Broadband Reflectometry: Theoretical Background - Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications - page 33

Fig. 3.6 Picture of the Tek-
tronix DSA8200 equipped
with the TDR80E04 unit [8]

3.3 Frequency Domain Reflectometry (FDR)

In FD, the parameter that corresponds directly to the time-domain reflection coeffi-
cient, ρ ( t ) , is the reflection scattering parameter, S 11 ( f ) . In this case, the frequency-

domain response function, R ( f ) , can be written as

R ( f )= V 0 ( f ) S 11 ( f ) (3.10)

where V 0 ( is the input signal [26, 38]. Traditionally, measurements performed

directly in FD rely on VNAs: in this case, the excitation stimulus is a sinusoidal
signal whose frequency is swept over the desired range of analysis.
VNAs usually have more than a single port; therefore, they can also be used for
transmission measurements. A generalized block diagram of a VNA is shown in Fig.
3.7, showing the major signal-processing sections. In order to measure the incident,
reflected and transmitted signal, four sections are required [3]:

f )

1. source for stimulus : most network analyzers have integrated, synthesized sources,
providing excellent frequency resolution and stability.
2. signal-separation block : this block is generally referred to as the 'test set' (which
can be either integrated within or separate from the VNA). There are two func-
tions that the signal-separation section must provide. The first is to measure a
portion of the incident signal to provide a reference for ratioing (this can be done
with splitters or directional couplers). The second function of the signal-splitting
hardware is to separate the incident (forward) and reflected (reverse) traveling
waves at the input of the SUT (for this task, bridges are often used).
3. receivers that downconvert and detect the signals : there are two basic ways of
providing signal detection. One relies on diode detectors which convert the RF
signal level to a proportional DC level. If the stimulus signal is amplitude mod-
ulated, the diode strips the RF carrier from the modulation (this is called AC
detection). Diode detection is inherently scalar, as phase information of the RF
carrier is lost. The other solution for detecting the signal relies on the tuned re-
ceiver, which allows preserving phase information. The tuned receiver uses a
local oscillator (LO) to mix the RF down to a lower intermediate frequency (IF).