Impedance Measurements

How does an Impedance Measurement differ from other measurements?

Terminals

Impedance measurements are typically, but not always, made on fundamental electrical components like resistors, capacitors, inductors, crystals, etc. that do not have coaxial connectors.  Devices like amplifiers that have coaxial connectors are typically designed to have an impedance that matches the connector impedance like 50 or 75 Ohms and can be tested using a network analyzer with high accuracy.

How you connect a DUT that does not have any connectors is important.  Whatever fixture you use is going to have it's own parasitic capacitance and inductance and if it is a single coax type will have some port extension electrical length.

The HP family of LCR meters that use the four BNC(f) connectors have a large number of test fixtures available.  Also the 43961A uses a standardized APC-7 type of test fixture.

For lower frequencies the test fixture is less critical and simple test leads can be used.  When using a swept impedance measurement, like the 4395, it's easy to see the effect of lead inductance as the frequency goes up, and set the stop frequency below this point.

Magnitude of Impedance

Components without connectors may have impedance values ranging from milli Ohms to hundreds of meg Ohms that are a function of frequency.  Coax and twisted pair impedances range from about 50 Ohms for common coax to 600 Ohms for open wire phone lines.

Impedance of Transmission Lines

Accuracy of Impedance Measurements

Different measurement methods give results with different accuracies.  One way to look at this is to plot impedance on the Y-axis and frequency on the X-axis of a graph and draw accuracy profiles.  See Agilent publication 8 Hints for Successful Impedance Measurements, App Note 346-4, pub # 5968-1947E, page 2, Figure 0-1 Accuracy Profile.

You can see that a network analyzer using either a reflection (bridge) or transmission impedance measurement method has good results for impedances in the 5 to 500 Ohm range over the widest frequency range.  The Auto-Balancing Bridge instrument has the broadest impedance magnitude coverage with 1 milli Ohm to 100 Meg Ohm impedance range, but with a frequency range limited to 20 Hz to 110 MHz.  TheV/I (V over I) method is close to the Auto-Balancing Bridge but is inside it for both frequency and |Z|.

Guard

Impedance Measureing Instruments

Low Frequency Bridges

HP, now Agilent, offers a number of LCR meters that are of the Auto-Balancing Bridge type.  The HP 4260 and  4332 are an analog LCR meters of this type and use three 5-way binding posts to connect to the DUT.  One of them is a guard that drives the shield on a coax cable so that no capacitance will be seen between the coax center conductor to the other wire lead.  Thus improving the accuracy for low value capacitance measurements.

Later instruments, like the 4274, 4275, 4284 use four BNC(f) connectors on the front panel and a more elaborate scheme that not only guards the connection to the DUT but also cancels magnetic coupling between the test leads.  This is not so important when a fixture, like the 16047A, is connected directly to the front panel, but is vital when making a remote reading, like when using an automatic tester or binner.

Note that the shield of one of the coax lines is NOT grounded but is part of the guard system.  You can check with an Ohm meter between the BNC-f shells and the chassis ground to confirm this.  So when making any test fixture were cables are used it's important to keep them insulated from each other (i.e. don't use non insulated coax like 0.141") and it's important that the cables are in a fixed relationship with each other.  So cable clamps are good and cables that are flapping around loose are poor. since the magnetic loop is changing.

The shields are connected together at the fixture end of the cable and that terminal is guard.  Guard should be floated relative to chassis ground.

ZM-4 DC bridge - designed for work on telephone lines
  
Heathkit IB-5281 LCR Impedance Bridge - manually nulled 1 kHz bridge, battery powered

HP 4260A ZM71A/U Universal Bridge - manually nulled 1 kHz bridge, line powered

 HP 4332A LCR Meter - analog 3 Ohm to 1 M Ohm, 3 pF to 1 uF, 3 uH to 1 H, line powered

 Marconi TF-2700 manually nulled 1 kHz Universal LCR Bridge, battery powered

Omega-T TE7 Antenna Noise Bridge - uses noise diode as test signal covering HF radio frequencies 1 to 30 MHz., battery powered

ZM-11 AC LCR Bridge - designed to test components used in typical tube type equipment including cap leakage, line powered

RF Bridges

<>HP Memory Org -Direct Measurement of Impedance - and the advent of S-Parameters

43961A V/I Test Set

The HP / Agilent 43961A is a test set originally made for use with the HP 4396A combo box Network, Spectrum & optionally Impedance Analyzer.  It covers 100 kHz to 1.8 GHz and will connect to many HP/Agilent network analyzers, but because of the V/I operation can only be used on the 4395 or 4396 network analyzers that have the option -001 firmware.  This is because the A port output is proportional to the Current through the DUT and the R port output is proportional to the voltage across the DUT.  Thus an A/R measurement yeilds I/V (=1/Z).  Also the math for the calibration and measurement are completly different from what is used for a reflection measurement.  For example with a reflection measurement the A/R value for an open and short have the same magnitude (normalized to 1.0 and only differ 180 degrees in phase.  This is very different from a V/I mwasurement where a short has near zero for V/I and an open is near infinity.

I'm working on some code that will allow this test set to be used on any HP / Agilent network analyzer that has IBasic.  Stopped work on this when I found the IBasic in the 2395 does NOT support the COMPLEX data type that is supported by normal Rocky Mountain Basic.

HP 4191 & 4193 patent 5345182 Impedance meter capable of performing measurements at high precision over wide impedance and frequency ranges Sep 6, 1994 324/649; 324/522; 324/654; 324/713 - the V/I method

4395A Network Analyzer using a reflectometer or coupler

The 4380A 8-Port test set can be used with the 4380S System software to measure the impedance of CAT5 LAN cable from 1 kHz to 500 MHz.  When the 4396A/B Network Analyzer is used 100 kHz to 1.8 GHz can be mesured.  Typical values for impedance are in the area of 120 Ohms for Unshielded Twisted Pair (UTP) and 50 Ohms for coax.

A Mini Circuits ZFDC-10-1 Coupler can be used for reflection measurements over the 50 kHz to 500 MHz range.  This can be done simply in the A port only mode, or with the addition of a power splitter in the A/R port mode that corrects for source level variations automatically.

4395A Network Analyzer in Transmission mode (Z:TRANS)

By inserting the DUT in series between the source port (or the output of a power splitter when using A/R) and looking at the forward transmission (S21) you can see the device impedance will effect the transmission plot.  Where the impedance is low the insertion loss will be low and so forth.  By using the MEAS/CONVERSION/Z:TRANS function the 4395A converts the S21 plot into either a linear or log |Z| plot.  This is the method used in the E5100() and E4916 Crystal Impedance meters.

E4980A Precision LCR Meter

This seems to be the only LCR meter now offered(July 2007).  Over $15k.  Smaller than the rack sized insturments like the 4284 and with most of their capability.

Patents

Also See