possible power losses. A Grounded Coplanar Waveguide (G-CPW) line was chosen, since
this kind of transmission line ensures good impedance control and can be implemented
in an outer PCB layer as needed in this case. A SMA female connector has been used to
feed the line.
The interface board is made on a FR4, 4-layers PCB. The substrate material is
characterized by relative permittivity εr = 4.6 ± 0.4 @ 1 GHz, TanD= 0.019 ÷ 0.026 @ 1 GHz.
A characteristic impedance of nearly 50 Ω is obtained using the trace width = 1.1 mm,
clearance from a coplanar ground plane = 0.3 mm each side. The line uses the reference
ground plane on layer 3, while copper is removed from layer 2 below the line. The height
of the trace from the ground floor is 1.335 mm. The calculated characteristic impedance
is 51.6 Ω, the estimated line loss is less than 0.1 dB.
The line geometry is shown below:
Figure 18:
Line geometry
Transmission Line Measurements
An HP8753E VNA (Full-2-port calibration) was used in this measurement session.
A calibrated coaxial cable was soldered to the pad corresponding to RF output; a SMA
connector was soldered to the board in order to characterize the losses of the
transmission line including the connector itself. During Return Loss / impedance
measurements, the transmission line has been terminated to 50 Ω load.
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