ekS - Instytut Agrofizyki im. Bohdana Dobrzańskiego PAN w Lublinie ...

visible for lower values of soil electrical conductivity than in the case without the
switch. In practice the arable soil electrical conductivity does not exceed 4 dS/m
and is not high enough to attenuate the TDR signal completely even in the
presence of the tested switches.
The electrical connections between the electrical components if the switches
should have electrical impedances equal to 50 Ω. Each element encountered along
the TDR signal path propagation introduces impedance discontinuities
responsible for secondary reflections that degrade the upper frequency limit and
attenuate the signal. When the time window of the sampling head is located at the
place of secondary reflections, they introduce additional measurement errors in
phase and amplitude measurements. The signal reflected from the tested SP16T
switches are presented in Fig. 49.
Fig. 49.
Reflections observed from the tested SP16T switches and TDR probe in wet sand
The signal distortion at the beginning of the reflectograms represent the
reflection from the switches, while the reflections from the TDR probe in wet
sand are at the right side of Fig. 49. The maximal amplitude of reflected signal
(17 mV) is for MMIC GaAs switch and it has inductive character with the
increase of characteristic impedance after passing the switch.
The reflectograms showing reflections of the needle pulse electromagnetic
wave from the rods of TDR probe are presented in Fig. 50. In the left column
there are ones when the probe was connected directly to the TDR meter, in the
right column the reflectograms were taken when the needle pulse propagated
through the selected channel of the prototype switch.
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