Testing analog prototype circuits: how to shield it from external influences

[Bigman]

Hi,

recently, I started to design some basic analog circuit (such as basic OpAmp circuits). Now, I am interested to see how much noise my circuit is producing by itself. In theory I know, I have to eliminate external influences to make sure that my measured results are really only the noise coming soleley from circuit. Besides calibrating and to "warm-up" the oscilloscope before doing the measurment, I have to shield my prototopy-circuti from external influences such as halogen-lamps, power supplies, etc.

For this shielding, I build a little box made of tinplate and copper. With the circuit inside and the oscilloscop outside the box, there is no way to use the osci's standard probes . So I mounted some BNC-connectors on the box grounded with the box-housing. I also soldered a capacitor and resistor in parrallel to "adjust" this new "BNC-probe".

Next, I made some BNC-cables to connect my tinplate box with the osci. I used coaxial cables (RG174).

When I connect my self-made BNC-cable ...

• ... ONLY with the oscilloscope (not with my tinplate box), I get a "clean" but unwanted 50Hz wave (btw: I live in Germany, and we have 50Hz power-supply);
• ... with the oscillioscope AND my tinplate box (closed), I still have this unwanted 50Hz wave.
• When there is no cable at all on the oscilloscope, I do NOT have this wave.

==> Hence, my setting is grap.

Despite I am convinced that my self-made cable are the major source of my problem, first I would like to ask for some of your recommendation and hints how you are handling such problems (on a low cost, hobbiest basis).

[tautech]

Think about WHAT you are looking for....not 50 Hz.
Select a faster timebase based on frequencies(or multiples of) your circuit is working at and ignore the 50 Hz.

The scope is just doing what it is designed to do...display electrical waveforms.

Drive it and get worthwhile information from it. 

[Bigman]

hmm... let's say I am building a simple OpAmp based audio-amplifier, which should only amplify an input signal of 1kHz. But instead of a clean amplified 1kHz signal, now I would get 1kHz superimposed by 50Hz.

Okay, for now I know there is this 50Hz in the air, which I could ignore ... but let's assume I am in the middle of a series of tests and my neighbour turns on his microwave. I would blame my circuit for a destored signal which indeed was caused by this microwave.
(Later on, when the cicruit design is freezed/finialized, I will put it a metal/shielded housing, but for the prototype which may be on a breadoard I wouldn't have such a housing).

[w2aew]

Is the ground of your circuit connected to the shielded box and coax ground?

[Bigman]

Is the ground of your circuit connected to the shielded box and coax ground?

Yes it is.
Attached a picture where I just choose a VCSEL constant current circuit for testing. The circuits GND is connected with (-) from the Battery as well as with the BNC-housing. And the BNC-housing is connected with the tinplate/copper box and the shield-wire of the coax-cable.

(I know that also my tinplate/copper box is far from perfect. But it took me a lot of work to make it and before I dump it, I wanted to listen to more experienced people. Also, as you can see in the pics above, the exact 50Hz I have also when just the coax cable is connected).