The Time Domain implementation in the VNA Master is trace based, which makes it very flexible to use. Each of the four traces in the VNA Master can be configured independently and can be in the frequency, time, or distance domain. Each trace can also be configured to represent any of the S‑parameters. The VNA Master (as an example) can simultaneously view S11 in the frequency, distance, and time domains using three traces. Alternatively, you can view all four of the S‑parameters in the distance domain or the time domain or both. This flexibility could be useful when tuning complex filters or analyzing long cable problems with multiple discontinuities.

With the ability to transform any S‑parameter, one question that arises is whether the time or distance that is plotted represents a one‑way or a round‑trip propagation. The one‑way propagation represents the transmission (or 2‑port) measurement, in which the signal is transmitted from one port, propagates through the device under test, and is received on the second port. One‑way propagation occurs when transforming S21 or S12.

The round‑trip propagation represents a reflection (1‑port) measurement, in which the signal is transmitted from one port, propagates through the device under test, fully reflects at the end of the device, and is received back at the same port. Round‑trip propagation occurs when transforming S11 or S22.

For reflection measurements, the VNA Master can handle the two cases of one‑way and round‑trip propagation differently in the Time and Distance domains. In the distance domain, the VNA Master compensates for the round trip reflection propagation by showing the actual length of the device under test (essentially dividing the distance by 2 for the reflection measurements). This compensation renders the distance reflection measurement as a One-Way measurement in terms of the distance value that is reported. In the Time Domain, you can choose to set the reflection measurement to be either One-Way or Round-Trip (press the following keys: Shift 8 (System), Application Options, Time Domain, and Reflection Calc in Time). When set to One Way (which is the default setting), the VNA Master compensates for the round‑trip reflection measurement as it does in the Distance Domain. When set to Round Trip, the VNA Master plots the response against the actual time that the signal travels from the transmission port to the receiving port, without accounting for the 2‑way propagation (reflection and return).

For example, look at the results of measuring a cable that is 3.05 meters (10 ft) long. For a transmission measurement, approximately 14.4 ns are taken by a signal when traveling from one end of the cable to the other end of the cable. For a reflection measurement, the time is twice as long, or approximately 29 ns are taken by a signal when traveling from one end of the cable, reflecting from the far end, and returning. Figure: Time Domain Measurements of a 3.05 m Cable Showing S11 and S21 shows a measured time domain response of a cable of this length for both reflection (S11) and transmission (S21). Note that for this example, the VNA Master Reflection Calc in Time parameter is set to Round Trip. The top trace of Figure: Time Domain Measurements of a 3.05 m Cable Showing S11 and S21 is the S11 plot showing the reflections from both ends of the cable (MK1 at the near end, and MK2 at the far end). You can see that the far end peak at MK2 is at approximately 29 ns. Looking at the bottom trace, you can see that the peak at MK3 (which represents the signal received at the end of the cable) is at approximately 14.4 ns.

Take a look at what happens in the distance domain for the same cable. As a user, you want the reflection and transmission measurements to show you where the end of the cable is located. Figure: Distance Domain Measurements of a 3.05 m Cable Showing S11 and S21 shows a measured distance domain response of this cable for both reflection (S11) and transmission (S21). The top trace is the S11 plot showing the reflections from both ends of the cable (MK1 at the near end, and MK2 at the far end). The bottom trace shows the transmission S21 measurement with the peak representing the signal received at the end of the cable (MK3). Looking at the signal at MK2 and MK3, you can see that the reflection and transmission measurements produced the same result for the length of the cable. The VNA Master compensated for the round‑trip condition in the S11 measurement so that the distance information matches the physical length of the cable, just as it does in the S21 measurement. Note that if the option parameter Reflection Calc in Time is set to One Way, then the time domain example shown in Figure: Time Domain Measurements of a 3.05 m Cable Showing S11 and S21 would look more like the result shown in Figure: Distance Domain Measurements of a 3.05 m Cable Showing S11 and S21.