Shield Current Induced Noise

The Rane Library has many interesting audio relaed technical documents. The most famouse of them is Note 110 Sound Systems Interconnections - it is a classic. Just few days ago I happened to look around at The Rane Library, and found some really interesting material related to audio systems and ground loops:

Grounding and Shielding Audio Devices document points out that the use of entirely balanced interconnection with both ends of the shield connected to chassis ground at the point of entry provides the best available performance. The document goes through several grounding practices. The Audio Engineering Society is developing a recommended practices document which also condones chassis-grounding balanced shields, among other things.

Pin 1 Revisited document tells that cable shields are essentially an extension of the shielding enclosure of equipment, and they should be connected directly to that shielding enclosure. To make equipment cheaper to build, manufacturers started connecting cable shields to the circuit board’s common trace, then took that trace to the chassis. The problem is that any voltage drop across the wiring that is common to both the shield current and the circuit’s path to ground will be injected into the audio circuitry.

SCIN: Shield Current Induced Noise tells about testing audio cable shielding properties on different cable types and different frequencies. The article also covers induced cable current effects. The article details this testing arrangement for testing SCIN:

 

3 Comments

  1. Tomi Engdahl says:

    Optimize mixed-signal Circuit/PCB design using noise modeling, part 1
    http://www.edn.com/design/pc-board/4458816/Optimize-mixed-signal-Circuit-PCB-design-using-noise-modeling–part-1

    Earth-ground noise sources

    Earth-ground noise is very difficult to quantify since it varies greatly depending on the application and length of cable runs. In order to start a path to modeling this type of problem, it helps to visualize RFI current paths to optimize shielding, ground connections, and ferrite placement. Earth ground connections within an instrument cabinet are not the concern here, although it can be an issue for circuits sensitive to signal levels below 10mV. Here we are considering ground connections a few feet or more apart. This includes two devices that may communicate with each other, but are plugged into different power strips. Connections less than 10m apart will likely not have much of a low frequency voltage difference (f 50V), so the standard caution to watch for ground loops with very distant connections apply. Be careful of rules of thumb, as shorter runs (<15m) may require shield grounding at both ends to address magnetic coupling concerns.

    A rough estimate for this type of noise model can start with amplitudes of 1-2 volts P-P at frequencies up to 100 MHz in series with a voltage source at 50/60Hz. The goal here is to provide as much immunity as can be afforded given cost and space, rather than a specific amount of attenuation unless you have already made field measurements of the phenomenon to gauge the voltage differential and spectral content.

    In order to quickly answer the question of "is a noise source going to affect my circuit?", we need a means to estimate what noise source amplitudes would be acceptable for most systems.

    Reply
  2. Tomi Engdahl says:

    Audio RF interference and ground loops
    https://www.youtube.com/watch?v=ezBLZjwpops

    A video tutorial on the causes and remedies of radio frequency interference and ground loop hum in the project and home recording studio.

    Reply
  3. Tomi Engdahl says:

    Case study: Why did an industrial controller fail the radiated immunity #test at numerous frequency bands? #TBT #interference #EMC #CableShield

    Case study: radiated interference to industrial controller
    https://www.edn.com/case-study-radiated-interference-to-industrial-controller/?utm_content=buffer03cba&utm_medium=social&utm_source=edn_facebook&utm_campaign=buffer

    As an EMC consultant, I seem to be running into more and more issues with ESD and radiated susceptibility. I believe this is due to the fact noise margins are gradually being reduced as supply voltages move from 5 to 3.3 to 1.8 to 1.2 volts. In addition, IC chips are scaling down in size, and quite frankly, designers still don’t understand basic EMC design principles, as I wrote up recently in an editorial for Interference Technology’s 2014 Test & Design Guide

    Generally, the first thing I like to do is to sniff around with a near field probe and current probe to get a feel for any radiated emission issues. Finding nothing major, the project engineer demonstrated how he could affect the controller using just a Family Radio Service (FRS) walkie talkie from about 10 feet away. I recently measured a typical FRS radio at a 1m test distance and it read about 2V/m. Using Equation 1, at 3m (about 10 feet), we’re talking just a 1.3 V/m field strength, where I’m assuming the actual power output from the FRS radio is 0.25W, the antenna gain is 0.7 and the distance is 3m.

    We actually performed most of the testing using that FRS radio. Initially, though, the resolution using the radio was too coarse, so a near field probe was connected to an RF generator, tuning it to one of the failing frequency bands (Reference 6). By probing around, we narrowed the issue down to one of several cables running through a mechanical arm on the machine.

    A shielded box with several cables running through grommets. Penetrating a shield with a cable without terminating the shield allows RF interference into the enclosure.

    we discovered the designer had failed to connect the cable shield! Once the cable shield was bonded to the chassis structure at both ends, the controller was completely immune to RF signals.

    It’s my experience that many designers seem unsure how and where to connect cable shields. I’m simplifying somewhat, but connecting the shield at one end provides a good E-field shield. Connecting it at both ends (to the same structure) provides a good H-field shield. Most digital circuitry relies on low impedance, low voltage switched currents. Therefore, it’s more important to shield for the resulting H-fields. On the other hand, things like switch mode power supplies utilize high impedance with switched high voltages and so E-field shielding is a practical solution. Additionally, connecting each end of the shield to two differing potentials – for example, one end to digital return and one end to chassis – can introduce a potential difference which can inject high frequency switching noise into the signal wires.

    There’s an additional point to be made regarding cable shields and that is the type and quality of the shielding material. Some less expensive cables use loosely formed shielding, with distributed gaps along the length. These should be avoided, due to poor shielding effectiveness. More expensive cables have a tighter weave on the shield with correspondingly better shielding performance.

    In conclusion, it turns out that most of the client projects in which I’ve been involved that fail one, or more, EMI tests are due to basic design issues, such as poor routing of clock traces, penetration of I/O cables through shielded enclosures, and poor termination of cable shields. For more on shielding and bonding, check the references.

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