Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 79

For modern power meter applications, the diode or multi‐diode power sensor is often the preferred choice. These sensors employ one or more diodes that rectify the RF signal and produce an equivalent DC signal. Occasionally, the DC signal is “chopped” or modified in such a way as to produce a square wave to the measurement portion of the power meter, typically a precision analog‐to‐digital converter (ADC). Chopping the signal helps compensate for DC offsets in the ADC input.

Older diode detectors used only a single diode, and the top 20 dB of the detector range was often described as the “linear” range; below that range, the diode would operate in “square‐law” mode where the output voltage would be a function of the square of the input RF signal. In the low‐power range, the output voltage would be linearly related to the square of the input voltage of the RF signal, thus be linearly proportional to the detected power. In such a region, they operated almost as well as the thermistor sensors but with much faster speeds and much wider dynamic range. At the top of their measurement range, in the linear region, the output circuitry and measurement algorithms are adjusted to compensate for the change to the linear mode of operation. However, in the linear mode, the power in the harmonics has a much greater effect, and a 20 dBc harmonic signal can have up to a 10% change in the measured power of the fundamental, even though it contains only 1% of the power. This is due to the peaking effect that the harmonic can have on the RF voltage. Out of the square‐law region (also known as the linear region, which is in fact where the power meter is not as linear in the usual sense of the word), the power meter may not give accurate readings for complex modulated signal or signal with high harmonic content or high peak‐to‐average envelope power.

More modern diode sensors use a multitude (two or more) of embedded diode elements, some of which are padded with larger attenuation to allow them to operate at higher powers and still be in the square‐law region. Complex algorithms in the power meter instrumentation detect when the power from one sensor exceeds the square‐law region and change to take their readings for power from one of the attenuated diodes. This extends the useful range of the power sensor over more common older diode sensors.