DIY of power divider and impedance measurement
Hello. I am still hanging around with HP4195A (Vector network analyzer, spectrometer, and impedance analyzer). This time, I would like to introduce how to make power divider by yourself. Then demonstrate how to measure electromechanical properties of ultrasonic transducers. Nowadays, power divider is integrated into VNA, but HP4195A need an external one for its high versatility of measurement. I hope this article is helpful for students who really have to deal with this type of old equipment and who are just curious how to measure the impedance.
HP4195A has separated ports of S, R, and T which might stands for “Source”, “Reference”, and “Test” respectively. It compares the differences of amplitude and phase to measure impedance. So it is necessary to divide the signal from the S equally to R and T (DUT). Although you could get a second-hand power splitter somewhere like eBay, I made the power divider to learn about this. Actually, what you will need are just three of “16.7Ohm resister”. Please see the figure below.
CMF5516R700BHEB (Vishay, Metal Film Resistors – Through Hole 1/2W 16.7ohms .1%) was used for this purpose.
Estimate maximum frequency of this power divider
The calculation above works only if a frequency is low enough to ignore the susceptance components of the divider. To estimate the maximum frequency of this power divider, I measured the 16.7Ohm resistance itself first. The following figure is what I actually got with different impedance measurement setup which guarantees up to 1GHz. From fitting with an equivalent circuit, I got the inductance of the resistor (CMF5516R700BHEB) of 27.5nH. You could calculate how this inductance actually bothers you.
Impedance of single 16.7Ohm resister
Measurement and Analysis of the impedance of Ultrasound transducer
I found a pair of ultrasonic transduces called “UT1612/UR1612” (bought in Japan, guess not available in U.S.) which is designed to have a 40kHz resonance frequency. So this one is used to test this power divider. This measurement will be useful to build a matching circuit by figuring out the equivalent circuit.
After calibration, I obtained the trace of |Z| and Phase on screen. The equivalent circuit fitting function of HP4195A derived following parameters.
- R = 294.677Ohm
- Ca 1= 51.947pF
- L = 63.2480mH
- Cb =1.74419nF
The impedance is then simulated by using COMPLEX function of excel with these parameters. The following shows how simulated parameters match with measurement data.
I also upload the excel file used in this fitting. Hope this could save your time.