Dear U3S whisperer,

I bought a U3S with the intention to increase 2m WSPR-activity, which is incomprehensibly low worldwide :-) I assembled the U3S-box and tested it initially on shortwave. The WSPR-spectrum showed a perfect straight signal on every band from 80m to 10m and I was received worldwide (reports from VK and the Neumaier-Station), great ! 

Finally I tried the 2m Band. Here is the received signal (from a 2m/10m Converter and a Perseus) with huge attenuation:



The little drifting upwards is possibly caused by the used converter (transverters-store.com/) . My problem is the peculiar "swing in" when transmission starts. Not every frame is decoded (missing 2038) , may be the "swing in" is the reason.
I`m using the simple txco with my U3S. Is there a way to reduce this effect ?

Thank you very much for help.

73, DF5FH
Henning

Hi Henning, 

I think that I'd try the OCXO instead of the tcxo, your screen shot also shows drift figures of -1 to -3 an OCXO would help with this.
Secondly regarding the swing in, perhaps look at your power supply? I run mine with separate 7805's for pa, heater and main board with thick wires to the 7805's and from them to the u3s, also I have caps fitted from input and output of each 7805 to its ground pin.
This is on my hf u3s's and have 0 drift and dead straight lines monitoring qrss in Argo. I am waiting for another u3s to arrive and that will be a dedicated 2m wspr beacon so I'd be interested in seeing if you solve this
Andy.
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

My problem is the peculiar "swing in" when transmission starts. Not every frame is decoded (missing 2038) , may be the "swing in" is the reason.
I`m using the simple txco with my U3S. Is there a way to reduce this

Henning,

As Andy says, it looks as if the supply voltage to the Si5351 may be changing.
But that small effect will not seriously affect WSPR, just may make the difference between a -30dB spot or nothing.
I'd say the TCXO is quite adequate, just see if you can get rid of your "swing in". Have you tried different park frequencies?
Drift is within acceptable limits, if it is the transverter maybe look at stabilising it's conversion oscillator?

73 Alan G4ZFQ

Hi Henning,
I struggled to get my U3S to work on 2m as well; I made many posts here.  I ended up using a Fox 924B series TCXO raised off the PCB on thin wires and various kinds of insulation to prevent it from getting warmed up by the PA transistor or the DDS chip.  But I never got the chirp at the start of transmission after I removed the crystal and installed the TCXO; instead there were frequency jumps as the TCXO temperature changed.
As far as I can tell, the DDS chip itself will not change frequency due to its voltage or temperature; it is a digital device.  But when it changes temperature due to its changing load, that change can also reach the crystal or TCXO, possibly via the copper foils on the PCB or the PCB itself, and that tiny temperature change then moves the crystal frequency a smidge, and that shows up in the output.
One difference in our configurations is that I'm running my U3S on 2m directly, without a transverter.  I'm curious why you chose to go the transverter route.  Does your transverter have a mixer that brings in a signal generated by another crystal or TCXO?
Cheers
Halden NR7V

Thank you for the answers ... I will experiment with the power supply and report soon.

@Halden: I`m running the U3S on 2m direct too. But the control-receiver for monitoring my own signal is a 2m/10m-converter followed by a Perseus-SDR.

73, Henning

Hi Henning
I looked at transverters-store.com and found a transverter there that uses a crystal local oscillator.  If that's the one you're using, you might want to check that oscillator circuit for tiny changes in voltage or temperature that could occur at onset of TX.  Per the reviews I also found, you might want to check the output to make sure the spurious emissions are sufficiently suppressed.
Cheers
Halden NR7V

So ... back from experimenting. What I did:

1.) I connected a very strongl regulated powersupply with thick wires direct to the U3S mainboard. Hmm ... there was no visible change at all. I played with the supply-voltage, reducing the voltage to 4.8V changed the frequency only for 10Hz. But at start of transmission the "chirp" seems to come from "infinity"...

