01 October 2014

Checking the calibration of my squarewave & sinewave calibrators and AD8307 rf power meter

At our recent QRP club show-n-tell activity I was fortunate to be able to check the calibration of my home brew instruments. For this exercise an HP Spectrum Analyzer (model unknown) belonging to Gary, N3GO, was used to check my 10MHz, -10dBm squarewave calibrator and my  11MHz -10dBm, sinewave calibrator (see a previous post for details of these two units).

 I have also been lent (on a longterm loan by Chris KD4PBJ) an HP Signal generator HP8657B. Herewith are the results of the calibration check of my homebrew AD8307 rf power meter.

As can be seen below. The results are not consistent.

10MHz, -10dBm squarewave calibrator

Fundamental -12.56dBm  (54.9uW)
Third             -23.05dBm  (5uW)
Fifth              -27.76dBm  (1.68uW)
Second          -39.9dBm
Fourth           -40.13dBm

Total =  61.78uW  This is equivalent to -12.09dBm

Thus this shows a reading that is 2.09dB lower than the nominal 'calibrated' value of -10dBm. The -10dBm level was calibrated using a DC calibration technique.

Next we measured the sinewave generator

11MHz, -10dBm sinewave calibrator

Fundamental -8.77dBm
Second          -39.69dBm
Third             -48.53dBm
Fourth           -58.34dBm

Thus this shows a reading that is 1.23dB higher that the nominal 'calibrated' value of -10dBm.

The tables below show measurement comparisons between the power meter and the scope readings. This result shows good correlation. It is reasonable to assume that the HP8657B signal generator is accurate to tolerances significantly in excess of the power meter or the scope. This shows a maximum deviation of 0.53dB. This is much better than the comparisons above!

Next I will ask Gary if we can run the tests once again. Check the HP8657B against Gary's Spectrum Analyzer.

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