The impedance bridge provides one of six common ways of measuring the impedance of an electronic or electrical component. The balanced impedance bridge provides that highest accuracy, typically at +/- 0.1%, and is thus the preferred method in electronics standards labs. The impedance bridge method is also applicable over the widest range of frequencies, from DC to 300 MHz or so, depending on the bridge configuration employed.

Impedance bridges are an older technology and are generally available as refurbished equipment. But if you are operating on a budget, operational refurbished impedance bridges can provide a cost effective means of measuring circuit or component impedances. Standard impedance bridges have a local oscillator that operates at usually 1 kHz; however, there are also RF impedance bridges that operate into the megahertz. In medicine, a special impedance bridge called a tympanometer is used to analyze the middle ear. When considering impedance bridges, follow this blueprint:

1. Find impedance bridge units that are refurbished.

2. Opt for an RF impedance bridge for higher frequency applications.

3. Buy an operating impedance bridge to measure the impedance antennas and transmission lines while in operation.

Action Steps
The best contacts and resources to help you get it done

Get a standard (low frequency) impedence bridge

When you need to measure the total impedance of an unknown load that may contain resistance, inductance or capacitance, an older but accurate choice is the impedance bridge. Bridge circuits can also easily be constructed in the lab to measure just about any device value desired, whether it be inductance, capacitance, resistance or Q.

I recommend: ValueTronics carries the impedance bridges that are designed for the audible range of 20 Hz to 20 kHz. Used-Line.com has an impedance bridge list with a variety of manufacturers represented.

Buy an RF impedance bridge for making impedance measurements at radio frequencies

RF impedance bridges are designed to give accurate readings when operating in radio frequency ranges up to 100 MHz, but the principle of operation is the same.

I recommend: Tucker Electronics sells the Delta Electronics model OIB-2 RF Impedance Bridge (refurbished). GenRad Sales carries the GenRad models 1656 and 1608A impedance bridges

Take impedance measurements on live circuits using an operating impedance bridge

If you need to determine the impedance of antennas or networks while power is running through them, then you need to get an 'operating' impedance bridge. These also work great for live transmission line sections and at the common point in directional antennas.

I recommend: Impedance bridge provider Delta Electronics makes the OIB series of impedance bridges, OIB standing for Operating Impedance Bridge. Ask them for a copy of Bulletin 1 and Bulletin 3, for useful operating impedance bridge information and tips on how to use your OIB. Megown also carries the Delta OIB series of impedance bridges, selling available working units.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Considering measurement accuracy and ease of use, the impedance bridge is the most recommended method of impedance measurement for DC to 100 MHz. For frequencies of 100 MHz to 3 GHz, the RF I-V method is best. Above 3 GHz, you're recommended to use the network analysis technique.