Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 123

Often, the performance of external components used to connect from the VNA to the DUT presents the largest contribution of errors to the measurement. These errors can come in a variety of configurations, and each has its own peculiarities that can affect measurements in different ways. The most common causes of external errors are cables and connectors.

Cables, connectors, and adapters are ubiquitous when using VNAs to measure most devices. The quality and particularly the stability of the cable and connector can dramatically affect the quality of the measurement.

The first‐order effect of cables is added loss and mismatch in a measurement. For short cables, the loss is not significant, but the mismatch can add directly to the source‐match and directivity of the VNA to degrade performance. With error correction, the effect of mismatch can be substantially reduced (to the level of the calibration standards quality) if it is stable, but cable instability limits the repeatability of the cable mismatch and often is the dominant error in a return‐loss measurement.

For transmission measurements, the effects of mismatch do directly affect calibration, though it is reduced to a small level by the quality of the calibration standards. Often, the major instability in a cable is the phase response versus frequency. Even if the amplitude of the cable is stable, if the phase response changes, the VNA error correction will become corrupted because of phase shift of the cable mismatch error. Methods for determining the quality of cable and the effects of flexure will be described in Chapter 9.