2.) When I put my finger on the crystal I see the frequency jumping for 100Hz or more. It seems to be very temperature-critical. Somewhere I read that it is a good idea to put a heatsink to the crystal to make the temperature-changing lazy. So I soldered a 2 Cent coin direct on the crystal. But again, no influence to the chirp at the beginning of transmission.

3.) Now I tried to measure the 27 MHz Carrier from the crystal. I connected my Perseus antenna-input direct with the 2 Cent coin and I found the crystal oscillating at 27.005 MHz. Then I tuned the Perseus into the wspr window and made a recording with Argo. This was very interesting:


 
During the pause between the transmissions the crystal frequency makes a big jump. But the crystal should oscillate accurate on one fixed frequency! What is the reason for this behaviour ? Currently I have no idea and have to think about it. Possibly somebody else has an idea for the reason ?

However ... I think it`s a good idea to order the oven-controlled version ... not primary for the oven but for the decoupled 27 MHz oscilator.

73, Henning

Hi Henning,
In your first post, you wrote that you were using a txco.  I thought you meant TCXO, so my comments were based on that.  But your recent post refers to a 27 MHz crystal oscillating at 27.005 MHz.  Could you clarify whether you're using a crystal or TCXO in your U3S?
Cheers
Halden

Hello Henning,
Looking at your first screenshot and the second it looks as though you are running from one wspr transmission straight into the next. Have you tried leaving a gap between transmissions? Also what park mode and frequency are you using? 
Andy
--
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

Hello Halden,
sorry, my fault, I scambled the definitions. I used the word "txco" without thinking. I`m talking about the original Si5351 synthesizer  using the kit-supplied 27 MHz crystal. I have no optional "SMD 27MHz TCXO" in use .

Sorry,
73, DF5FH
Henning

Hello Andy,
I`m using parkmode 0. I have only one enabled mode screen so the parking frequency should be the 2m frequency itself?
Yes, I tried transmitting every 4 Minutes but this didn`t changed the "chirp"-behaviour. Later I will make Tests with all park modes.
I`m not sure yet, but I have the feeling that the park-carrier is only replacing the WSPR-transmission but it is off during the seconds between two tranmissions. I will check this detailed ...

73, DF5FH
Henning

Hi Henning,
In that case, what you're seeing is very much like what I saw before I installed the TCXO which changed chirp and drift to discrete frequency jumps.  Eventually, I lifted the TCXO a couple cm off the PCB, which helped reduce my frequency jumps.  That lead me to wonder whether the TCXO was necessary at all.  But I didn't go back to the crystal to find out.  If the chirp is due to the crystal temperature changing, then lifting it a couple cm off the PCB, connecting it with thin strands of wire (enough to conduct the small current but not enough to conduct much heat), and wrapping it and its coin with foam might help.  Using thin wire would reduce the heat coming in from the foil on the PCB and the PCB itself, and wrapping it with foam would reduce the heat coming in from the PA transistor which warms up at key-down.
Before lifting the crystal, you could back off on the PA's bias and see if the chirp magnitude changes.  This might indicate whether heat from the PA transistor is part of the equation,.
Cheers
Halden

So ... back from experimenting. What I did:

1.) I connected a very strongl regulated powersupply with thick wires direct to the U3S mainboard. Hmm ... there was no visible change at all. I played with the supply-voltage, reducing the voltage to 4.8V changed the frequency only for 10Hz. But at start of transmission the "chirp" seems to come from "infinity"...

2.) When I put my finger on the crystal I see the frequency jumping for 100Hz or more. It seems to be very temperature-critical. Somewhere I read that it is a good idea to put a heatsink to the crystal to make the temperature-changing lazy. So I soldered a 2 Cent coin direct on the crystal. But again, no influence to the chirp at the beginning of transmission.

