For Cable Testing should I use a time domain reflectometer (TDR), a network analyzer, or both?
Cable testing can be accomplished in many different ways. The optimum test equipment depends on the desired results and how the cable specifications are written. In some scenarios both a network analyzer and a time domain reflectometer (TDR) may be beneficial as they both have unique strengths.
TDRs are simple to use and can provide insight into cable faults. The TDR process can be thought of as a kind of "radar". The TDR launches a pulse into a cable and samples reflections (or echos) at different times. The time and amplitude of returned echoes is displayed graphically on a time/distance scale. This permits faults to be located along the cable.
TDRs are characterized by limited distance resolution. This may present a problem if accuracy is needed to phase-match cables. Also, TDRs lack the inherent accuracy and associated vector error correction available in a network analyzer. Furthermore, TDRs are limited to the time domain, they have no frequency domain capabilities. They are intended for fast, easy measurements. The TDRs absence of phase data is often a deciding factor in the decision to use a network analyzer. Time domain equipped vector network analyzers have frequency domain, voltage standing wave ration (VSWR), and impedance parameters.
Network Analyzer Strengths:
Vector network analyzers (VNA) add several features when compared with a TDR. VNAs are more accurate and can perform vector error-corrected measurements of reflection and transmission coefficients. Additionally, they perform swept frequency domain measurements. This is an important distinction, especially if VSWR is specified. The network analyzer's ability to provide VSWR (or return loss in dB) versus frequency can be critical to many applications. In addition, VNAs can be used as a TDR when equipped with the appropriate time domain option or fault location option (option 010 or option 100 - analyzer dependent).
Frequency-dependent reflections can be measured with a VNA in several different formats. This flexibility is not available with a TDR. Also, phase information is available with the VNA, making possible Smith Chart displays, impedance measurements, and the ability to accurately perform phase matching of cables.
VNAs are more accurate due to vector error correction achieved through the use a various types of calibration methods. They also have far greater dynamic range due to the swept tuned receiver architecture. TDR receivers are very broadband and noisy compared with a network analyzer. Dynamic range is critical for cables which have lossy performance and/or excellent VSWR.
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