Recently, I had a chance to work on ultrasound transducers, which requires an impedance measurement of a piezo-electric element for the design of pulse circuits. This old equipment has been sit in our lab for a long time. Now, it is time to be fully utilized.
I have seen many students who believe this equipments is just garbage. But actually not. This equipment is designed so that it can be used as impedance, spectrum, and network analyzer by providing three terminals, source (S), reference (R) and test (T). This machine basically compares the amplitude and phase of signals between R and T with various frequencies from S terminal. So, when S terminal is connected to a power splitter and one signal goes to R and another goes to T through DUT, it works as the impedance analyzer. If you add directional bridge after the splitter, it works as the network analyzer. This, “exposed” terminals make this instrument highly versatile. The power splitter suitable for your interested frequency is mandatory to do “something” with this. If your target frequency is not that high(<100MHz, you can fabricate splitter by T or Pi shaped resistor bridge.
Measurement of Ultrasonic transducer
Here is the practical example how to measure an impedance of an ultrasonic transducer. The signal comes from S terminal is split to R and the transducers. The green board shown at the center of the right figure is to convert two BNC cables to one BNC port for ease of connection. Following figures show how the impedance analyzing function looks like with an ultrasonic transducer.
The calculation of lumped constant and simulation are implemented.
Data extraction is really harsh!
What caught your eye when you take a glance at this instrument is yellowish floppy drive located below the CRT. According to manuals, the disk format is called “LIF file”, commonly used only in HP devices. Of course, you cannot read this by Windows. Freeware called “OmniDisk” can generate disk image (binary data) by using a special floppy driver, and “Lifutil(GitHub)” or “LifUtil(Keysight)” extract files from the disk image. Unfortunately, LifUtil does not support the file format HP4195A generate (DATA file?). As text file is supported, You may be able to get the data if you could save the measurement data in text file. But, for me, this is already complicated enough. So I decided to switch to GPIB communication.
Software for data transfer from HP4195A
I found out getting data through GPIB is super easy. The Ascii command you will need is “FMT1;X?A?B?“. Then HP4195 send back the data for frequency, trace A (Yellow), and trace B (Cyan). You can also read sweep conditions by alternating “X?” with “START?” or “STOP?” (Refer Appendix.F of the manual). To send this command, you need interface devices such as “GPIB-USB-HS” or “Prologix“. I built the software to get data automatically. So I would like to share the code and executable file here (HP4195A).