Vector Network Analyzer for a penny

Do you want to measure impedance up to 100MHz with 2-3% error? If yes, than continue reading, else exit.
There are at least four methods how to measure impedance. The difference of each method is in error profile within the frequency range.

Step A: Build the well known Wheatstone’s bridge with 50-Ohm resistors. You will need six 1% 49.99 Ohm SMD resistors for the bridge, calibration, and generator termination. You can use your transceiver instead of the signal generator. Borrow a portable two channel digital scope with phase angle read-out. Channel A-B. There will be digital voltage read-out on the screen, as well as phase. The sum expense is $1 for the resistor bridge and small PCB.

Step B: Draw the schematic diagram. Write down the two formulas for voltage, phase, and impedance.

j = - arctg (X/R).
M = the formula for magnitude, s21.

Step C: Measure the signal generator voltage with first probe. The second probe will measure the voltage and phase angle delta of the unknown impedance. Measure at the bridge branches. Both branches are grounded. Both probes will load the bridge with same capacitance, and the bridge will be balanced. Calculate the unknown impedance from angle and magnitude =>. You are finished.

Step D: The above measurement has impedance error of 1% up to 30MHz. An error of 3% up to 100MHz was measured with HP digital scope and 16pF probes. If you have high errors, check the probes and vertical amplifiers. They should be identical. The results were compared with 6GHz network analyzer with excellent agreement. Measurement of 2pF reactance at 1.95MHz or 30MHz is not critical. This type of measurement seems to work quite well for low frequency range. There is no lower frequency limit.

GL VA3DIW 2002

The method was successfully copied by a company in IL.

Back to Main