AIM 885 produces internally inconsistent results

AIM 882 produces internally inconsistent results and AIM 882 produces internally inconsistent results – more tests raised some issues with AIM882 software.

AIM885 has been released.

I opened a SCN file received from another party in AIM882 and saved it with a new name, so creating a fresh CSV file.

I then opened the first SCN file in AIM885 and saved it with a new name, so creating a fresh CSV file.

The CSV files were then compared.

Screenshot - 30_03_2015 , 08_34_41

Above is the file comparison in KDIFF3. Note that apart from the first record, five of the nine fields are different in the AIM885 save.

But are they correct?

Lets look at the second record.

Freq(MHz),SWR,Rs,Xs,Zmag,Phase(deg),Rho,ReturnLoss,%ReflectedPower
7.145101,  4.581875,  96.964500,  104.138402,  142.291676,  47.043117,  0.641697,  3.853394,  41.177563

Calculating SWR from Rs,Xs…

Screenshot - 30_03_2015 , 08_35_56

… we get SWR=4.47 yet AIM885 has calculated 4.58. Though it is not far out, it is out by far more than attributable to representational errors in good software. The Rho, ReturnLoss, and %ReflectedPower are also inconsistent with Rs,Xs.

It is a further sign that I cannot trust AIM885 to calculate results consistently.

Accuracy of AIMuhf system – AIM865A vs AIM882 vs AIM885 on a ferrite cored inductor

This article builds on Accuracy of AIMuhf system – AIM865A vs AIM882 on a ferrite cored inductor by adding a measurement using AIM885 of the same test inductor.

 

Inductor1

Above is the test inductor. It comprises 6 turns of 0.5mm PVC insulated wire wound on a BN-43-202 binocular balun core. Continue reading Accuracy of AIMuhf system – AIM865A vs AIM882 vs AIM885 on a ferrite cored inductor

A new impedance calculator for RF inductors on ferrite cores

Some time ago I published a calculator for estimating the impedance of RF inductors on toroidal ferrite cores (Calculate ferrite cored inductor).

Screenshot - 23_02_2015 , 08_09_25

The calculator (input form above) use the core dimensions and complex permeability as the basis for calculation.

There are some popular cores that are not simple toroids and so not suitable for direct use with that calculator. For these cores, a practical method is to measure the inductance constant Al (inductance of 1 turn in nH) at low frequency (where µ=µi), and using the µ’,µ” characteristic from the datasheets, to calculate the impedance at the desired frequency. Note that µ’,µ” are usually frequency dependent for ferrite materials. Continue reading A new impedance calculator for RF inductors on ferrite cores

Speaker wire is so popular as an RF transmission line

I received a sample of speaker wire from a correspondent who asked me to characterise it.

Even if I had the time, a 50mm sample isn’t sufficient to characterise in a meaningful way… but let’s have an abbreviated look which will highlight the pitfalls of this stuff.

SpeakerMicrometer

First thing to do is measure the conductors, stranding and diameter. There are 14 strands and several measurements fall just below 0.15mm diameter. This is probably nominal 0.15mm with new drawing dies which are a little undersize. Continue reading Speaker wire is so popular as an RF transmission line

Accuracy of AIMuhf system – AIM865A vs AIM882 on a ferrite cored inductor

In other posts, I have commented on the apparent inconsistency of AIMuhf measurements.

One of the devices I often wish to measure is a ferrite cored choke such as those used for a Guanella 1:1 balun. A small test inductor was made to provide a common device for measurement across instruments, and versions of software, though small, it has similar characteristics to the larger inductors more commonly measured.

Inductor1

Above is the test inductor. It comprises 6 turns of 0.5mm PVC insulated wire wound on a BN-43-202 binocular balun core.

Clip 124

Above is a plot of predicted inductor impedance. The model is calibrated for assumed self resonance around 15.5MHz based on measurement of the inductor using the latest production version of AIM software, AIM882.

Note that the curve is smoot, R rises to a peak at the self resonant frequency, and X increases linearly at low frequencies until the effect of equivalent shunt stray capacitance turns the curve over and it passes through zero. Measurement with other instruments show this to be a quite good model for this type of inductor over this frequency range. Note though that tolerances on ferrite cores is quite wide and for #43 mix, variation of 20% in measured R and X is not out of the question.

