOY!!!
). Undismayed, I lashed up a basic PLL with a 10MHz OCXO on a solderless breadboard using two 74HC390 ICs to divide the OCXO frequency down by 10000. This was then followed by a week or so of frustration as I tried to get this lashup to phase lock for more than ten minutes at a time. With a maximum tuning offset of just 10Hz or so and a 10000 frequency ratio, the process of waiting for lock was rather glacial to say the least.
Eventually, when it finally occurred to me to monitor the PPS signal itself, I discovered a
very peculiar "quirk". Effectively, with a 50/50 mark space ratio, the 'mark' half of the PPS pulse would start slowly shrinking in width about 7 to 10 minutes from startup (the repetition rate stayed at 1KHz throughout) and then disappear briefly up its own fundament another 7 to 10 minutes after that before reincarnating itself as a 'Full Fat' 50% ratio square wave to start the next cycle of 'weirdness'.
No wonder I'd been slowly going round the twist trying XOR and 4046 PLL techniques to get the OCXO to lock to this bastard GPS module! I don't know whether it was just some weird peculiarity of this particular (or batch of) module(s), possibly only occurring when the default PPS rate was altered from its normal one or ten millisecond wide pulse at 1PPS but since I was never going to entertain the even more glacial prospect of phase locking to a 1Hz pulse, I never bothered investigating this weirdness any further. I did start a thread topic on this but no one ever came back admitting to being caught out by this peculiarity.
Obviously, that was a case of "Scratch one cheap bastard M6N module and be a little less cheap about my next U-blox module purchase.", hence my spending a little over 15 quid on an even smaller M8N based module. Despite its small size, it actually possessed both an IPEX and an SMA antenna socket. Unfortunately, it was an out and out fake (no flash, nor it seemed, any SAW filter). However, the lack of flash was just a matter of piggybacking a CR2032 cell holder onto the module and accepting its weird response to signals from the external active antenna and an indoor active unit I'd picked up at the Blackpool amateur radio rally (Hamfest) which showed very similar signal strengths when I knew for a fact that the external antenna was at least 10dB up on the indoor antenna.
Despite this signal strength anomaly, it did seem to perform quite well, and being an M8N unit, I could at least program the PPS to output 100KHz by which to phase lock the OCXO using just a single 74HC390 to get the required divide by 100 which seemed the optimum middle ground between using a 1KHz pulse (I was curious enough to try this with a module that wasn't going to sabotage the proceedings by having the PPS disappear up its own backside every ten to twenty minutes) and a 10MHz pulse feeding my 7486 XOR gate based phase detector now that I was free of the need to use a crappy 4046 PLL detector as I'd been forced to resort to with that bastard M6N module.
I actually got as far as laying out a vero-board for mounting into a neat aluminium case and soldering a support spacer to the serial port pins of the module to mount it on end right next to the FTDI adapter, fitting the aforementioned CR2032 battery holder and removing the SMA connector (I was going to use an SMA socket with IPEX ended flylead on the back panel for this build). However, before I got around to this stage of the build, I spotted another neat looking M8N navigation module for just over £7.50 delivered (there was postage on this one) which looked worth taking a punt on.
This unit was also a fake but only in so far as it too lacked working flash. The signal strength reporting behaviour was a lot closer to that of the original M8N on both antennas but something like about 6 dB down rather than the 16dB down (on the external antenna) of the previous fake. This came with the bonus addition of a rather neat 1 inch square active patch antenna with a one inch length of IPEX connector terminated co-ax.
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[EDIT 20191129]
I decided to connect this first M8N module to the PC (it rather conveniently includes its own built in micro USB port) to refresh my memory of witnessing 55 to 56 dB C/N signal strength reports that I would occasionally observe. This time, however, the signal strength reports seemed little to no better than my latest module's reports. Also, I discovered it had lost its ability to save configuration settings to the FLASH (or the I2C-EEPROM or the SPI-FLASH). I'm absolutely certain it was able to save its settings without selecting the bbRAM option otherwise I'd remember lashing up a CR2032 to it which I'm certain I hadn't felt any need to do.
This came as a bit of a shock to me since it brings my powers of recall into question. Quite possibly the loss of this feature may be down to collateral damage that went unnoticed at the time when I blew the PPS output with a 12v jolt, possibly a latent fault which only now has finally revealed itself. Anyway, even when I bypassed the 5 metre antenna extension cable to maximise the signal strength reports, I never saw it go above 53dB. By comparison, when I did this test with the current module, I only saw brief reports of 52dB peak strengths so not as big a difference as I thought.
Possibly, in the intervening 6 months, the mag mount patch antenna and its 5 metre co-ax may have deteriorated due to damp ingress in spite of such antennas normally being weather-sealed for use as a car roof antenna. However, the impossibly ultra low loss behaviour of my 5 metre RG174 extension cable (just 2dB when the loss per metre figures for this cable indicate an expected 6dB +/-1dB of insertion loss) still seems to apply with both the oldest and the newest examples of M8N based navigation modules.
