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14 March 2022

When You Need a TDR and When You Need a WavePulser

Figure 1: S-parameters and TDR responses are two means to
the same end, characterizing interconnects.
Oscilloscopes are used to measure signals, usually as voltages vs. time, and signals come from active devices. But interconnects are passive structures that don’t produce their own signals. To characterize interconnects, you need a stimulus-response system.

There are two, principal types of stimulus-response systems used to characterize interconnects: Vector Network Analyzers (VNAs) used to measure S-parameters in the frequency domain, and Time Domain Reflectometers used to measure impulse responses in the time domain. Each uses a different type of incident signal and a different formalism, but as long as the interconnect is linear, passive and time invariant, both S-parameters and impulse responses yield the same information content in different formats and can be translated from one into another.

So, which do you need? We’ll look briefly at what each does and what are the criteria that might require you to have one versus the other.

When you need a TDR

When we use a step edge as the stimulus and look at the response, we refer to those measurements as Time Domain Reflectometry or TDR measurements. What scatters back is referred to as the Time Domain Reflected signal, in addition to which is the transmitted wave, the Time Domain Transmitted signal. TDRs can be 1- or 2-port, the latter able to give you differential TDR measurements.

While simpler to use than a VNA, a TDR instrument is more limited as to the bandwidth and rise time of the signals used to stimulate the interconnect.

You can use a TDR if:

  • You’re measuring bandwidths > 5 MHz but < 15 GHz, single-ended or differential
  • You’re measuring rise times > 35 ps (65 ps Gibbs’ ear adjusted)
  • You care about measuring:
    • Impedance profiles of interconnects
    • Time delay of interconnects
    • Effective dielectric constant of uniform transmission lines
    • Impedance of a discontinuity at a specific rise time
    • Excess inductance/capacitance of a discontinuity
  • You’re doing 1- or 2-port measurements
  • You don’t want to learn about S-parameters
  • Cost is a concern 

Teledyne Test Tools offers the T3SP15D TDR instrument.

When you need a WavePulser 40iX

To obtain S-parameters (scattering parameters), sine waves are used as the stimulus. Sine waves have three figures of merit: a frequency, an amplitude and a phase. For all linear, passive, time invariant interconnects, the frequency sent in will be the same coming out, what changes will be the amplitude and phase, so we characterize interconnects by how they affect the amplitude and phase of the scattered waves compared to the incident waves across a wide range of frequencies. This is formalized as the scattered wave coming back on one port (S11), the scattered wave going through (S21), plus the scattered waves into other ports (S31, S41, etc.). 

While traditionally S-parameters have been measured by a VNA, Teledyne LeCroy offers the WavePulser 40iX High Speed Interconnect Analyzer, which is a hybrid instrument that can calculate either TDR or S-parameter measurements from the same acquisition.

You need a WavePulser if:

  • You’re measuring bandwidths > 5 MHz but < 40 GHz
  • You’re measuring rise times > 9 ps (25 ps Gibbs’ ear adjusted)
  • You’re doing 3- or 4-port measurements
  • You want S-parameters for de-embedding, emulation, etc.
  • You care about measuring:
    • Crosstalk
    • Behavioral model of a differential channel
    • Frequency-dependent delay or loss of a channel
  • Cost is less of a concern

When you need a VNA

There are some cases where a WavePulser will not suffice, you will need a traditional VNA to produce measurements that meet your requirements.

You need a VNA if:

  • You need metrology grade S-parameters (> 70 dB dynamic range or < 0.1 dB accuracy)
  • You’re measuring bandwidths > 40 GHz
  • You care about measuring:
    • Narrow bands < 5 MHz
    • Low frequencies < 5 MHz
    • HighQ resonators
  • Cost is not a concern 

Watch Dr. Eric Bogatin weigh the options in our on-demand webinar, What Do I Need and Why—TDR, S-parameters, or Both? 

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