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Using Calibration to Optimize Performance in Crucial Measurements
With the introduction of high-definition video signals operating at data rates up to 1.485 Gb/s for 1080i and 2.97 Gb/s for 1080p, a new generation of BNC connectors needs to perform at significantly higher frequency than standard BNCs in broadcast studio and transmission applications. To meet the broadcast industry’s stringent HDTV standards such as SMPTE 292M and 424M, Radiall has designed a new 75 ohm HDTV BNC connector that offers a true 75 ohm interface with low VSWR and return loss over a frequency range from 0 to 6 GHz.
The new connector design needs to keep a suitably low VSWR to ensure signal integrity. The increased frequency range—from 3 to 6 GHz—presents concerns over testing the connectors. Commercially available 75 ohm calibration kits are specified only to 3 GHz. However, kits should be usable at higher frequencies once they are validated to such frequencies. This article explains how Radiall validated its existing 75 ohm calibration kit for performance to 6 GHz. Specifically, Radiall wanted confidence that it could accurately measure VSWR and return loss for the connector.
Calibration kits are essential to adjusting a vector network analyzer (VNA) and other test equipment for accurate measurements. A kit contains precision components of known characteristics to allow the VNA to be accurately calibrated and to remove the effects of adapters and other components from the results. Once calibrated, the VNA reveals valuable information about a component’s VSWR, return loss, impedance and other characteristics by evaluating reflected and transmitted energy.
To validate the 75 ohm calibration kit at higher frequencies, Radiall compared its performance to that of a 50 ohm kit at frequencies up to 6 GHz. Since the 50 ohm kit is specified up to 18 GHz, comparing the VSWR of the two kits allows determination of the suitability of using the 75 ohm kit to characterize the new connectors at high frequencies.
Figure 1 Calibration measurement dynamic.
Figure 2 Measurement of 75 Ω matched load with 75 Ω calibration kit.
50 and 75 Ohm Calibration Dynamic Measurements
First, the RF dynamics of the two kits were evaluated by measuring return loss and VSWR. As shown in Figure 1, the results were quite close. For broadcast market testing needs, a dynamic lower than -50 dB between 0 to 3 GHz and -40 dB between 3 to 6 GHz are acceptable. The dynamic of both the 50 and 75 ohm calibration are better than 45 dB for return loss and 1.01 for VSWR over the entire 0 to 6 GHz frequency band. Thus, there is a high-level correlation in terms of reflection due to the connection component between the VNA port and calibration plane.
75 Ohm Matched Load Measurement
The 75 ohm calibration kit was then used to measure a 75 ohm matched load. The return loss and VSWR results are shown in Figure 2. The VSWR of the matched load is lower than 1.1 in the frequency band from 0 to 6 GHz. These results show that the calibration done with the 75 ohm calibration kit is suitable for measurements up to 6 GHz.
Figure 3 50 and 75 Ω air line VSWR measurements by using 50 and 75 Ω calibration kit.
50 and 75 Ohm Air Line Measurements
Using the two calibrations, Radiall measured 50 and 75 ohm air lines. Since air lines are a perfect coaxial line for measurements by minimizing the effects of a plastic dielectric, it can again correlate the results. Taking into account geometrical dispersion between 50 and 75 ohm air lines and measurement dispersions, the VSWR of the air lines are lower than 1.1, as shown in Figure 3. The conclusion can be made that the measured VSWRs are comparable. The two lines have a VSWR lower than 1.1 up to 6 GHz. Equally important, more than 99.8 percent of the energy passes through the connector, with a low 0.2 percent reflected.
To validate that the 75 ohm calibration presents the same reference (calibration) plane as the 50 ohm calibration, Radiall measured the phase for the two air lines. The maximum phase difference between the two air lines is less than 2 degrees at 6 GHz. Both calibration kits measured with the same phase.
Figure 4 VSWR measurement of the Radiall HDTV to 6 GHz.
As a result of the process described here, Radiall is able to test and characterize the new BNC connector design through 6 GHz. Figure 4 shows that the new design provides considerable headroom in meeting design goals and ensuring the high-performance needed for evolving broadcast applications.
By testing to see the correlation between an 18 GHz, 50 ohm calibration kit and a 3 GHz, 75 ohm kit, Radiall is able to confidently validate the performance of the 75 ohm kit to 6 GHz. The 75 ohm calibration kit allows the VNA to be calibrated for testing to 6 GHz with accurate results. In fact, the test results will be worst case; products tested will perform better than the measured values.
Radiall USA Inc.
33 1 49 35 35 35,
RS No. 300
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