3.) Now I tried to measure the 27 MHz Carrier from the crystal. I connected my Perseus antenna-input direct with the 2 Cent coin and I found the crystal oscillating at 27.005 MHz. Then I tuned the Perseus into the wspr window and made a recording with Argo. This was very interesting:


 
During the pause between the transmissions the crystal frequency makes a big jump. But the crystal should oscillate accurate on one fixed frequency! What is the reason for this behaviour ? Currently I have no idea and have to think about it. Possibly somebody else has an idea for the reason ?

However ... I think it`s a good idea to order the oven-controlled version ... not primary for the oven but for the decoupled 27 MHz oscilator.

Correct,

Perhaps if you take the OXCO route as you have documented characteristics for the Si5351 kit at VHF, it would be a useful half way step to repeat the testing with the OCXO
but with the heater unpowered to purely to observe the improvement brought about by alternative Osc circuit.Before employing the heater option.
Alternatively as others have stated the cheap TCXO's generally have a robust output not to be affected by loading changes.
The FOX has being used in the past, if you search the complete forum thread, another device characterised is :-

https://groups.io/g/QRPLabs/topic/28195580#28671
https://ka7oei.blogspot.com/2018/02/better-frequency-stability-for-qrp-labs.html

The page title is for the ProgRock, but in actual fact the improvements are made solely to the Si5351 module and is applicable to all QRP-Labs kits using the Si5351 kit.
It's smaller than the Fox alternative, but may be worth consideration if you can work with the device?


Alan

73, Henning

https://ka7oei.blogspot.com/2018/02/better-frequency-stability-for-qrp-labs.html

It's smaller than the Fox alternative, but may be worth consideration if

you can work with the device?

A method of mounting and connecting devices like these.

https://groups.io/g/QRPLabs/message/27233

73 Alan G4ZFQ

Thanks Alan,
Missed that one....
Not used on TCXO but in another application I never was a fan of soldering direct to the pads of one of the 'cheap' Si570's in case they tore off under the stress of a 'full sized' component lead and would leave everything floating with a strand from 16/.02 first.
It would make a grown man cry if it was one of the expensive variants....

Cunning & clever, good to see"Traditional" Veroboard is alive an well even in the age of SMD. :-)

Next time the lid is off.......
Alan

I never was a fan of soldering direct to the pads of one of the 'cheap' Si570's in case they tore off under the stress of a 'full sized' component lead and would leave everything floating with a strand from 16/.02 first.
It would make a grown man cry if it was one of the expensive variants....

Alan,

A long time ago I received a couple of "A" grade Si570s after asking for a sample!!
On one of them when using a thick wire to dead bug it I cracked the ceramic substrate around the corner ground pad. I covered it with epoxy glue and it's carried on working ever since..
Apart from that modern components survive a lot of abuse.

73 Alan G4ZFQ

Hi Henning,
as you have tried all the obvious ‘fixes’, I did wonder if what you are seeing is a characteristic of your receiver. I have seen this effect in high speed data receivers due to the audio coupling capacitors charging at the start of a data bit. Can you get an audio output from earlier on in your receiver, say before any audio amplification?

73 Ken g4apb 

Alan,

good save.

Two staples no shack should be without, Veroboard and Araldite.... :-)

A.

Hi Henning,
I received my 3rd u3s today, this one will be 2m only, one bs170 at 9v pa, OCXO fitted with gps module for calibration. 
I should have it built and oven set in about 4-5 days. I'll see what my U3s shows when txing wspr on 2m.
Andy
--
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

"When I put my finger on the crystal I see the frequency jumping for 100Hz or more."

Is this frequency jump from your hot fingers? I'm wondering if it has another source, because your
plot shows a fairly fast change of frequency.
I'm wondering if some local RF fields are re-biasing the oscillator, causing quick drift.
I'd be tempted to try grounding the crystal case. It is currently floating, and can act as an antenna.
And perhaps adding some shielding above/below the Si5351 + crystal. A tin hat & underpants.

It is also possible that local RF fields affect the power supply regulator feeding Si5351. Twist those
Vcc & ground wires together to minimize loop area.