Joel P. Dunsmore

Handbook of Microwave Component Measurements


Скачать книгу

power source‐match is that when the DUT is not matched, there will be a reflected signal that, while it will be detected by the reference channel and thus compensated for in gain measurements, will cause the a1 wave to vary from the value one sees when the port is terminated in 50 Ω and thus cause error or ripple in the drive power to the DUT, as illustrated in Figure 2.7. The figure shows, in the dark trace, the incident power at a1 for a load at the test port. It is not perfectly flat because of other mismatches in the system after the reference coupler. The light trace shows the result of an open at the test port. This large ripple is because of the poor power source‐match and generates an error in the incident signal of nearly 1 dB. Since this is power measured at the a1 receiver, it is related to the ripple in the output power, but other loss or reflections between the reference splitter and the test port can affect the output power. Other reflections past the reference splitter will add to the power source‐match but are not represented in the a1 measurement. In the case of a linear device, which S‐parameters presume, this is of no consequence; but in the case of non‐linear devices, such as amplifiers in compression, this will directly affect the reported output power. In such a case, the drive power from the VNA can be higher or lower than the displayed power setting, so the amplifier will be further, or lesser, in compression and the power reported will be in error if based on the input drive level.

Graph depicts the measured incident power into a load termination and an open termination for a vector network analyzer with a coupler in the reference channel. Schematic illustration of the block diagram for measuring power source-match. Photo depicts the line stretcher used for match measurements. Graphs depict the measurement of long line indicating power source-match using an external coupler terminated in a short as a two-resistor power splitter (upper) and a trace for a coupler in the reference path (lower).

      (2.2)equation

      where VSWR is the peak‐to‐peak ripple in dB found at the output of the monitoring coupler and LCM is the loss in the main arm of the monitoring coupler. In the upper trace of the previous example, the p‐p ripple at low frequency is about 3.0 dB, and the mainline loss of the external coupler is about 1.6 dB, so the power source‐match is

      (2.3)equation

      This is almost exactly the power match expected from a 50 Ω splitter (83.3 Ω or −12.05 dB). The lower trace shows a power source‐match for a directional‐coupler of around −21.6 dB at low frequencies, and −18 dB at higher frequencies.

      The power source‐match is normally set by the ALC loop in the source, and as such the reflections from the DUT are sensed by the detector diode in the ALC loop such that the source power is adjusted to maintain a constant voltage at the detector.

      2.2.2.3 Source Output Impedance