Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 94

The ratio source‐match is that match that will affect the results of a ratio measurement, given a DUT is not perfectly matched to the reference impedance. The value of the ratio source‐match, most commonly called the raw source‐match or uncorrected source‐match, is derived from a combination of the quality of the reference channel signal‐separation device and any mismatch between this device and the input port of the DUT. This is always the value used to compute the uncertainty or accuracy of a gain or return loss measurement. But this match applies only to parameters that have a ratio of some receiver to the reference receiver. In this case the reference receiver is used to measure the change in source drive power incident to the DUT, and the error‐correction math removes the ripple at the output of the DUT associated with the measured incident ripple in power. Thus, for linear measurements such as S21, where the output signal is linearly related to the input signal, the ratio is not affected by changes in incident power to the extent that the reference receiver properly measures the actual incident wave at the DUT.

Errors in the measurement of the actual incident wave are mostly attributed to the ratio source‐match of the VNA. The ratio source‐match can be determined during the calibration process and is shown in Figure 2.5 for two cases of reference channel signal separation: the upper trace is using a two‐resistor power splitter, and the lower trace is using a directional‐coupler. While the detailed response is different, the overall quality is quite similar between the two cases.

Figure 2.5 Ratio source match: trace when using a power splitter (upper) and trace when using a directional‐coupler (lower).

When a splitter is used, since the splitter uses equal 50 Ω resistors in most cases, the input match to the splitter (as it appears from the source) is nominally 50 Ω, and the loss through the splitter is about 6 dB. The mathematical process of taking the ratio has the effect of creating a virtual ground at the common node of the splitter, so the ratio source‐match is a measure of the quality of the internal 50 Ω resistor.

Interestingly, for the case where the reference comes from a coupler, in the absence of other sources of mismatch after the coupler, the ratio source‐match will be identical to the directivity of the reference coupler. This makes sense as directivity is a measure of the reverse signal leaking into the coupled port, and this signal adds to the reference receiver reading even though it is not part of the incident signal, thus causing an error in ratio measurements.