The actual impedances of the calibration standards used are of no importance. All that matters is the resulting accuracy of the calibrated analyzer. Therefore no attempt was made to measure the calibration standards.
The picture on the previous page shows 2 sets of BNC standards I made. Following the HP/Agilent naming convention a BNC-F standard would be a standard that would connect to a BNC-F connector (the standard would be BNC-M). The supplied AIM4170 calibration standards would therefore be type BNC-M and tests were also done on these standards. The home made BNC-F standards were made from BNC-M PCB mount connectors that I sliced off on my lathe. The BNC-M standards were made from some old high quality silver BNC connectors with a removable pin and a screw apart body. Attempts were made to have both sets of home made standards have the same offset plane.
As was previously done with the HP standards a BNC-M to SMA-F adapter was connected to the AIM4170 and the standards were adapted from BNC-M or BNC-F to SMA-M for the test (requiring a larger line extension correction). After doing a custom calibration on the analyzer, I adjusted the line extension till the SMA-F connector gave the highest impedance when nothing was connected to it. The same 2 high SWR loads were used and the data for the HP standards is repeated here again. All the line extension (LE) settings are shown for 50 ohm and VF=1. The reference plane for the my home made standards was not quite as easy to correct for as with the HP calibration standards indicating some small anomolies with my standards. The AIM4170 standards were even harder to correct as the arc on the Smith Chart did not converge to a dot with adjustments in the line extension indicating some anomolies in the accuracy of the standards. However a best guess at the line extension giving the best "open" was made. Since the AIM4170 uses open/short/200 ohm where all the other calibration standards used open/short/50 ohm, I decided to change the 200 ohm resistor in the AIM standard to a 49.9 ohm 1% MF 1/8W resistor. The results are shown on the line "AIM4170 open/short/50 ohm cal" and the results for load #1 are slightly better but the results for load #2 are slightly worse.
Additionally the best line extension fit of -8.5 cm. for the AIM4170 standards did not agree with the physical measurements of the connectors which should have produced a line extension of about -6.2 cm. Attempts to reconcile this discrepancy resulted in me slicing the connectors in half showing an inner pin of .132" in diameter and an inside diameter of the shield of .200" dia. For typical dielectric constants of 2 to 3 this would yield an impedance of about 18 ohms. These are clearly not 50 ohm connectors and when I tried to do a line extension with a cable having an impedance of 25 ohms the results were better. Unfortunately there was also a 50 ohm adapter that needed to be corrected for requiring 2 different line extensions to do the proper correction. Nevertheless the supplied standards are probably good enough for all but the most critical users and the accuracy is great in the HF range and still very good up to 170 MHz.
Load #1 = Four 22 ohm 5% 0603 SMT resistors in parallel + the BNC to SMA adapter.
Load #2 = Four 2.7k ohm 5% 0603 SMT resistors in parallel + the BNC to SMA adapter.
AIM4170 with Load #1 Using:
HP85033D standards for cal. @ 170MHz. Z = 5.87 + j9.122 ohm LE = -0.9 cm. VF = 1
BNC-F home made std. for cal. @ 170MHz. Z = 5.68 + j9.371 ohm LE = -5.3 cm. VF = 1
BNC-M home made std. for cal. @ 170MHz. Z = 5.72 + j10.81 ohm LE = -5.5 cm. VF = 1
AIM4170 standards for cal. @ 170MHz. Z = 5.60 + j15.48 ohm LE = -8.5 cm. VF = 1
AIM4170 open/short/50 ohm cal. @ 170MHz. Z = 5.78 + j15.11 ohm LE = -8.5 cm. VF = 1
AIM4170 with Load #2 Using:
HP85033D standards for cal. @ 170MHz. Z = 73.7 - j204.0 ohm LE = -0.9 cm. VF = 1
BNC-F home made std. for cal. @ 170MHz. Z = 74.5 - j202.8 ohm LE = -5.3 cm. VF = 1
BNC-M home made std. for cal. @ 170MHz. Z = 75.6 - j203.8 ohm LE = -5.5 cm. VF = 1
AIM4170 standards for cal. @ 170MHz. Z = 74.8 - j206.2 ohm LE = -8.5 cm. VF = 1
AIM4170 open/short/50 ohm cal. @ 170MHz. Z = 74.8 - j197.7 ohm LE = -8.5 cm. VF = 1
This concludes the analysis of the AIM4170.
Measurements with Different Calibration Standards
Page content last updated Mar. 12, 2008
Copyright © 2008 Larry Benko, W0QE
