The SFD series and STD series waveguide detectors offered by SAGE Millimeter, Inc. are square root small signal detectors. What this means is, for the RF power in the range of -45 dBm to approximately +0 dBm, the detector output voltage is proportional to its incident power measure in dBm. Because of the behavior exhibited, these detectors are also called “square law” detectors. In “linear” operations, the detected voltage output is proportional to the square-root value of the input power. In other words, the power to be detected vs. the output voltage is squared, not linear.
Based on above, the Figure 1 chart shows a typical detected voltage vs. input power of a V band detector with model number SFD-15SF-N1. From the plot below, the square root relation of P ∞ V2 can be observed clearly.
Figure 1. Typical Detected Voltage Output vs. Input RF Power
Due to the small signal detection nature, these detectors are not designed and fabricated for power measurement, unless the input power is in the range. However, they can be used for power measurement if proper directional couplers, such as SAGE Millimeter’s SWD multi-hole directional couplers are used. For instance, the detector can be used to detect the power range from -15 to +30 dBm if a 30 dB directional coupler is used. The hook up schematic is shown in figure 2. Once the P vs. Vout lookup table is developed, these detectors can be used as power detector.
Figure 2. Use Small Signal Detector as Power Detector
The STD family is just the SFD family + STF family so that better port isolation is offered for more reliable power detection.