Vector network analyzer basics
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© 2017 Pico Technology
pv106ug r1
4.7.7.1.4
Reactive terminations and discontinuities
Reactive elements can also be determined by their response. Inductive terminations produce a positive
pulse. Capacitive terminations produce a negative pulse.
Simplified representation of the response of a reactively terminated line
Similarly, the position and type of discontinuity in a cable, due to connectors or damage, can be determined.
A positive pulse indicates a connector that is inductive or damage to a cable, such as a removal of part of
the outer screen. A negative-going pulse indicates a connector with too much capacitance or damage to the
cable, such as being crushed.
Simplified representation of the response of a line discontinuity
4.7.7.2
Time domain from frequency domain
An alternative to traditional TDR is where the time domain response is determined from the frequency
domain using an Inverse Fast Fourier Transform (IFFT). Several methods are available for extracting time
domain information from the frequency domain. The main methods are
lowpass
and
bandpass
.
4.7.7.2.1
Lowpass method
The lowpass method can produce results that are similar to the traditional TDR measurements made with a
time domain reflectometer using a step signal, and can also compute the response to an impulse. It
provides both magnitude and phase information and gives the best time resolution. However, it requires that
the circuit is DC-coupled. This is the method supported by the PicoVNA 106.
The lowpass method uses an Inverse Fourier Transform to determine the impulse response in the time
domain from the reflection coefficient measured in the frequency domain. The DC component is
extrapolated from the low-frequency data to provide a phase reference. Alternatively, if the DC termination is
known it can be entered manually. Once the impulse response is computed, the step response may be
determined from the time integral of the impulse response. In the step response mode the trace is similar to
that of a TDR, except that there is no step at t = 0. When the time domain response is derived from the
frequency information the value at t = 0 is the impedance of the transmission line or load immediately
following the calibration plane. The value is referenced to 50 Ω, the characteristic impedance of the system.
For example, an open circuit would appear as a value of +1 unit relative to the reference value and a short
circuit would appear as a value of –1 unit relative to the reference value (see example TDR plots above).
