With the introduction of high-definition video signals operating at data rates up to 1.485Gb/s for 1080i and 2.97Gb/s for 1080p, a new generation of BNC connectors needs to perform at significantly higher frequencies 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 designed a new 75Ω HDTV BNC connector that offers a true 75Ω interface with low VSWR and return loss over a frequency range from 0GHz to 6GHz.
The new connector design needs to keep a suitably low VSWR to ensure signal integrity. The increased frequency range — from 3GHz to 6GHz — presents concerns over testing the connectors. Commercially available 75Ω calibration kits are specified only to 3GHz. But kits should be usable at higher frequencies once they are validated to such frequencies. This article explains how we validated our existing 75Ω calibration kit for performance to 6GHz. Specifically, we wanted confidence that we 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, incident, impedance and other characteristics by evaluating reflected and transmitted energy.
To validate the 75Ω calibration kit at higher frequencies, we compared its performance to that of a 50Ω kit at frequencies up to 6GHz. Since the 50Ω kit is specified up to 18GHz, comparing the VSWR of the two kits allowed us to determine the suitability of using the 75Ω kit to characterize the new connectors at high frequencies.
RF dynamics
We began by evaluating the RF dynamics of the two kits by measuring return loss and VSWR. As shown in Figure 1 on page 54, the results were quite close. For our testing needs, a dynamic lower than -50dB between 0GHz to 3GHz and -40dB between 3GHz to 6GHz are acceptable. The dynamic of both the 50Ω and 75Ω calibration are lower than -45dB for return loss and 1.01 for VSWR over the entire 0GHz to 6GHz frequency band. We thus have a high-level correlation in terms of reflection due to the connection component between the VNA port and calibration plane
75Ω matched load
We then used the 75Ω calibration kit to measure a 75Ω 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 of 0GHz to 6GHz. The results show that the calibration done with 75Ω calibration kit is suitable for measurements up to 6GHz.
50Ω and 75Ω air line
Using the two calibrations, we then measured 50Ω and 75Ω air lines. Since air lines are the “ideal” for measurements by minimizing the effects of a plastic dielectric, we can again correlate the results. Taking into account geometrical dispersion between 50Ω and 75Ω air lines and measurement dispersions, the VSWR of the air lines are lower than 1.1, as shown in Figure 3. We can conclude that the measured VSWRs are comparable. The two lines have a VSWR lower than 1.1 up to 6GHz. Equally important, more than 99.8 percent of the energy passes through the connector with only a low 0.2 percent reflected.
To validate that the 75Ω calibration presents the same reference (calibration) plane as the 50Ω calibration, we measured phase for the two air lines. The maximum phase difference between the two air lines is less than 2 degrees at 6GHz. Both calibration kits measured with the same phase.
As a result of the process described here, we are now able to test and characterize our new BNC connector design through 6GHz. Figure 4 shows that the new design provides considerable headroom in meeting design goals and ensuring the high-performance needed for evolving broadcast applications.
Conclusion
By testing to see the correlation between an 18GHz, 50Ω calibration kit and a 3GHz, 75Ω kit, we were able to validate the performance of the 75Ω kit to 6GHz confidently. The 75Ω kit allows the VNA to be calibrated for testing to 6GHz with accurate results. In fact, the test results will be worst case. Products tested will perform better than the measured values.
Charles Populaire is an R&D manager, Mansour Mbaye is an R&D RF technical expert and Fabien Bourgeas is an R&D product designer at Radiall.
