Why the LNA oscillates when I touch coaxial cable inner and outer conductor connections?

[huytergan]

an antenna at the input, followed with filter and lna then soldered to the coaxial cable (without any connector at the board).

When I touch at the output of the LNA region where the decoupling and output matching circuit components are, and also very close to the coaxial cable soldering points(2 ground and 1 center pad), so I might be touching the coaxial cable’s outer conductor(gnd) as well;
i see the oscillation appears at spectrum analyzer. When I dont touch, doesn’t appear. I wonder what does that mean, i prevent the ground connection of the LNA and it starts oscillation? or Rf signal leaks to the DC line?

Thx guys

 

[betwixt]

I guess your finger isn't matched to 50R.

Brian.

 

[D.A.(Tony)Stewart]

Emitter followers tend to generate positive feedback with dielectric loads. read capacitive.
What impedance or pF would you estimate your finger is? My fingers are well-calibrated.

 

[huytergan]

I guess your finger isn't matched to 50R.

Brian.

I get it my finger changes the matching, however what confused me is, I read somewhere that about hand sensitivity, if I touch to it and if it oscillates, might it be saying me that dc lines carries rf signal? and mismatching with my finger is that doing it?

 

[D.A.(Tony)Stewart]

I get it my finger changes the matching, however what confused me is, I read somewhere that about hand sensitivity, if I touch to it and if it oscillates, might it be saying me that dc lines carries rf signal? and mismatching with my finger is that doing it?

Anywhere you can manually trigger oscillation means you have found a way to inject positive feedback in a circuit regardless if it normally, just DC or AC or RF by regenerative feedback and can be suppressed by adding some series losses or emitter degeneration. The physical layout has all sorts of RLC parasitics that must be examined and added to the mental picture of the logic diagram, to fully understand. e.g. nH/mm and pF/mm with impedances.

 

[huytergan]

Anywhere you can manually trigger oscillation means you have found a way to inject positive feedback in a circuit regardless if it normally, just DC or AC or RF by regenerative feedback and can be suppressed by adding some series losses or emitter degeneration. The physical layout has all sorts of RLC parasitics that must be examined and added to the mental picture of the logic diagram, to fully understand. e.g. nH/mm and pF/mm with impedances.

Thank you