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Post by resonant on Apr 12, 2014 17:19:50 GMT -5
Hello, I have purchased a used AIM 120 and it seems like a fine device. After calibrating, when comparing the frequency and return loss readings on a fixed antenna, with an HP 8753 E and Rhode Schwarz ZVL-3 network analyzers they do not track. Is there a fine tune for the frequency display. Also, is Z Mag the impedance reading? and what are the series/parallel circuit displays for?
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Post by Bob on Apr 13, 2014 23:04:01 GMT -5
The AIM4170 frequency is typically within +/-30 PPM ( 30 Hz per MHz) without any calibration. It can be fine tuned if an accurate frequency counter is available. Connect the AIM output to the freq counter input and set the AIM function to Constant Freq. The specified frequency can be any convenient number like 5 MHz or 10 MHz.
Adjust the AIM freq until the reading on the freq counter is exactly what you expect. Then click the AIM Cal button on the constant freq dialog window. This correction will be saved and applied to all subsequent freq values.
The AIM only needs to be calibrated at one freq since all the programmed valued are directly proportional to the calibrated value. That is, there is no offset or non-linearity to worry about.
Zmag is the magnitude of the impedance. Zmag = SquareRoot[(Resistive*Resistive) + (Reactive*Reactive)]
On the right side of the graph:
Rs = Resistive part of the series circuit
Xs = Reactive part of the series circuit
Then:
Zmag = Rs + j*Xs
Phase = ArcTangent(Xs/Rs)
Rp and Xp are the corresponding components in a parallel equivalent circuit.
Rp and Xp are related to Rs and Xs by mathematical formulas, the two sets of measurements are equivalent but sometimes one representation is more meaningful.
For example, when measuring a coil and it's associated resistance, the value Z= Rs + j*Xs makes it easier to visualize the circuit values for the coil and its series resistance.
When measuring a resistor and it's associated capacitance, the values of Rp and Xp make it easier to visualize a resistor with a parallel capacitance.
When measuring an antenna, the freqs at which the phase crosses through zero depends more on the accuracy of the instrument calibration with the short, open and standard load than it does on the actual programmed frequency. The programmed freq usually has negligible error when testing an antenna.
The cal loads are very critical and the overall instrument calibration is sensitive to a lot of factors, such as connectors, adapters and interconnecting cables. When comparing different instruments, be sure the calibration is done in a consistent manner. This is not easy to do and it is often difficult to rationalize the data on different instruments.
-73/Bob
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Post by resonant on Apr 14, 2014 6:00:20 GMT -5
Bob,
Thanks, that helps a lot.
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Post by smokchsr on May 4, 2014 0:02:11 GMT -5
One thing to do if you didn't already is to use the same calibration loads for all equipment. When I need very accurate results from the AIM120 I'll use an Anritsu OSL to calibrate to. Once I do that the AIM tracks the Anritsu very closely.
Also try doing both custom and standard calibrations. I haven't quite put my finger on the exact weirdness yet, but sometimes it seems my "high precision" custom calibrations are just wacky, while a "standard" calibration with the same loads and cables will look just fine.
Also as far as the frequency, I've checked my AIM and found it to be within 1Hz in the AM band (that's not to say they all are). I haven't had a reason to look at freq error up in the VHF region yet, guess I should have when I had it connected to the monitor.
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