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Friday, 1 November 2013

The art of Calibration (Once More)


After playing some with frequency counters in software, I thought back to the frequency standard that I have from back here:

I thought it worth a quick check that it's still (as near as damn it) on frequency. It's an oven controlled oscillator which means that the actual gubbins that creates the signal is inside a small metal box that heats up to a fixed temperature; thus eliminating temperature related drift.

There is a voltage adjust pin on the OCXO giving you +/- a few Hz. How well I can actually calibrate it remains to be seen, but for the purpose of this exercise being fairly close is good enough. When I made the original box I included a multi turn trimmer to alter the voltage on this pin and calibrated it against the radio I had here at the time - an FT950.

I have to assume that the Kenwood radio I got here:

has been accurately aligned, it also contains a 1ppm (part per million) local oscillator - so it should be a good reference for me.

So, tuning the radio to my 10MHz reference signal with a grounded antenna connection I see this:

The spectrum analyser sees this:

Both pictures above are at the same Spectrum Analyser configuration, however the information on the screen differs. You can see the centre frequency in the top image and the bandwidth in the lower one. Each horizontal square is ten Hz, so it looks like I am very close according to this instrument.

I am going to make some kind of GPS disciplined frequency counter at some point, it might also be good to make a GPS disciplined voltage adjustment for the OCXO like this one:

I'm going to order the PCB for that project and see how I get on, I've also found what I think will be a suitable GPS receiver with a 1pps (pulse per second) output on eBay - that's winging it's way from China.

However, there's always the problem that a man with two watches never knows the correct time....

Interesting, huh?

1 comment:

  1. These pieces of information really help readers to understand better on the art of calibration. Calibration is indeed very important. This is a process in which tools and equipments are set or made sure to be of standard quality. Thanks for sharing this very informative post.