I've been running u-centre with this first module connected on the end of the antenna extension cable for the past 12 hours or so and my impression now is that its signal strength reporting is a pretty close match to this last fake module which I'll be using in my GPSDO project rather than being some 6dB down as I originally stated.
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Although it lacked an actual SMA socket, it did at least have the solder lands to take one. Since this was a little bulkier than the other module, I decided to rearrange its mounting in the case and solder a right angled SMA-F connector to it to mount it onto the back panel. I'd acquired a few different types of SMA sockets just after I'd purchased the genuine M8N module and this seemed a good way to make some use of this stock of PCB mount SMA sockets.
I've been bogged down with this DIY GPSDO project over the matter of how best to implement the necessary supply of 5 volt DC required. I've been testing a couple of dc-dc converter modules bought from Banggood a month or so back with this project in mind since I want to avoid the need to bolt any heatsink tabs on analogue regulators to the case and the effect of varying levels of heat when powering it from 9 to 15 volt DC wallwarts.
That and the fact that I don't want to have to deal with more than an absolute minimum of fixing screws whenever the need to repair/modify this homebrewed GPSDO arises. Indeed, being able to slide the whole board out of its case and be able to power it up with
less fear of overheating a voltage regulator during testing is the prime motivation to finding an effective voltage stabilisation solution.
Initially, I was just going to use a well filtered dc-dc converter module to directly provide the required 5 volts but I'm now thinking of using low noise LDO regulators from a dc-dc converter providing somewhere between 6 to 7 volt output. However, that plan is now in hiatus since I blew the small, low ripple output converter in a meter probing accident and the other, larger unti was generating almost 1 volt pk-pk ripple on its output (until I solder blobbed an open circuit jumper bridge to connect the Shottky diode to the output inductor, reducing the level to a mere 400mV).
Hopefully, this was just a rogue bad part (I've ordered a couple more similar units to experiment with along with another four examples of the smaller module I burnt out with careless probing). In the meantime, I've decided to connect just the FTDI usb2 serial converter and the GPS modules together which I can power over the usb link, allowing me to run some tests whilst I wait on those dc-dc converters, an "LCR-T4 12864 LCD Graphical Transistor Tester Resistance Capacitance ESR SCR Meter" bought simply because it was so cheap at just £4.73 delivered, an "Original Hiland DIY M12864 Graphics Version Transistor Tester Kit LCR ESR PWM" (a high end version at £14.28 with discount), an "Original Hiland DIY M8 LC Tester Digital Inductance Capacitance Meter Kit" (12 quid with discount - it doesn't test ESR but it covers an extended range down to 1pF and 0.1μH values which the ESR meters don't touch) and a pack of three terminal PCB mount screw terminal blocks that are
not supplied in that LC tester kit.
All the above items were ordered just a week ago from Banggood. I placed individual orders for each item. You don't get any extra discount by integrating them all into a single delivery so it's best to keep them separate to avoid needless processing and shipping delays due to any items not being in stock. With a bit of luck, I might start seeing a trickle of small parcels by the end of this week.
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[EDIT 20191129]
For those of you wondering why I even mention my BG orders of kits and components, the reason is simple - I realised, whilst trawling through my 'salvage' extracted from smpsus and motherboards looking for capacitors and inductors to make up filters to reduce the ripple noise of these dc-dc converters (and that originating in the 9 to 15 volt wallwarts I plan on using), that a convenient way to measure inductance and capacitance (and also ESR) would facilitate my picking out the required parts from my 'salvage' which undoubtedly will include pretty well all the required items, saving me having to specify then order brand new parts at both monetary cost and, perhaps more distressingly, waiting time for the damn things to be delivered.
Also, of course, I can use this kit to measure the inductance of ferrite beads and home made inductors as well as test suspect components, including diodes and transistors. For DIY electronics such as this GPSDO project of mine, they're a surprisingly desirable investment which will likely recoup its capital cost in rather short order.
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Harking back to a comment I made near the start of this missive, Since I've been running tests with the latest fake M8N module over the past two days, I've noticed much less wander in the position deviation map display than any of the other modules, including the original genuine module since I reduced the elevation filter setting from the previous "optimum" value of 10 deg (default having been 5 deg) down to just 1 deg. Surprisingly, this seems to reduce the range of wander to just around the 2 to 3 metre (radius) mark versus the more typical 5 to 7 metre wander.
My externally mounted active mag mount patch antenna has a pretty good all round view of the horizon. It is still slightly obstructed to the East (uphill slope with ground clutter from neighbouring properties and some localised obstruction from the gable ended attic bedroom, the window of which gives me access to the roof of the bay window below upon which a ballasted metal draw with eight foot of antenna pole clamped onto it to clear as much of this obstruction as possible is sat).
I'd like to raise it another 18 inches but until I can reroute the 5 metre cable to allow the SMA connection to a 5 metre extension cable to be kept in the dry, this desire has to remain on hold for the time being.
