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I want to make a aluminum shield box (this is my first time!), I made some holes to host some female BNC connectors. Now my question is, should I isolate the BNC connectors from the box? Or should the outer shield of the coaxial cables touch the shield box?

Update

What I am trying to do is making a test fixture to measure leakage current of a diode. I found the picture below on the net:

enter image description here

If I have 2 female BNC connectors and an aluminium box, what will the representation of that metal shield connected to the LO of both ammeter and source in the picture above in my case? does it mean bnc's should betouching shield box ??

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    \$\begingroup\$ Talk about jumping in at the deep end !!!!!!!!!!!! Aaagh. Short: Don't ground them. Longer: Aghhhhhhhhhhhhhhh Longer still: Try and make the fixture so small and leads short and ... that it is almst a probe extension. Fig I-14 Page I-18 in Keithly Model 6845 instruction manual but many nearby sections relate. \$\endgroup\$
    – Russell McMahon ♦
    Commented Dec 16, 2011 at 5:19
  • \$\begingroup\$ Ohhh I am getting confused!! I cant understand what should I do about that connection between metal shield and LO :( So by not grounding them you mean I should put some insulators inside and outside of the box when I want to put the bnc female connecteros in the whole of the box? \$\endgroup\$
    – Dumbo
    Commented Dec 16, 2011 at 9:14
  • \$\begingroup\$ I now see you probably have lo & hi in a common coax with BNC connector.That's not how I had visualised it. | LO connects to shield as shown. HI may be the internal conductor of a coax with HI as it's output. IF SO you connect LO via BNC ground to shield at point of entry of HI through wall. (My answer assumes that hi was a near zero length extension.) Now LO from psu is tricky - which is where my agh came from. A photo of the setup may help. The object is NOT to connect LO from prog source at a different point on shield than lo from picoammeter. ONE ground connection to shield only. \$\endgroup\$
    – Russell McMahon ♦
    Commented Dec 16, 2011 at 10:33

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Ground them all!!!

In the large majority of cases the BNC connectors should make good electrical contact with the box and the whole assembly should be at ground potential.

There are specialist cases where this is not so but by the time you encounter them you will know why you may wish to do it - and will sit agonising over whether you in fact should and at which end and more.

The material below relates to why sometimes you shouldn't or what happens when you do or what issues relate. BUT for a start, ground them all, and read up on the why not's for future reference.

For later years:

Ground loops in testing:

Excellent discussion

enter image description here

  • A ground loop results when two or more separate ground paths are tied together at two or more points. The result is a loop of conductor. Connecting the ground lead of an oscilloscope probe to the ground in the circuit-under-test results in a ground loop if the circuit is “grounded” to earth ground. Typically the metal chassis of both scope and device under test are connected to safety ground and internal power supply common. Scope probe ground connects to scope chassis at the input BNC connector.

    In the presence of a varying magnetic field, this loop becomes the secondary of a transformer which is essentially a shorted turn. The magnetic field which excites the transformer can be created by any conductor in the vicinity which is carrying AC or changing current. The potential difference seen on oscilloscope probe ground can range from microvolts to as high as hundreds of millivolts.

Mysterious ground Related enough to need to be aware of.

Video ground loop breakers - Expert anti-agonising devices.

Ground loop isolator

SP-G01 CCTV Video Ground Loop Isolator, Coaxial Cable, BNC

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And another

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Good intro discussion

  • What is a Ground Loop

    A ground loop occurs when there is a potential difference between inter-connecting equipment such as the camera and DVR and is due to an imbalance in the power loading of the two systems. The potential difference cause a loop current to flow between the two inter-connecting equipment and as a result any interfering source such as 50 Hz mains power is picked up by the ground current flowing through the braiding of the coaxial cable instead of it acting as a shield to protect the interfering source from entering the coaxial cable.

    The 50 Hz mains power or hum is then passed on along with the wanted video signal from the camera and displayed on the monitor as a rolling hum bar

enter image description here

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  • \$\begingroup\$ thanks for nice info, Russell. Would you mind checking my update in the question to see which way is applicable in my case? I am not sure and I am afraid to blow up the instruments! \$\endgroup\$
    – Dumbo
    Commented Dec 15, 2011 at 14:52
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Answering the question: don't isolate the ground. But also consider using separation transformers/baluns.

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  • \$\begingroup\$ Can you please explain a bit more what do you mean by separation and baluns? \$\endgroup\$
    – Dumbo
    Commented Dec 15, 2011 at 17:10
  • \$\begingroup\$ Baluns - look at my links above - that's what they probably are \$\endgroup\$
    – Russell McMahon ♦
    Commented Dec 16, 2011 at 10:25
  • \$\begingroup\$ Yep. The pic above shows signal isolator, which is probably a 1:1 transformer inside. That transformer is called balun. \$\endgroup\$
    – Tomas D.
    Commented Dec 16, 2011 at 12:56

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