Rubidium Lab Frequency Standard - Design Pt 1

Due to the bank holiday I finally had a bit of spare time today so I decided to try and make a start designing my Lab Frequency Standard.  I say start but in truth I have been researching this for some time.  I even ordered many of the parts so I had everything I would need to build a prototype.

Feature List

Whilst I was tempted to add all sorts of features such as a LCD display I decided to keep it simple. Below is a list of the feature I felt were essential.

1. For it too be mains powered so not to tie up my bench PSU.
2. A solid enclosure which is available in various sizes so I could make other devices to match.
3. A combined power and status LED.
4. Both 10Mhz and a 1Hz outputs.
5. Independently buffered outputs.

A 24v power supply and the enclosure were easy to find and order online but I opted to prototype the the other parts of the system before designing a PCB.

Prototyping the Status Circuit

I decided to start with the status LED circuit, I had originally thought of using a micro controller but in the end I went with a simple transistor based circuit. After a bit of searching I found a project of which part of the design was just what I needed. Luckily I had all the parts so I knocked up a quick prototype and it worked perfectly.

Prototyping the Frequency Divider

I had originally intended to use a dedicated frequency divider chip to get the 1Hz output but they were a little on the pricey side of things.  After a bit of a search I came across TVB's site where he talks about using PIC microcontrollers to do the same job at a fraction of the price.  You can even order a pre programed PIC from him,  I decided however to opt for programming my own.  This is made much easier by the fact he has published all the HEX on his site.

The only issue currently is I cannot find my old programmer and I have not had much luck getting a cheap one I purchased on eBay to work although I think that might be a windows 8 issue.  Once I have overcome this issue I will get this aspect prototyped as well.

Rubidium Frequency Standard - Testing

Now I have this thing I guess I better check it works.  Luckily it had  pin out on the side and the data sheet was easily found on the internet so setting it up was pretty straight forward. According to the data sheet it needed a supply of between +19v and +32v (+24Vdc nominal) capable of 1.7Amps.  For the test I used my bench supply, the positive connects to pin ten and the return to pin eight.  You also need a way of knowing when the standard has locked, this done by measuring the voltage of pin 6, I opted for a multimeter but you could just as easily used another channel on your scope.  Finally you need a method of measuring the out put, I used my TTi TF830 frequency counter and my Rigol DS1054Z Oscilloscope.  These were connected to pin 1.

So time for the moment of truth, I powered up the standard, and it drew about 1.5 Amps for the first two or three minutes then it dropped down to 0.47 Amps once it had reached temperature.  A few moments later the voltage on the lock pin (its labeled BITE on the pin out) dropped from 4.7V to just above 0V indicating it had locked. During it reaching lock the the counter had been varying its readout by a few tens of hertz once it reach lock it kept flipping between 9.9999999 and 10Mhz.

After a further ten minutes it displayed 10Mhz continually.  I am inclined to believe that this final 1hz variance was down to the counter having not yet reached temperature as it had only just been switched on rather than any change in the output of the standard.

It is also worth noting that neither the scope or my counter have an oscillator any ware near as consistent at the standard so I am taking there readings with a pinch of salt although things look good.  All in all I think its safe to say the Frequency Standard works.

Next stage will be to build an enclosure and power supply, I may also add a couple of buffers and maybe a divider to get a 1hz pulse as well.

Rubidium Frequency Standard - The basics

Today the rubidium frequency reference I purchased on eBay arrived from Australia after only eight day which was surprisingly quick. In fact so quick I have not got round to sorting out the other bits and pieces I need to make my lab frequency standard.  In the mean time here is a brief description of what it is and why I wanted need one.

What is it?

The device in question is a Datum LPRO-101 10Mhz Frequency Standard, the manual for which can be found on-line pretty easily for those of you who want the detail but in a nut shell it outputs a very stable 10Mhz signal . Note this is not a primary reference like a caesium atomic clock or a hydrogen maser, but a secondary reference, i.e. it needs to be calibrated but once it is it holds its frequency with a very high degree of stability. As always there is lots more information on the internet, for a basic overview I would start with Wikipedia.

Why do I need one?

Well to be honest I probably don't need one but it does have lots of uses.  One of the main reasons I want one is to improve the accuracy of some of my test gear, in particular my universal counter which does not have an oven-controlled crystal oscillator but which can take a 10Mhz external standard input, this would increase its accuracy significantly.  I also fancy building a very accurate stand alone NTP server, but I guess the main reason is so I can say I have my own atomic clock.

Ok I'm sold how can I get one?

Rubidium frequency reference's can be purchased brand new for around £1700 pound which is way out of my budget, they do however come up quite frequently on ebay, these are normally second hand units removed from mobile phone base stations or SDH equipment.  Either way they tend to go for between £100 and about £150 pounds.

Once I have all the parts I will build the lab standard, this will really just consist of a PSU, lock monitoring circuit and maybe a divider to get other frequencies, I will most likely be publishing how I built it as an Instructable.

Wythall Radio Rally

Today I spent a very enjoyable morning mooching around Wythall Radio Rally with a couple friend from fizzPOP, I wish i had know about this event before, this was apparently its 30th year so there really is no excuses.  I will however look on the bright side and think of how much money I have saved.  Having said that I managed to limit my spending, although it was a close run thing and I nearly ended up being a lot poorer as I spotted a HP bench multimeter I really liked the look of, I decided to have a think and when I went back it was gone.  I ended up coming away with just a TTi 1.3Ghz frequency counter for just £30 (something I have been looking for on ebay for some time).

I also ended up purchasing  23 25v 22000uf capacitors, he wanted £1 each but I managed to talk him down to £15 for the lot.  Whilst they are a little lower capacitance than I had been looking for the will hopefully be suitable for my battery tab welder project which I will be starting as soon as the SCR I have ordered on eBay arrives.