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  1. #1
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    Engineer Finger Test

    I have RF design (three section Wilkinson splitter) that performs overall ok, but recently I acquired a small and very performant impedance and SWR measurement gadget that I am using not to test my antennas. I decided to try my splitter with this gadget, and to my surprise (it is 1:4 splitter design as shown at the picture below) Port 0 exhibits somewhat higher then 2:0 SWR and impedance curve somewhat jumpy (a big sine looking curve) in 2.4 - 2.5 GHz range (Test 1), while readings taken from P1, P2, P3, P4 are perfectly fine (test X).

    - Test 1 - impedance/SWR test gadget connected to P0 with P1,P2,P3,P4 terminated with 50 ohm
    - Test X - impedance/SWR test gadget connected to P1 or P2, or P3, or P4 with the rest of ports (including P0) terminated with 50 ohm

    While being at Test 1 I tried "engineer finger" test and looks like placing human finger (dry) on top of isolation resistor R200A (middle section of first split from P0) brings readings to values similar to test X. I assumed that my finger presents capacitance in 0.1 pF - 0.7 pF range, and I saw people utilizing caps in parallel with isolation resistors in Wilkinson splitter designs in order to improve bandwidth, but soldering capacitor in parallel with this R200A didn't change a thing (tried values from 0.5pF to 100pF), and this R200A is identified as THE place that affects the whole P0 picture and does not affect any other ports.

    Any advice/suggestion in terms how can I "replicate" this human finger test? I know that hands-on RF engineers aged >50 will understand what I am asking.

    Click image for larger version. 

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  2. #2
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    Re: Engineer Finger Test

    No doubt capacitance plays a part but in addition our body is an antenna which picks up RF (and/or mains hum). Then in some way, shape, manner or form, we affect an adjacent circuit.

    Instruments won't necessarily detect the effect, yet it triggers some borderline response as in:

    * improving performance temporarily

    * causing oscillations, or 60-cycle hum

    * inhibiting operation temporarily.

    Etc.



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  3. #3
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    Re: Engineer Finger Test

    I expect that the "finger test" adds in the first place losses rather than capacitance. As long as you don't monitor transmission along with reflection, the result is almost useless.

    Measuring complete s-parameters (at least magnitude) would be also necessary to assess achieved design performance.



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  4. #4
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    Re: Engineer Finger Test

    I think the finger adds a capacitance in parallel with the resistor, but also between the two pads toward the ground. Probably as far as the phase difference between the two splitted lines is low the parallel capacitance doesn't play a role. By the way it should be a R-C series rather than a pure capacitance. If your working frequency is high enough you can try to place some indium on the tracks close near the pads R200A is connected to, in order to add capacitance to ground. It should works. To have a look to the effect you can touch the track with the metallic tip of a plastic tuning screwdriver.

    Did you do any simulation ?



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