GPS Modules Accuracy and 1pps

Looking for a GPS Module with 1pps to use on the Ultimate U2 / U3 kits. Then read the Specifications and avoid the StarIII Chipset, I looked into these some time ago and concluded that they were never intended for timing applications.

Last week I became concerned about StarIII module being supplied with reference to them having 1pps output. Not many suppliers tell the lie but at least one Chinese source does.
OK there is LED that flashes at something like 1pps but that is an indication that the thing is on and searching for or located the odd satellites. It is certainly not an indication of 3D lock or that an accurate PPS is available. Little information is available from suppliers or manufactures and some suppliers have got it wrong, you have been warned avoid SRFIII like the plague.

I searched the Manufactures Spec. Sheets and produced this comparison of some known good and bad GPS Modules.

GPS Module Timing Accuracy From Spec. Sheet

In order of timing accuracy

Trimble
PPS UP501
PPS output +/- 50 ns (RMS) accuracy

Half as good as Trimble.

50Hz in 1,000,000,000 (RMS?)

EM406a
Time: 1us synchronized to GPS time

40 times worse than Trimble

1Hz in 1,000,000

VK16E
timing accuracy: 30 us

More than 1000 times worse than Trimble. (Could be +/- 2mS Jitter according to GPS Passion)

30Hz in 1,000,000
187.5Hz in 6.25MHz
3.75KHz in 125MHz

MTK3339
Timing Accuracy (1PPS Output) 10 ns RMS Jitter

No actual statement of accuracy. Well at least they admit there is jitter

The last two Modules use the SiRF StarIII chipset it seems that there is no true PPS.

To quote from GPS Passion “A jitter this large definitely confirms that it’s not 1PPS but a simple digital output crudely toggled by software at opportune moments. You can already have a similar accuracy from the first start bit on the TX pin “

8th December 2013
Things move on, and now the U3 Huff Puff type stabilising has been added to the U3, average the errors for long enough and you end up with a reference. It works a treat with an accurate GPS, not that it needs taming, the thermal drift of the DDS and Reference Crystal benefits.

Propagation Study and Psychology

I didn’t intend to start a debate when I mentioned the blatant difference in the number of reports from my low power QRSS and WSPR tests actually ZERO unsolicited QRSS reports against copious every day for WSPR.
I have been running low power tests on HF and MF for several years, and the results are the same. Reports of QRSS reception require humans, decent equipment and skill, WSPR is relatively more simple.

With QRSS the receiving station once he has bothered to look, bothered to adjust the DSP software, bothered to understand displayed Morse, then he has got to be bothered to post a report to a site or send a report by e-mail.

In fact I have known ever since I was first licensed that psychology is involved in signal reporting.

The other human imperfection reporting system I have known since I was a young licensee is the OPA, Old Pals Act system. I have only ever mentioned this in closed circles. Well you have to get on with folk down at the Radio Club despite their foibles, otherwise how would you get to be Chairman / President.
However when you get older you don’t care about speaking your mind, lots of those involved are dead and why should those still alive be allowed to get away with their blatant rudeness.
The prime case of the OPA system is the ‘our best pal moved to VK syndrome’, I have observed it for years with more than one local who went down under. When the VK station has his sked with his pals he puts up with all levels of signals QSB noise etc. missing bits are filled in and ignored, the little net continues. Reports are 5-9 for all the group no matter how modest their set up. Just let another local call in on the sked, funny thing the VK gets the Call Sign, Name, QTH then “can’t hear you mate sorry” he is ignored and soon goes away.

Another local group on 40m found some other locals calling in to their net a nuisance and had a code, QRM can’t copy a thing try channel 2 if the interlopers found them after the QSY it was time to end the net, it just dissolved.

My own experience of OPA rudeness was another local who moved to VK and was calling the sked one Saturday morning and getting no results. I called him, exchanged the haven’t spoken for a while stuff then he said, “have you heard Tom, Dick and Harry this morning”. Nope. “Oh dear maybe we have got the time confused, can you ring them”. Well I don’t have phone, (it was when home phones were rare). “Can you go to a phone box and call them”. Well its a fair way away, you know I live in a village. “Oh go on please get the car out and go give them a ring mate”. So I did and when I got back the group were on with him, I called in, ‘so the phone call worked then’. “Sorry can’t hear you mate”.

