When power sensors are designed, attempts are made to produce the lowest VSWR (SWR) so that the user will have the best match when they use it. However match is never perfect because no device including any power sensor has a perfect SWR. For this reason, two power sensors of equal quality and calibration will read differently in a user’s system. In this brief, we’ll take a look at the reasons for this along with ways to achieve the best power measurements.
As a result of manufacturing limits, part & component variations, line lengths and detector properties, the VSWR of any power sensor is not perfect and varies dramatically over frequency. Two high quality, high performance power sensors will exhibit different VSWR characteristics at different frequencies. Figure 1, a chart from a LadyBug LB5940A Power Sensor data sheet, shows the specifications and typical data for a production run of power sensors. In the data sheet, the company also provides a table with limit (red line in Figure 1) and typical data for several frequency ranges to make uncertainty calculations easier for users. It is also apparent that as the frequency increases, VSWR increases making it very important to understand match when working with higher frequencies.