Monday, November 26, 2012

Pescada de rabo na boca...

This Portuguese expression it's equivalent to a "no win situation" in English, as the image states. By the way, it's also a traditional dish!

This relates to the AD8307 power meter, without calibration it's "mostly" useless, well you measure something in this case power but you will never know how close or how far you are from a real value...or you blindly trust the datasheet.

Most people that build this circuit tend to calibrate against a know source, that is also my idea in the mean time for simple tests and for relative power it works, sort of, let's exemplify:

An unknown power source to the input:

 Now let's attenuate by 10dB the same source:

Let's do some math:

(Supposedly the power meter should give me 20mv/dB)

So a 186mV drop translates to: 9.3dB

I had measured my 10db pad as 10db at 28Mhz and 9.7 at 70Mhz on the MFJ...again not calibrated. Also my signal source is full of harmonics (noise generator)...

So I have a pad that should be somewhere near 10db, more or less 1 db, no big deal?! Could be the difference between 100 or 125 Watts...

Should I assume the pad is correct and calibrate the power meter to the pad?

...I could measure the attenuator pad on DC against my voltage reference, but the voltage reference it's not calibrated and would only give me the DC behavior......OK...back to the beginning......I will calibrate one of this days against a know calibrated signal generator.

Have fun and measure, doesn't matter if the voyage is 100 or long as you make it!

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