You can calculate Z at other frequencies using Calculate ferrite cored inductor (from Al).

Screenshot - 25_03_2015 , 10_29_18

Above is an example calculation at 30MHz. (Al was determined by measurement of inductance with an RLC meter at 10kHz to be 90µH with 6t, so Al=90000/6^2=2500.)

AIM865A

The instrument was calibrated using a standard cal, and the test fixture was a 920mm RG58 cable with SMA connectors, and a N-M to SMA-F adapter on the AIMuhf.

Screenshot - 24_03_2015 , 19_47_11

Above, measurement with AIM865A. R is higher than predicted, but not excessive. X is pretty much in line with prediction.

AIM882

The instrument was calibrated using a standard cal, and the test fixture was a 920mm RG58 cable with SMA connectors, and a N-M to SMA-F adapter on the AIMuhf.

Screenshot - 24_03_2015 , 19_47_44

Above, measurement with AIM882. R is a little higher than predicted, but not excessive. X is pretty much in line with prediction.

Comparison

Screenshot - 24_03_2015 , 19_57_33

Above is the AIM882 plot with the AIM865A plot overlayed. Though the colours are the same, the AIM865A curves are readily identified by higher R and higher self resonant frequency.

For AIM882, R is higher, and the resonant frequency is significantly lower than indicated by the AIM865A measurements which is difficult to explain as the system was calibrated with the same test fixture, same OSL devices, and within minutes of each other.

The other notable feature of the AIM865A plot is the ripple on R, it is very apparent below 15MHz and that is not a feature of the DUT, it is a result of the AIM system.

Release notes do not make any disclosures that might explain the apparent difference in the two plots. They cannot both be correct, one or both are wrong!

It does appear that changes were quietly made to AIM882 which may have improved its accuracy, it is certainly different to AIM865A.

It is concerning that the author does not fully disclose problems that were identified and the impact that may have made on historical measurements.

Clearly the lesson to be taken from this is that the AIM system is unreliable, the author:

  • does not fully acknowledge defects;
  • does not detail how all defects may have tainted historical measurements; and
  • does not give information to assist in salvaging good data from tainted historical measurements where that is possible.

For those reasons, all historical measurements must be regarded as having very limited value and future use is against the background that the measurements might later be show to be worthless.

Slot-it CA20A rework

The Slot.it CA20A is a sidewinder configuration with the Mabucci style MF08 motor with extended shaft at both ends. The shaft ends extend out to about the mid point on the rear tyres with about 1.5mm clearance to the tyre.

The supplied PT32 tyres are quite elastic, and fly off the rim at high speed, making contact with the extended motor shaft and gouge the tyre, not to mention slowing the car. Continue reading Slot-it CA20A rework

Hobbyking Swamp Dawg build – #1

SwampDawg01

I couldn’t resist the Hobbyking Swap Dawg as an opportunity to play with SimonK ESC firmware with forward / reverse enabled.

ESC selection and implementation

I selected a Hobbyking F-30A ESC, which might seem a bit of overkill but it is a closed air space which reduces its dissipation capacity. The F-30A have proven themselves reliable with SimonK on a range of BLDC motors. Continue reading Hobbyking Swamp Dawg build – #1

SimonK on Hobbyking 40A ESC 4A UBEC 9261000003

ESC02

Following on from BLHeli on Hobbyking 40A ESC 4A UBEC 9261000003 – #3, further tests were conducted on the ‘chugging’ ESC.

The 9261000003 is a relatively low cost ESC with 6S rating and fast FETs.

At Simon’s suggestion, the BEMF caps were removed. This eliminated the chugging effect on both the DT750 and 4822-690KV where it had been previously observed. The 4700pF BEMF caps are in a vertical row at the bottom right of the pic above.

I used to routinely remove BEMF caps (a carry over from using WiiEsc), but found on some tests that it made insignificant difference. That might be the case for some motors and smallish caps (1000pF), but in this case, the combination of these challenging motors and largish BEMF caps were incompatible and removing the caps solved the problem. Continue reading SimonK on Hobbyking 40A ESC 4A UBEC 9261000003