Re: U3S with TCXO #u3s

Hans Summers

Hello Mont, all

A reminder that curing drift on an Ultimate3S is possible, at least to a large extent, by other methods than a TCXO - and that this was described in an age old App Note I wrote back in the mists of time, AN001 at 
Since your assembly doc specifically mentions this TCXO, does it mean that you "specially tested and selected by me, to make sure that it uses analog (amooth) frequency correction. No jumps."  ?? 
I am referring to the TCXO used in the QCX+/QCX-mini module - this is the one carefully tested.  
Yikes...   I know that here you have been referring to the QCXPTCXO, but that's not clear in the Synth docs.

Yes, the problem is the synth docs were written some years ago, and predate the QCXPTCXO which only came on the scene in mid 2020. When there are hundreds of documents it is hard to keep everything straight between them.  
Accordingly I really should remove any mention of the FOX924 from the Si5351A Synth kit manual and will do so soon.

How about replacing that text with the Manufacturer and Model # of the TCXO you have tested and use on the QCXPTCXO board?

It isn't something that is available from a components distributor. The procurement of these was via a tortuous path which is far outside the scope of this post... 

The only way for people to get these components would be if I sell just the component alone on the QRP Labs shop; but I am reluctant to become a parts distributor, let alone a parts distributor of one-off SMD parts. I already have enough problems being a kit producer! 

So therefore the best way is to use the QRP Labs TCXO module, which was extensively tested and characterized and found to be a very suitable fit for QRP Labs kits. The cost is also low particularly considering no SMD soldering is needed and that it is a known, tested item, specifically in regard to this "stepping" nature of some TCXOs, which is a performance characteristic of great importance to low bandwidth weak signal mode operations like WSPR, but is not something available in any datasheets. 

For reference, attached are some of the charts from my testing. The derived specification from my testing:

Accuracy at 25C: 25,000,000 Hz +/- 5Hz
Output: 1.05V peak-peak
Temperature stability: +/- 0.25ppm over 4-65C tested
Supply voltage: 1.6 - 3.3V
Current consumption: 1.25mA

It doesn't constitute $million laboratory testing but, I did put some reasonable time and effort into it and believe the results justify calling it a 0.25ppm TCXO and suitable for our purposes (no sudden frequency stepping during a 10's of C temperature test). 

73 Hans G0UPL

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