Monday, 4 May 2015

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.

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