TDR and VNA Basics
Two basic measurement techniques exist for signal integrity characterization of gigabit interconnects in digital systems " Time Domain Reflectometry (TDR) and frequency domain Vector Network Analysis (VNA).
The TDR instrument is a very wide bandwidth equivalent sampling oscilloscope with an internal step generator. The TDR sends a step stimulus to the DUT, and based on reflections from the DUT, the designer can deduce a lot of information about such DUT properties as location of failures, DUT impedance and time delay, and gain insight into the topology of the system. The user can also use Time Domain Transmission (TDT) measurement to measure crosstalk or to characterize lossy transmission line parameters, such as rise time degradation, return loss, and skin effect and dielectric loss. However, one cannot observe the frequency dependent behavior of the system directly on the instrument; additional software is required to achieve that.
TDR is visual and intuitive due to the transient nature of this measurement technique. The incident step propagates through the discontinuities in the DUT, and the reflections indicate the exact location of discontinuities and their size. The fast TDR rise time (25-35ps in currently available Agilent and Tektronix TDR oscilloscopes) ensures that a wide range of frequencies is captured during this broadband measurement, Figure 1.
Figure 1. Time Domain Reflection and Transmission (TDR and TDT) block diagram. A similar diagram can be drawn for reverse measurements (from port 2 to port 1).
Any of these measurements can be performed in a differential or single ended fashion. In differential, common or mixed mode, the measurements require at least 2 synchronized sources and a 4-port measurement setup, as shown on Figure 2 below.
Figure 2. A differential, common mode or mixed mode measurement would require a 4-port measurement setup.
The VNA uses sine waves as a stimulus, and uses a very narrow band filter at the receiver end. The measurement is performed by sweeping the source and the receiver in a synchronized fashion, making it a swept-frequency, steady state measurement. As a result of this measurement, the user obtains information about frequency domain performance (often referred to as S-parameters) and losses in the system (insertion and return loss). However, in order to gain insight into the topology of the DUT, as one does with a TDR measurement, one must convert the data into time domain using additional software.
Figure 3. VNA measurement block diagram. A similar diagram can be drawn for reverse measurements (from port 2 to port 1). A differential, common mode or mixed mode measurement would require a 4-port measurement setup.
When comparing the equations for time and frequency domain measurements, one cannot help noticing that they look very similar.
In fact, the two measurements are related via Fourier Transform. The time domain and transmission data can be obtained from frequency domain S-parameter measurements via Fast Fourier Transform (FFT) with the use of proper windowing and signal processing techniques. The VNA data can be converted into time domain either with the time domain option on the VNA, or with additional software (such as TDA Systems' IConnect or Agilent's PLTS). At the same time, frequency domain data, such as insertion and return loss, and frequency dependent crosstalk can be obtained from the TDR oscilloscopes using the same software mentioned above. The correspondence between the time and frequency domain waveforms is shown on Figure 4 below. The differential stimulus, differential response (upper left) quadrant is most commonly used and most important for digital design and signal integrity analysis. The upper right and lower left quadrant are what is referred to as "mixed-mode" S-parameter quadrants, which are used in digital design least frequently.
Figure 4. Correspondence between time and frequency domain waveforms
In recent years, differential network analyzers have been introduced, which allow measurement of insertion and return loss in differential, common and mixed mode. At the same time, new TDR add-on modules on the market, such as those from Picosecond Pulse Labs, allow measurements up to much faster rise times and frequencies.
The dynamic range of a VNA tends to be substantially higher than that for a TDR measurements (up 110 dB for VNA vs. 50-60 dB for TDR). It makes a big difference in microwave design world. However, in signal integrity and digital design, this difference is not as significant, as vast majority of the measurement tasks can be achieved with 40-50dB of dynamic range, and 80% of the measurements with even less. When evaluating this statement, it is worth remembering that -40dB measurement translates into mere 1% crosstalk in time domain, which many times can be simply ignored; or that a recent standard such as Serial ATA requires insertion loss measurement only on the order of "10dB. The larger dynamic range is obtained in VNA mainly through the use of narrow band filtering at the receiver, but the digital averaging in TDR achieves the same effect of increasing the dynamic range of the measurement.
The better frequency domain performance of a VNA comes at a price; a user typically is paying at least twice as much for a VNA compared to a TDR-based system with a similar rise time and bandwidth. When choosing an instrument, it is also important to remember that because of windowing and signal processing required to convert the data from one domain to another, a 20 Ghz VNA will not provide as fast of a rise time as 20 Ghz 35ps TDR; at the same time, a 20 Ghz, 35ps TDR will only provide meaningful data up to about 10-12 Ghz in frequency.
In summary, TDR is more intuitive to digital designers, and hence gets used more for digital design and signal integrity analysis. It is broadband in nature, and captures all the frequencies up to the highest defined by the rise time at the same time. It enables the user to window out the unwanted parts, but this has to be done with care to ensure the multiple reflections (discussed below) do not distort the measurement picture. The VNA, on the other hand, is more accurate, narrowband, and more intuitive for designer with RF or microwave background. The higher accuracy also comes at a substantial cost difference vs. a TDR with a comparable bandwidth.