OK, one more personal experience, the CK report Californian Kilowatt. I was an early user of Slow Scan TV, we had to build our gear, give the authorities a description of it and only then they gave you a special permit to use it. I answered a CQ with a test card, QRP of course, back on phone came a US station he had read my call and location. “Your a noisy picture turn up the power”.
Your my first US station on SSTV can you give me a report please,(on phone), followed by a picture with lettering, QRP SSTV 10 Watts.
“I see you are running QRP, only 10 Watts come back when you’ve gotten a linear”.
He was reading every word of my SSTV picture cards but still refused to give me a report.

So when you put the radio on in the shack be sure to put a white coat on too so you can analise the inner meaning of any reports you might get.

Hans Summers QRPLabs U3 Tests

Here are the photos of the U3 on test showing the really nice blue screen display and with my hand to reference the true size.

It really is a little beauty. Currently I am setting up the 20MHz clock oscillator and checking free running with no GPS, frequency stability. It is very good, I have no Xtal heat sink fitted and see no problems. The dirty finger nail is ‘an in joke’ here since I did a picture of poking a dirty finger on a switch and the photo didn’t do it justice.

Rear view showing the 600m LPF, those scruffy ‘almost impossible to wind’ inductors don’t look too bad.

I thought my LPF’s were small but the U2 / U3 ones (green) are tiny, the one on the right is the even more impossible to wind 2200m LPF, a little fatty, but it fits the board with a little overlap.

The U2 circuit board layout gave problems of interference to the GPS, my test lead for the U501 is shorter than the one I used on the U2 and I forgot to put ferrite beads on the wires at either end, so far I have seen no problems.

QRPLabs U3 Launched

I am not going to do what almost all the authors do. See a new product, copy and paste the photos from the site, copy the published spec and promote what they have never seen or used.
I didn’t do that with the U2, I couldn’t bring myself to write much about that at first, it was a bit of a disaster and I hope there are none of the v2.01’s left on air polluting the airwaves. If you are running one, shame on you.

After several months and versions the U2 v2.06b is a pretty good stable usable MEPT and I am pleased to say I had a hand in pressing for the improvements.

The U3 has a new PCB with an Earth plane and upgraded display. It only needed new drivers to produce an upgraded unit, unfortunately somehow errors crept into the code and the launch unit comes with bugs. Hans is toiling hard, into the early hours, to get those sorted but really the time for that is before launch, Hans is too independent and too hasty I am afraid, you just cant question his enthusiasm. There is nothing like using independent Beta Testers to find the bugs that the programmer missed, before launch.

It can be assembled in rapid time,I am not going to try and beat the record, I have first started this blog post with a photo of what comes in the little parcel.

Right Click and select View Image for full view.

After assembly I shall continue with honest, true test results.

A Little Later in the Afternoon.

Whoa,Hans called in the designers from all departments, RF PCB Design, Mechanical, Ergonomic and Artistic. The new display, sandwich construction with main board and daughter boards which don’t overhang looks and handles like a professional bit of gear. The layout with GPS connector on the opposite side to the PA and PCB Ground plane must improve the RF performance.

I tested the supplied software version v3.0a, yes it had a stray last character left on the display after sending Hell as Mode 1. With no GPS connected I noted the Heartbeat indicator pulsing steadily indicating a perfect 3D satellite fix!! Once connected the GPS calibrated the DDS up, then down steadily on each transmission. Most of these problems already reported by the first users to receive kits.

Luckily I have now got myself equipped for flashing the AVR chips, so there is no need to wait for replacements anymore and Hans sleepless night produced v3.0b. Someone joked ‘careful there are only 26 letters in the Alphabet’ but I don’t think we need to worry, initial tests indicate that all is well now.

This is a true and honest review so I must mention errors and omissions, human errors creep into the packaging if a foolproof system is not used. My U2 was missing the resistors, the U3 had two contrast potentiometers enclosed and the LPF kit had no wire.

Tomorrow I will do some setting up and take some more pictures.