OCXO drift with U3S #u3s #ocxo


Sverre Holm
 

I just assembled an OCXO and have had it running for a few days with the U3S at 18 and 14 MHz. The U3S is in the QRPLabs enclosure. It turns out that I need to have the lid off in order to reduce drift to 0. Otherwise I will get drift in the -1,-2,-3 range. This seems to indicate a heating problem, could it be that it gets too hot with the lid on?

And how should I interpret the drift? Does it mean that the 27 MHz reference is drifting down in frequency during the transmission with about 27/14 times the reported drift? Could it be that I haven't found the proper sweet spot for the setting of the OCXO?

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Sverre

 

LA3ZA, http://la3za.blogspot.com


M0RON
 

Hi Sverre, 
I have built 4 of the OCXO, how did you set the OCXO up? Mine are all inside a qrp-labs.com case with no issues. 
I used Argo software and a FT 817ND as receiver with a lead from spk out on the 817 to mic in on laptop, set u3s to tx fskcw as stated in manual and watch the signal frequency in Argo drop then stabilise. At some point it will rise, this is too far,back off a touch and you will be fine. My OCXO u3s don't drift although sometimes a spot may say -1.
Good luck 
Andy
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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.


Sverre Holm
 

Hi Andy,
Thanks for the reply. Yes, I set up just like you describe. But I am still afraid that I didn't quite hit the sweet spot because of the drift I am seeing. 

The fact that I am seeing negative values for drift, seems to indicate that my 27 MHz clock is not at the minimum frequency. And since the setup for the OCXO specifically looks for the minimum, I am afraid that I am not at that sweet spot. But before I tear it apart to redo the calibration, I thought I would look for some confirmation of that and other advice as well.

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Sverre

 

LA3ZA, http://la3za.blogspot.com


John Canfield <bucket@...>
 

When I was using the plain old Si5351A synthesiser module (no GPS correction) I was getting drift reports of 0 or 1 - it was very stable. My U3S is in the aluminum case but I have a LM7805 voltage regulator on a little board with a large heat sink (converting 13.8V to 5V) which is probably keeping the heat very constant in the case.
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John, WB5THT


M0RON
 

One other thing, are you using the gps to calibrate the u3s? The OCXO oscillates at a lower frequency, maybe the OCXO is set correctly and it's working it's way to its settled point.
I use a big calibration step and 240 seconds on first start up after the Argo setting up then once it's settled down to around 26.998 mhz or so it rarely needs calibration and after around 20 mins warm up time rarely shows a drift on wsprnet. 
Hope you sort it soon,
Andy
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The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.


Sverre Holm
 

I'm afraid I've tried this. The calibration is now set to Step 01, Time 060, but the step size was larger initially. The frequency has more or less stabilized to 26,998,733 which should be about right.


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Sverre

 

LA3ZA, http://la3za.blogspot.com


Sverre Holm
 

The solution to the problem I had of drift with the OCXO was much simpler than I had thought: To my embarrassment connecting the heater to + 5 Volts made all the difference!

It turned out that I had done the OCXO initial adjustment using my old U3 which had the heater connected to the Vcc. When I moved it to my newer U3S I got drift. But it was all resolved when the heater was connected.


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Sverre

LA3ZA, http://la3za.blogspot.com