Often times, you would like to measure the characteristics of a DUT by connecting to it with a cable. Unfortunately, the cable is not ideal and degrades the measurement of the DUT. Frequency Gated by Time (FGT) is a feature that allows the unwanted characteristics of the devices surrounding the DUT to be “gated” out of the measurement. FGT first transforms the frequency data into the time domain, “gates” out the unwanted time domain data, and then transforms the gated time domain data back into the frequency domain.

The screen capture in Figure: DUT at Far End of Cable Measurement – Frequency Gated by Time (FGT) shows the measurement of a DUT at the far end of a 30 cm cable. The top‑left graph shows that the cable and DUT together have a worst‑case return loss of approximately 13 dB. The top‑right graph shows the time domain step response of the cable and DUT along with a gate that is placed around the DUT from 29 cm to 33 cm. The bottom‑right graph shows how the gated time domain step response appears. The mismatches and reflections of the cable leading up to the DUT are now gone. The bottom‑left graph shows the Frequency Gated by Time response of the DUT with the degradations of the cable removed. The worst‑case return loss is improved by approximately 21 dB.

 

Another feature that is related by FGT is called “notch”. Instead of gating the desired portion of the time domain response, notch allows you to suppress an unwanted portion of the time domain response.

For illustration purposes, the screen capture in Figure: DUT at Far End of Cable Measurement – Notch with Frequency Gated by Time places a “notch” on the DUT location, leaving just the cable in the time domain response. The bottom‑left graph shows the FGT return loss of the cable (approximately 15 dB) with the DUT “notched” out of the response.