so strange phenomena in my PCB, need HELPPPPPPPP

In Layout, signal is transmited from the left A & B(red in picture). And it is difference signal(I/Q) of vco. The output point of signal is C in the right(red in picture).

The distance between two components nearby is about 0.1mm.
The components style is 0402.
Capacitance is about *** pF(<50pF)
Inductor is about ***nH(<50nH)

Here I test impedance looking from C to "A&B" in Smith chart.
If I press my finger on these components, impedance changes immediately.
But if I press some other thing such as paper on these components ; and then press my finger on paper, impedance is all the same.
If touch these components using forceps, impedance is same too.

Could someone tell me the reason? Why does impedance only change when directly press components using finger?

Frequency of singal is from 1700MHz to 1900MHz


I am eager to know the answer.
Thanks a lot!!!!!!

My guess is that you are shorting the components with your skin resistance. Human skin is a conductor. The resistance is dependent on the moisture content of the skin, and the thickness of the epidermis. When you press on an object, you compress the epidermis and lower the resistance.

Typical skin DC resistance varies from about 500ohms to 9Mohms. You could measure yours with a multimeter by just pressing your finger tip against two probes set side-by-side. The impedance of your finger tip at the frequencies you are working with would be even less than the DC resistance because of the capacitive effects of the very efficienct conductor represented by your dermis and bloodstream (remember that your blood is a salt solution). At high radio frequencies, the interior of your finger represents a large conductive plate bridging the DC resistance of your epidermis.

Your body also represents a path to ground, even if not directly coupled via bare skin - almost anything conductive touching or in very close proximity, (you can verify this by taking a piece of metal foil taped to a wooden stick and moving the metal close to or touching the circuitry.), will have an inductive/capacitive effect at these frequencies.

SiGiNT

Hi sigint,

I discounted the ground coupling effect in this particular instance because he said that he doesn't see the impedance change when he puts a piece of paper between his finger and the circuit.

I agree that the body is a large coupling capacitor to ground, but in this particular case body capacitance doesn't seem to apply.

-HouseCat

Thanks for all of you. Somebody else say that finger just like capacitance in AC. So I test my finger using network Analyzer. It's about 30pf in 1GHz. Is it because "dermis and bloodstream" ?

Waltz_Fan said:

Thanks for all of you. Somebody else say that finger just like capacitance in AC. So I test my finger using network Analyzer. It's about 30pf in 1GHz. Is it because "dermis and bloodstream" ?

Click to expand...

Yes - it is because the inside of your body is a conductor, and your skin is an insulator. A capacitor is two conducting plates separated by an insulator. All you need to complete the capacitor is an external conductor - a probe lead, chassis, or any other conductor completes the structure.

"All you need to complete the capacitor is an external conductor - a probe lead, chassis, or any other conductor completes the structure."

Sir, I am a little puzzled about what you say above.
Here I correct the phenomena I see on network anlyzer.

In range about 2GHz, when press my finger on Anylzer's port1 at full tilt(nearly bleeding ^O^), impendance of S11 is about 35ohm+0.8pf; when loosing finger a little, resistor goes up and capacitor goes down.

Based on your opinion, I know why resistor go up.

But why capicator would go down? Is it because space of "dermis and bloodstream " is reduced? Maybe I think... What about your opinion?

Thans a lot!!!

The capacitance goes down because you are increasing the distance between the two condutors. For a parallel plate capacitor -

C(pf)=.2248 x Er x A x (n-1) / d

Er=dielectric constant of insulator
A=area of each plate
n=number of plates
d=distance between plates

For the case of human body capacitance, the above is only good for illustrating what is happening (you don't really have parallel plates with the human body as part of the circuit) - but it should help you understand what your are seeing.