GPS and frequency stability with a U3s

John Canfield <bucket@...>

Does the GPS correct the reference frequency in a U3s? I noticed my frequency drifting a bit in WSPR reports with the standard 27 Mhz crystal board. Yes I do park at 150 Mhz. The main reason I bought the GPS kit was for precision frequency and frequency stability so I don't understand. 

I have done quite a of searching here but haven't seen this question addressed. 

73 de John WB5THT 

Even if Si5351A is replaced, it becomes "si5351 ERROR".Why?


Even if Si5351A is replaced, it becomes "si5351 ERROR".Why?

Side cutters #tool


Have my 40m QCX ordered and am waiting (somewhat) patiently for the status to switch from "complete" to "shipped", but in the mean time I want to ask a tool question from other builders.

What is your favorite brand / type of side cutters for trimming leads after soldering?

I usually just pick up a decent pair of 4-5 inch ones with a small head. They work well for a while, but after a few projects the blades end up getting notched near the tip from all the trimming making them more difficult to use, and in need of replacement. Just wondering if anyone has any recommendations for a good long lasting pair of side cutters.


Re: U3s Frequency drift / stability #u3s #wspr #problem



Another aspect is needing several calibration cycles before the PLL is well calibrated.  in my case it was 5 or more. Since then my U3S is sufficiently stable from a cold start, with none of the advanced features you have. Do try parking up at vhf as suggested. I did not learn enough about this in the manual, but from messages here. Somewhere there is a message from Hans suggesting higher cal parameters for a newly installed unit or controller. It would be nice to have this instruction in the manual.


Re: QCX a Phenomenal Transceiver

John Kirby


Thanks to Hans the QCX is an excellent learning platform

To cover three bands ..
With few exceptions I built and aligned a 30m QCX to specks...

1) I did not install LPF on QCX PCB because you will need different LPFs for different bands and Hans has several (to include switchable) QRPLabs options
2) I installed T1 on bottom of PCB for ease of solder/, turns out no fiddling was required for 40, 30, 20
3) C1 requires fiddling to cover three bands (two bands no problem) and of course must be tweeked each band changeI I have two variable caps on PCB ... Note too the variable caps supplied with kit do not tolerate much heat and/or fiddling I destroied several and ended up using two variable ceramic units
4) I fiddled with L4 enough to determine I needed a better understing of class E amplifier "efficency" so  L4 = STET for 30m

Remember too based on much discussion here on the list... different LPFs produce different RF output levels, fiddling with LPFs can be a delightful learning experience

Replacing the onboard audio filter with different designs and bandwiths was an OUTSTANDING learning experience

72 73

Re: Shack clock contrast adjustment


As I look at it more closely, in the drawing on page 6, the jumper is not installed in the same place as the resistor would be.  Looking at the board itself, there are four holes in a line, the outer pair to accommodate a resistor, and the inner pair to accommodate the jumper.

I'd appreciate it if someone could 'splain it to me.

Re: U3s Frequency drift / stability #u3s #wspr #problem

Bruce K1FFX

Hi, Jose -

I might suggest lowering the Park frequency.  With my U3S unit (on 20m and 30m), I found it was initially drifting downward in
frequency.  When I set Park to 150,00,000, it was then drifting upward in frequency.  I experimented with 
different settings of Park, and ended up at something like 141,300,00 ... that worked well for my setup
(I also put a heat sink on the crystal on the synthesizer board and put some foam around the whole
synthesizer assembly ... have a look at Hans' Application Note on this topic.

73 -

Bruce K1ffx

Re: OCX Display

terry hughes

i removed ic2 (the MCU) and powered up.

the display remained blank

does this mean that the lcd is broken

terry gm4dso

GPS Module wanted

Paul Bowen





I use VK2828U7G5LF modules with my GPS projects. Amazon prime quick expensive get it by Friday. Ebay get it early next year at half the price.


I have found some other modules take an age to lock inside the house where as VK2828U7G5LF works well inside.










