Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 118

Most VNAs have a maximum test port operating level on the order of 10–15 dBm, with a damage level on the order of +30 dBm. Beyond the operating level, the receiver will be in substantial compression, so the data is not valid. Many VNAs provide internal receiver attenuators that allow reducing the power to the receiver, providing operation to much higher levels. The maximum input power to test port couplers are often rated higher than the maximum level of other components behind the directional‐coupler so that with proper padding and isolation, the VNA can be operated to levels as high as +43 dBm, depending on the model. Operation above these levels is possible but requires substantial external components including external couplers to ensure the power level at the VNA components is below the power damage level. Details of high power test configuration are shown in Chapter 6.

Another common practice is to add fixed attenuators between the DUT output and the VNA test port. This works well so long as the total attenuation between the test port and the DUT is less than 10 dB. Adding attenuation after the test port coupler degrades the directivity by two times the attenuation (in dB), as will be shown at the end of Section 2.3.2. In practice, up to 10 dB of external attenuation can be added and compensated for with normal calibration techniques. If between 10 and 20 dB are added, the system becomes somewhat unstable, and for more than 20 dB of added attenuation, different techniques for calibration must be used, and the S22 measurements become unreliable.

For testing devices that require high‐power drives, it is common to add an amplifier to increase the power level normally available from the VNA. One method is to simply add a booster amplifier to the port 1 output and drive the DUT directly from the booster amplifier. This generally results in poor measurements of DUT due to mismatch and gain errors in the booster amplifier. In this approach, it is common to add a booster amplifier, normalize the S21 trace, and then add the DUT and measure the resulting gain relative to the normalized booster amplifier response. However, the normalization has errors due to mismatch between the booster amplifier and the load port. And the measurement has errors due to mismatch between the booster amplifier the high‐power DUT. Further, the input match or S11 of the DUT cannot be reliably measured because booster amplifier isolation eliminates the ability to measure a signal reflected from the DUT. A second error in gain measurements often occurs with this direct approach due to gain drift or gain compression of the booster amplifier.

A better systematic approach is to add the booster amplifier behind the test port coupler, and use a second coupler as the reference‐channel tap to generate a signal proportional to the booster amplifier output signal that can be routed to the reference channel. In this scenario, a directional‐coupler rather than a power splitter is typically used to provide lower loss after the booster amplifier. The output of the reference channel is directed through the test port coupler of port 1 so that the S11 of the DUT can be accurately measured. In almost all cases, an accurate measurement of high‐drive‐power devices requires a booster amplifier followed by a reference coupler. Chapter 6 provides a detailed discussion of high‐power amplifier measurements, including several alternative block diagram configurations to support various power levels.