AIM4170 measurement vs. Spectrum Analyzer

[k0to]

Yesterday I measured the voltage drop across two impedances in series using (a) HP 8640b sig generator and HP 8405 vector voltmeter and (b) HP141T-HP8553B +tracking generator to measure the voltages [expressed in dBm]. One of the two impedances was a 50 ohm standard in a BNC. The other was a ferrite core [type 31] RF choke fabricated from ferrite cores and RG400U coax.

The impedance of the choke was more than 5000 ohms at 1.8 MHz the frequency of interest. The self resonant frequency of the choke was 2.4 MHz. The dB attenuation [voltage drop] across the choke impedance was the same for both (a) and (b). It was about -42 dBm +/- 0.5 dBm. I used small, 1/4 resistors to check the readings and the values for the resistors were the same on each set of instruments and as expected using classic voltage divider math. A 6800 ohm resistor provided -42.5 dB of attenuation.

I next used the AIM4170 to directly measure the impedance of the choke. I used the fixture shown in the manual and scanned from 1.8 to 2.0 MHz. The impedance curve was considerably lower than 5000 ohms; closer to 2500 ohms. I made several measurements using the AIM4170 [845 software] with essentially the same result. I then measured the 6800 ohm resistor in the fixture and the result was a straight line from sid-to-side at about 6800 ohms. It appears that measuring a reactive load produces very low readings while measuring a resistive load produces expected readings

Can anyone suggest why there would be such a difference between the AIM4170 measurement of the choke and the two voltage divider measurements of the choke.

Tod, K0TO

[Bob]

Hello Tod,

Inductors are fairly complex devices with a combination of inductance, capacitance and resistance. Usually at low frequencies, the inductive parameter dominates and the component looks approximately like an ideal inductor with some series resistance.

All inductors have capacitance associated with them and eventually you reach a frequency where the device acts like a parallel resonant circuit. A little below this frequency, the inductive reactance increases rapidly. The value of Zmag will be sensitive to the stray capacitance in the test fixture, so different instruments may give significantly different readings near the series resonant frequency.

An advantage of the AIM and VNA is they give a graphical picture of the impedance over a wide frequency range so you can locate the points where the reactance changes between inductive and capacitive.

You can check the accuracy of all your instruments by using inductors with close tolerances and measuring well below the series resonant frequency.

This ap note shows how the AIM data agrees closely with theoretical data.
w5big.com/Q-Measurement.pdf

--73/Bob