From: <> On Behalf Of m0bmn via Groups.Io
Sent: 12 December 2018 19:29
Subject: [QRPLabs] GPS Module wanted


Hi All

I hope its ok to ask via the group but does anyone in the UK have a GPS module that they don’t want or use, if so let me know your price please, looking for one to setup with my 20m QCX, fed up of setting the time for WSPR.

All the best

Paul M0BMN

U3s Frequency drift / stability #u3s #wspr #problem


Hi ¡
I know that there are already lots of post about but I have been reading quite a lot these days and Im still not able to find out what happen with my U3S.  I have been playing during few days with several configurations and the U3S is beating me up.  The unit was on my drawer during long time because of this issue, but Im back again encouraged to make that working properly...

- Problem: 
I'm only working on WSPR mode.  After certain time (depending on the Cal steep config.) the TX freq gets out of the RX window; what is more, without stopping at any point inside RX winding,  Guess the the goal an stabilization inside the RX window.
I start always with a Ref. Freq 27.004.000 with good results......
Also, a reset of the Atmega has been done.
- Set up: 
     U3S board ver. 2. // Atmega  V 3.08
     PSU 5V from regulated feed.
     Si5351A (including small cooler on the crystal.
       6x  LPF.
     GPS controlled (SKM52)
     U3S config.
          Frame Start: 10 - 02
          Cal Step Time: 10 - 120 (tryed with several configs here and final result is teh same,  out of RX winding)                  Park mode: 2 - 150.000.000
          Sys Frq: 20.000.000
          Ref Frq: 27.004.000
     Monitoring my TX frq. with SDR

What Im missing?  thanks for any light you may give.

73 de EA2KV, José

GPS Module wanted


Hi All

I hope its ok to ask via the group but does anyone in the UK have a GPS module that they don’t want or use, if so let me know your price please, looking for one to setup with my 20m QCX, fed up of setting the time for WSPR.

All the best

Paul M0BMN

O3S Power Drop On 630M

Bob Cutter

My new unit, on 630M, seems to drop about 30% power during the WSPR transmit period into a dummy load. The final is a single 170.  Peak power during calibration is 800mw but I reduced to 700. 

I suspect heating. Any suggestions?

72, Bob KI0G 

Re: QCX a Phenomenal Transceiver

Kees T


I agree that it's a great unit.

Would you expand a little on #5  ....making the 30m unit a 3 band unit. Switchable ?

73 Kees K5BCQ

U3S ref freqency?


hi group,

i have a U3 which i use for wspr since a couple of years. Works flawness no problems at all.
For a fellow ham i build a U3S but untill now i’m not satisfied at all.

I measured the output frequency direct via an attenuator on a freq counter.
The difference i use as ref. Frequency.
Result is very strange. Most of time one or no spots, or only spots from one station.
U3S is connected on a GPS. It seems that or the output frequency is not correct or the U3S is drifting of frequency.
Do you have an idea, or encounter the same issue?

best 73 Ton PA1PXL

Re: #wspr U3S for 2m WSPR: How to remove the frequency-swing at start of transmission #wspr


Tried the Fox924B 27.000 tcxo today, I have two synth with this chip on board and tried both, performance was abysmal!!!!.
2 hours, not a single decode. 
In WSJT-X the signal can be seen to slowly drift downwards and then step up before another downward drift. As this occurs during the transmission cycle the decode fails. At the point that the step is seen, on the noise graph what should be a nice single peak changes to a ragged double peak and then back again. Note also noise either side of fundamental, does not happen with the standard synth. 
Both synth showed identical behaviour. The standard crystal synth with no insulation performed much better than this, tomorrow I'll fit the OCXO module and see what happens. 
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.


