The Wheatstone Bridge: How Does It Impact VNA Measurements? (.PDF Download)

A very wise engineer once told me, “If you want to make an accurate measurement of something, use a bridge.” I have found this advice to be true on more than one occasion. A bridge can be made in many different ways, as long as two paths are conjured up to make a comparison: a reference path and a measurement path.

Here, I’ll only consider the Wheatstone bridge with resistive elements. How might it be used to measure an unknown resistance or complex impedance, or to measure the forward and reverse components of a “traveling wave” over a broad range of frequencies? This last capability is an essential part of a vector network analyzer (VNA), a tool used to determine the S-parameters of a device under test (DUT) by measuring the reflected and transmitted components of incident RF waves in a 50-Ω system.

Figure 1 shows a Wheatstone bridge driven by a 1-V dc battery. Resistors R1, R2, and R3 are each 1,000 Ω. R4 takes on the values of 800, 900, 1,000, 1,100, and 1,200 Ω. The plot to the right of the schematic shows the voltage difference between the two arms of the bridge going from −55 to +45 mV, with a 0-V balance occurring when R4 is 1,000 Ω. This makes sense of course. When R4 is 1,000 Ω, the voltage between the resistors of each arm is half of the supply voltage and the difference between them is zero.

1. This figure depicts a Wheatstone bridge circuit. The plot reveals the voltage difference between the two bridge arms.