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Saturday, 17 November 2012

The Art of Calibration

Well,

Been thinking some more about power meters and the calibration of them. Quite tricky really, as you need an known RF source to calibrate a meter against it, but to have an accurate and known RF source, you need a calibrated meter to measure it. Recursion: see recursion.

The first thing I have done is to build a power meter that was published in QST, it's designed by Roger Hayward, KA7EXM and is PIC based. This is a stand alone meter for simply measuring from about -80dBm to +7dBm:


Then to calibrate this baby the text says... "assume for a moment that you have an accurate signal generator capable of -70dBm to +7dBm"..... well I don't!

So, a newly found homebrew chum in VK land pointed me at an article by Bob Kopski, K3NHI where he presents a simple calibration source using a CMOS clock oscillator. It kind of looks like this:

So the idea here is that you set the variable resistor such that a digital volt meter connected to the test point reads exactly 158mV. Then in the example above the output goes through a attenuator pad such that the output port is a known -20dBm signal at 10MHz. As I need at least two reference points to calibrate anything (because of the interpolation needed in software) I figured I could use a different pad at the output to create another one of these at, say, -30dBm or even -50dBm. Here's one of them on the bench under test:


Creating a pad is quite simple and there are a number of calculators on the Internet, here's the one I used:

http://chemandy.com/calculators/pi-attenuator-calculator.htm

I'm one BNC socket short of the party right now, so I'll post more to let you know how I get on, but here's one of them in a box ready for use:

Fun, egh?

2 comments:

  1. Hello G0MGX,

    I have tried to create the RF Calibrator using the 10 Mhz CXO, But I am not able to bring down the voltage at the test point to the specified 158mV. Seems like my choice of CXO is creating problems. Voltage level at the CXO output is 2.5 v and the Trimmer Pot is not helping to reduce the testpoint. Is it necessary to use the CMOS version of the clock oscillator ?

    Vinod ES

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  2. Hi, Vinod

    Searching through the site, I saw your message now, I do not know if you've solved the problem, but in any case there is an improper connection in the schematic diagram, the test point must be connected after the variable resistor, ie the right side of it, while in drawing up, the same is on the left side, which does not allow any adjustment. OK.

    Renan, PP7HP

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