[SOLVED] Finger Tuning on Reactive Load

[chiques]

I'm trying to design a 50Ohm ---> reactive load circuit. I've had some success but my return loss is not the best when I build my circuit. If I put my finger on the capacitor the response looks like I want it. My question: What element would represent a human finger across a multilayer ceramic capacitor? Is that just a high resistance value with some capacitance?

My circuit:

Linear Reponses for my simulated, real and finger versions:

Smith Chart (impedance) of the S11 port:

 

[chiques]

I think my finger was lowering the resistance. I verified in the simulator.

Lowering the resistor element

I ordered some resistors to validate on the fixture. I will post my results.

 

[dick_freebird]

Adding a finger across two conductors will add shunt C
and R. Highly variable w/ moisture in and on the skin.

You might find it more reliable to do something like a pair
of matched LC oscillators, one burdened by a replica and
one by a finger-exposed "real deal", and use the frequency-
pull against a PFD to get an error voltage that you can then
compare against "valid pull" criterion. The reference osc
might be tuned slightly lower than sense osc, for some
falsing-margin.

Kinda like metal detectors do, only with finger-C rather than
by eddy-loading an inductor.

Absolute measurements and "meat " don't always play well
together, as "meat" has a lot of variability.

 

[ge]

Finger across the capacitor is parallel resistance or larger series capacitor or shunt capacitor.
Am I seeing 13MHz?
Dick_Freebird is absolutely correct. A finger is a very rough estimate on the bench of where the circuit wants something. I use it often on lower power/low voltage circuits.
A finger can also act as an attenuator, which is not always wanted.

 

[D.A.(Tony)Stewart]

My fingertip was once calibrated in pF and Mohm depending on area or pressure, which often proved useful for sensitivity tests with stray impedance, crosstalk and attenuation.

 

[chiques]

Finger across the capacitor is parallel resistance or larger series capacitor or shunt capacitor.
Am I seeing 13MHz?
Dick_Freebird is absolutely correct. A finger is a very rough estimate on the bench of where the circuit wants something. I use it often on lower power/low voltage circuits.
A finger can also act as an attenuator, which is not always wanted.

Yes, the center is frequency is 13MHz.