Waveguide Joint Design and Validation for use in Acoustic Vector-corrected Network Analysers

Abstract

The Vector-corrected Network Analyser (VNA) has been an indispensable tool for many decades in the field of RF and microwave engineering where the availability of calibrated and traceable results is taken for granted. However, calibrated and traceable measurements are not so easily available in the acoustic domain. In an effort to allow such measurements, a dual-port acoustic vector network analyser (AVNA) has been designed and built. The calibration of this test and measurement system hinges on the repeatability of the uncalibrated system and its acoustic waveguide joints. The first generation of acoustic waveguide joints have been 3D printed with a high precision printer and designed to include alignment pins, O-rings, and a reinforcing 'clip' system to allow for a consistent bolt torque without damaging the plastic waveguide. We show that the resulting variation in measurements between cycles of disassembly and reassembly is acceptably small and will allow for calibration of the system. Three-dimensional printed titanium joints show increased rigidity and ease of use but similar repeatability.

Full Two-Port Vector-Corrected Network Analyzer in the Acoustic Domain

“…The repeatability of measurements using the acoustic network analyzer has been shown to have a worst-case standard deviation of 0.4 dB [11]. The difference between the forward and reverse orientations for S 11 , S 22 , S 21 , and S 12 is less than 1 dB in most cases.…”

“…The smaller-size couplers operate over 10-20 kHz and have a guide dimension of 6 mm by 6 mm, and the larger set operates over 1-2 kHz with the same dimensions used by Pennington. The performance of the acoustic waveguide flanges has been carefully studied using statistical methods [11]. This is done by taking a number of measurements of the same DUT and disconnecting it in between each measurement.…”

“…This is done by taking a number of measurements of the same DUT and disconnecting it in between each measurement. It is then possible to determine the standard deviation in these measurements and then use this value as an indicator of the repeatability of the joint [11]. The authors concluded that even the order of tightening the flange bolts affects how repeatable a connection is.…”

“…A Line, or Thru of nonzero length, was also built, and since it is of known length, it is possible to know the phase change through its length for each frequency, making it a potential acoustic standard. The zero-length Thru is achieved by connecting both ports together repeatedly by using a specific bolt pattern and torque [11]. The "Reflect" is achieved by closing off the port with a 5-mm mild steel plate, using the same bolt pattern and torque.…”

“…An acoustic vector network analyzer (AVNA) is a device for measuring traveling waves, much like an impedance tube, but it has speed and accuracy advantages over the impedance tube [9][10][11]. An AVNA is capable of swept measurements at ≈200 points per minute.…”