Alan G4ZFQ

All output frequency variations I see are reflected by a move of the 27MHz crystal frequency.
I've done a few more observations and I think I'll stop now.
U3S with my particular synth module with a 27MHz crystal (and a 8x8x3mm aluminium block glued to the Si5351) Some of this confirmed by using the same module in a VFO with 50 ohm load:- Note these effects are not seen with a suitable TCXO.
Big changes in output frequency seem to have two effects, a quick change in frequency, maybe a Hz or so, followed by a slower drift.
When the U3S goes into calibration mode the 27MHz can change upwards by a Hz or so depending on the frequency of the preceding transmission. This would seem to limit the accuracy of the GPS calibration.

Small changes in frequency seem to have no effect. At 144MHz my synth is not particularly stable. The cyclic variation, +/- a few Hz, I saw on WSPR is there without modulation. 10x8x3mm aluminium glued to the crystal seemed to make no difference.

The VFO uses a different GPS adjustment, the 6.75MHz is on Clock 2 permanently, although it does move with large frequency changes I guess the huff-puff averages it better. My checks with PPS show the VFO settling rather closer to nominal (a few 10ths of a Hz at 27MHz) than the U3S. I'm fairly certain the clock 0 output moves less on big changes of frequency when GPS mode is enabled (but no PPS).

73 Alan G4ZFQ

Re: QCX a Phenomenal Transceiver

John Kirby

ham 1957

Re: QCX a Phenomenal Transceiver

John Kirby

I ... second st3v3  ... plus...  third, forth, etc

3) documentation OUTSTANDING
4) ease of alignment a delight
5) with little effort the 30m chsssis becomes a three band xcvr
6) bypass the outstanding on board CW filter with different designs produce other modes/personalities
7) mountain topping
8) the desire to build more QCXs
9) purchase different kits from QRPLabs

I feel certain others can/will add QCX experience/personalities/delights


72 73
ham 1967
retired 1997

Re: #wspr U3S for 2m WSPR: How to remove the frequency-swing at start of transmission #wspr


Thanks for that Alan, 
I did wonder if that was the case, as all spots over about an hour period, 8 transmissions into a dummy load, reported the same -4 figure, on slightly different frequencies.
It looks like the standard synth then is good enough for 2m work, I suspect that the other synth boards will produce better results. 
I have arranged for a group of amateurs in Gloucestershire to assist in experiments once I go on air, I live underneath a hill so vhf is not omnidirectional for me. 
The tcxo equipped synth draws slightly more current but it is only slight it'll be interesting to see its drift. 

Good fun.
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

Re: QCX a Phenomenal Transceiver

Hans Summers

Thanks Steve, nice feedback!

73 Hans G0UPL

On Tue, Dec 11, 2018 at 8:36 PM <st3v3nd007@...> wrote:
For the record, I am retired going on 75yrs old and not affiliated with any businesses and giving my two cents about the QCX transceiver.
I have built lots of kits but not one as nice as my 40M QCX. I bought this gem a year ago November and decided to build it a few weeks ago.
I took my time only soldering a few parts each day and one torroid a day taking care to ensure all solder connections are correct.  I
also used IC sockets on all the ICs to prevent any possible overheating of the ICs. Soldering iron temperature was set to 750 degrees. Did
the alignment carefully to specifications. Smoke test was good and power into my QRPoMETER dummy load was 4.3 watts at 13.6 volts.
I set the voltage down to put out about 4 watts for the SKCC weekend sprint and worked 61 stations from Maryland including California, Texas,
France, and many Midwest to East coast stations. Today I tested for maximum output. Here are the results:

Configuration: QCX, LDG Z100Plus tuner, OHR QRP wattmeter, and G5RV antenna at 28 Feet. Power from Alinco DM330 Power supply.
9 volts = 2 watts
12 volts = 3.8 watts
13.6 volts = 4.8 watts
14 volts = 5 watts
15.25 volts = 6 watts
The LDG tuner does a wonderful job tuning to a good match. Thinking about building a another one on a different band. Can't wait to
see what the multiband transceiver QRPLabs is working on.

Shortcomings: It would be sinful to complain about a $49 product that offers so much.

St3v3 in Maryland