Model XC0900A-20
Rev F
Application Information
Directional Couplers and Sampling
Directional couplers are often used in circuits that require the sampling of an arbitrary signal. Because they are
passive, non-linear devices, Anaren directional couplers do not perturb the characteristics of the signal to be sampled,
and can be used for frequency monitoring and/or measurement of RF power. An example of a sampling circuit is the
reflectometer. The purpose of the reflectometer is to isolate and sample the incident and reflected signals from a
mismatched load. A basic reflectometer circuit is shown in Figure ap.n.1-1.
V
input
1
2
LOAD
Reflected
Wave
4
3
V
VR
I
Figure ap.n.1-1. A Reflectometer Circuit Schematic
If the directional coupler has perfect directivity, then it is clear that VI is strictly a sample of the incident voltage Vinput
,
and VR is strictly a sample of the wave that is reflected from the load. Since directivity is never perfect in practice, both
VI and VR will contain samples of the input signal as well as the reflected signal. In that case,
VI = C + CDTΓejθ
Eq. ap.n.1-1
and
VR = CD + CTΓejφ
Eq. ap.n.1-2
where C is the coupling, D is the directivity, Γ is the complex reflection coefficient of the load, T is the transmission
coefficient, and φ and θ are unknown phase delay differences caused by the interconnect lines on the test board. If we
know VI and VR, we can easily calculate the reflection coefficient of the load. One should notice that in order to make
forward and reverse measurements using only one coupler, the directivity must be really low. In specific customer
applications, the preferred method for forward and reverse sampling is shown in Figure ap.n.1-2.
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