AIM 882 produces internally inconsistent results – more tests


AIM 882 produces internally inconsistent results raised some issues with AIM software.

This article documents some further tests.

A simple test referenced to 50Ω

Load is 25+j0Ω at the end of 1.14m of RG58C/U, instrument calibrated at its connector.

AIMuhf is connected and powered up, and a scan made of the test load.

Screenshot - 24_01_2015 , 07_28_59There are several things about this display that raise questions, but lets look at the glitch in SWR (Zref=50) at the cursor (blue vertical line). It is suspicious for a start because there is no corresponding large glitch in the Return Loss curve below, and since they are related there is an obvious inconsistency… one or both are wrong.

Let us calculate SWR(50) for the impedance shown in the right hand side.

Screenshot - 24_01_2015 , 16_53_29

SWR is 2.03, 50% higher than shown on the AIM display.

Return Loss is 9.36dB, 43% lower than shown on the AIM display.

What might we expect for the scenario. Lets solve the scenario using TLLC.

Screenshot - 24_01_2015 , 17_03_01

TLLC suggests Z=89.30+j23.50, the measured Rs,Xs shown by AIMuhf is believable but the calculated SWR(50) and Return Loss have large errors, and the related curves are also flawed.

Changing Zref

Zref was changed to demonstrate another failure of the AIM software.

Screenshot - 24_01_2015 , 07_14_50

Above is the AIM display of the same measurement with Zref changed to 101.7Ω which should result in a near perfect match (SWR=1 and very high Return Loss) where Xs passes through zero

Good, Return Loss is very high at the cursor (remember that it is plotted upside down, Return Loss increases downwards) (though the right had side shows Return Loss as just 11.74dB, a huge error).

But SWR is 1.698, it obviously has not updated from the previous display and it is not even correct as SWR(50)

Screenshot - 24_01_2015 , 17_13_39

Above, SWR(50) of the cursor impedance 101.815-j0.043Ω is 2.04 (20% higher than shown in AIM) and Return Loss 9.34dB (20% lower than shown in AIM).

Screenshot - 24_01_2015 , 17_11_39Above, the values that SHOULD be displayed for Zref=101.7 SWR(101.7) of the cursor impedance 101.815-j0.043Ω.

Making a new measurement with Zref=101.7Ω

Screenshot - 24_01_2015 , 17_19_00Now the display seems to reconcile, SWR and Return Loss look believable but the glitch has gone, and frequency of resonance has moved 500kHz higher than previously reported… very hard to believe.

This does not encourage confidence in AIMuhf.

This version has been out there for almost three months with these issues, and some or all of the issues may have pre-dated that.


  • AIM 882 software produces internally inconsistent results and is not to be relied upon.