Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 96

The effective output match of the VNA source is the same as the power source‐match over the region within the ALC loop bandwidth of the source signal. In that region, the ALC loop responds to reflection signals as described earlier. Outside the ALC loop, the source presents a different output impedance. This is the reflection that would be measured in response to a signal not related to the source output signal. This source reflection is important in cases where the DUT presents other signals reflected into the source, such as from a mixer, or intermodulation products from the input of an amplifier. This value represents the manner in which these other signals would be reflected back out of the source. It can be measured directly as any other reflection coefficient, using a separate VNA reflectometer; an example measurement is shown in Figure 2.11, where the internal source is set to 1 GHz. Here, the internal VNA source frequency is shown as a large spike at 1 GHz on the source output‐impedance measurement. This is again a case where the coupler in the R‐channel used to sample the incident wave provides better match (lower trace) than a two‐resistor splitter (upper trace), at least at low frequencies where mismatch after the reference splitter is minimal.

Figure 2.11 Measured source output impedance away from the source frequency: a trace using a power splitter in the R channel (upper) and a trace using a coupler (lower).