I've come to the conclusion that when the GPS antenna has a good all round view of the horizon, it's better to reduce the elevation angle filter value to a minimum in spite of the perceived wisdumb that such low elevation signals are so degraded by ground reflections as to render them unreliable. I've actually found the reverse to be the case. It's not quite so obvious with the 5 degree default or even with the "Best Compromise 10 degree" setting but once you go for "Better Kwalitee" satellite signals using a 30 or 40 degree elevation filter, the deviations get worse not better.
A 30 to 50 degree setting might help when the antenna is in a bit of a canyon where unwanted reflections can create chaos with the satellite signals (Timing is everything in these GNS systems) but this can only gain an improvement in an otherwise intolerable situation. If you've been able to site your GPS antenna to gain a good sky view, don't waste the extra satellite signals by needlessly filtering out those close to the horizon.
Also, it's worth taking note of the fact that atmospheric bending (mostly an effect of the ionosphere I understand) means the actual signal paths will arrive a degree or two higher than the satellite's reported position data would lead the navigation engine in your GPS module to report (I've noted elevation angles of -1 and -2 degrees being reported just prior to the SVs disappearing off the plot).
If you observe the way satellites hove into view as they come up over the horizon with signal strengths well above the minimum required to be decoded whilst still being reported as at zero degrees of elevation, the conclusion is that such signals are way better than the perceived "Wisdumb" in this matter implies. Just consider that at the moment the Sun's disk just contacts a true horizon at sunset, it has actually just completely dipped below the horizon as observed in the absence of any atmosphere. Anyway, that's my tuppence worth on the subject; make of it what you will.
I've taken several photographs this afternoon and evening which may be of some interest, showing some U-centre screens as well a few of the modules mentioned above which I will attach to this post. From past experience with this blog's web forum posting UI, they may not appear in the submitted order so if the following descriptions look confused, check the filenames.
The first, 9930, is worth a close look at R22's stats. The next three (possibly the first three from past experience) numbered 9942, 9943 and 9944 are just showing the state of play after about a six hour run. The fifth (or 4th), 9949, shows the last GPS module mounted on stand-offs over the OCXO module on the half built GPSDO PCB with the supplied active patch antenna parked on top of the OCXO simply to include it in the shot.
The module is being powered from my desktop PC via the FTDI usb2ser adapter just above the GPS module with the backup battery (CR2032 cell in its holder) just to its right. The OCXO and the other ICs aren't yet connected to this temporary arrangement. The yellow wire coming from the CR2032 Cell holder is threaded through the bottom right TPH with a short length of silicone rubber insulation (also yellow) for protection and is soldered onto an SMD diode which is soldered to the 1K resistor hanging off a solder joint onto the end of the 1K smd resistor that connects to the Vbk pin (pin 22) of the M8N module itself.
The original 80 milli Farad supercap represents a vampire loading of 1.2μA on my backup battery but since the draw by the module is some 14.7μA anyway, it's no big deal so I've resisted the urge to remove or disconnect the supercap - I could end up doing more harm than good.
The diode leakage when powered up seems to be around 0.1μA in this circuit which eventually raised the coin cell voltage from this afternoon's start point of 3.128v up to 3.152v over an 18 hour period. However, it had reached this voltage several hours ago so appears to have reached its limit. ISTR reading that such cells in backup use could tolerate charging currents of half a μA indefinitely but after spending the past hour or more on a fruitless search of CR2032 datasheets, the best I could track down was this little nugget from a Maxell datasheet claiming the limit on such inadvertent charging current being 10mA !
That's all a far cry from my recollection from the days when manufacturers used to offer more comprehensive data than the typical sales blurb sheets you see today! According to Maxell, my 1K resistor is a 5 times larger resistance value than needed. That was based on using a 5v supply - in this case it's a 3.3v supply so it might actually be a hundred times the resistance requirement based on the voltage difference calculation they used. It seems I can now take note of Dr Strangelove's philosophy and stop worrying about exploding CR2032 cells.
The final image shows the three previous GPS modules I'd used. Displayed in the order left to right, is the original M8N module as per the subject line of this topic, then the next one is that bastard NEO-6M that was not fit for purpose, finishing with the very petite fake as a nine bob note M8N module, all prepped up to be stood on end into the main PCB, replete with a CR2032 coin cell holder and cell stuck to the top of the U-blox module on its backside with double sided tape and wired to the 80mF supercap via a 1K resistor and smd Shottky diode with the SMA socket removed from the PCB.
On the LHS of the picture, the passive 1 inch square ceramic patch antenna is laid out alongside of its donor module whilst at the top of the picture, the smallest of cutdown ceramic patch antennas is shown next to its partner in crime. The most expensive of the M8N fakes, BTW, wasn't supplied with any such accessory.
All in all, my quest to build a cheap DIY GPSDO has worked out a little more expensive than anticipated. Still, it's been quite a learning experience at relatively little cost despite all of that and the lessons still aren't over. I've yet to complete my little "Basic GPSDO" project and I've no doubt I'll still be learning more once I have it up and running.
JBG