2023. április 9., vasárnap

Rubidium frequency standard 1.

It took nine years, to pull this project out from the drawer:


When I tested my HP 53131A and HP 53181A (already sold) frequency counters (also checked my oscilloscope and function generator), I realized, that all of the equipment a bit off in frequency (even the HP with its HS Oven).

I need some lab standard. As my lab is in a basement of a big building and the only access for the open sky is on the street side, where I can't put antenna, so I think the GPSDO is out of question. The other option would be a DCF-77 based solution (maybe based on this: https://www.qsl.net/dl4yhf/dcf77_osc/index.html), but it would take a while to experiment

So the best thing I can use is the FE-5680A rubidium standard, what I bought at the $50 era (now you can get it around $200).

I bought a Geppetto Electronics breakout board for it https://www.tindie.com/products/nsayer/fe-5680a-breakout-board/. Before try with it, I looked around and realized, that some of the FE-5680A modules on sale has internal 5V supply, and supplying it externally, may harmful for the unit.

So I just pick a DB-9 screw breakout for the first test.

It started without additional supply. I was able to get the lock signal, but nothing else.
I was able to identify:
Pin 1 - 15V
Pin 2 - GND
Pin 3 - Lock indicator
Pin 5 - GND
But nothing else. Most probably the narrow 1pps signal is present on the Pin 6, but wasn't able to get it for the first try (later, I'll retry it)
Looked around many sources. It look like it is an older model.
Found a few links. Somewhere in the documents were some information about Mathias Bopp's pdf file named: precise reference frequency rev 1_1.pdf
Actually I have the link, just the file is not downloadable from there. Here come handy, that I'm in the IT for a long while. Some digging in the Internet Archive resolved this.
It looks like, this document contains the most useful information about my specific unit. Probably other people will need it, so I republishing it here: precise reference frequency rev 1_1.pdf
About this specific unit:
The Good:
It has the DDS board inside. Theoretically it can be programmed to any frequencies from 10Hz to 20MHz.
The Bad:
The RF signal is not present on the DB-9 connector, so modification needed. In addition the phase noise (jitter) of the DDS not as good as a VCO controlled OCXO could be. It could cause problems at high frequency usage.
The Ugly:
It is a facepalm situation when I first seen how some of the ham operators killed the other ways proper RF connector on the DDS board.

One of my parts box I found an IPX - SMA pigtail cable. Tried it, and the IPX connector perfectly fit into the RF connector on the DDS board - Win 1!

As I'm still not an RF guy, don't have RF converters, cables, etc.
Found locally SMA - BNC adapters to run my first measurements with frequency counter and oscilloscope. I was able to pick it in one day. - Win 2!

Ok, now connect everything together and measure.
Trying to screw the SMA adapter to the pigtail cable. Doesn't fit. What?
Then realized. Wasn't careful enough. I just took a quick look to the pigtail cables I have. Now checked more thoroughly. It is RPSMA and not SMA.  Ouch.

Anyway. Measure, to get some results:

Wow! It looks like my frequency counter is spot on. Showing 0.75Hz difference between the rubidium standard and the OCXO in the counter (from this, I can see that the standard is set to 2^23 Hz instead of the 10MHz) - Yes, probably I can get better results, when this unit is boxed, with the proper connectors and left switched on for a while.
Let see, how it looks like on my Rigol oscilloscope.

It show 8.4210MHz. I definitely not happy with that. I'll try to figure out, how can I adjust it a bit.

I stand here with this unit.
What is next:
  • Figure out the RX/TX connection and set it to 10MHz
  • Add a PSU and box the unit.
  • Do some performance measurements (actually I'll need an SA, what I don't have right now)
Further plans/ideas (maybe or maybe not):
  • Lock an OCXO to the 1PPS signal to enhance the phase noise performance, if necessary
  • Create a user interface (MCU, probably VFD display, rotary encoder), to be able to change the output frequency. This most probably need the modifications described in the pdf above to get a better performance from the output filter.
To be continued...