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How can I measure conducted RF ?

Discussion in 'Electronic Design' started by Roland, May 21, 2004.

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  1. Roland

    Roland Guest

    Hi

    I have a circuit board and 5W VHF radio in a plastic enclosure. When I key
    the radio is sometimes screws up operation of my DTMF decoder/ FSK chip
    and adds noise to the synthesized audio I am sending out over the radio.

    Is there a way to measure the conducted RF on my board. When I try looking
    with my scope, there is RF on every trace including ground. My scope leads/
    scope probes, everything has RF on it.

    I have up to 20 lines coming in from sensors,battery,radio....


    Roland
     
  2. James Meyer

    James Meyer Guest

    OK, suppose you *could* measure the conducted RF. The next step would
    be to reduce the conducted RF.

    Why not take the easy way out and just start by reducing the conducted
    RF without measuring it?

    The first step would be to put both the radio and the circuit board into
    individual *metal* boxes. Then both boxes could go in your plastic box. That
    may be enough to cure the problem and you won't have to measure anything.

    Jim
     
  3. Paul Mathews

    Paul Mathews Guest

    Just do a google search on 'EMI sniffer' and you'll turn up lots of
    info on how to build your own. Very easy. Very effective.

    Paul Mathews
     

  4. Thanks Paul

    I will try building it. Here is the link if others are interested
    http://www.reed-electronics.com/ednmag/archives/1998/060498/12DF_04.pdf


    As far as putting everything in a metal box, I tried it with limited success
    .. I think my problem is that the RF is conducted in the wires and gets in
    that way. I could add ferrite beads/ caps to all lines and better shielding
    but it gets costly. So I want to try and find the worst offenders first.


    Roland DeLuca
     
  5. Think about what you are doing. You are trying to measure a electric or
    magnetic fields, without your measurement device becoming either a
    source or sink.

    The sniffer probe may work, but when you move it near the board, it too
    may well affect any measurement, or pick up electric field radiating off
    one part of the board, and put it on another part.

    It would be nice if there were some kind of aerosol LCD mist that could
    be sprayed, and stoboscopically illuminated with modulated laser light
    to image the RF.

    Practically, the best thing is to design or shield. Keep a ground-plane
    near the board for the RF to have a (relatively) low-capacitance path
    to. If you have coils, see they are shielded. Put a ground plane under
    your sensitive circuit, and ground the plane with short wires at many
    points until your troubles go away.
    Think of hot RF traces as glowing. You don't want to put anything near
    that will reflect the light around, you want to absorb it.

    --
    Scott

    **********************************

    DIY Piezo-Gyro, PCB Drill Bot & More Soon!

    http://home.comcast.net/~scottxs/

    **********************************
     
  6. Paul Mathews

    Paul Mathews Guest


    The other posters are right to suggest that EMI can be baffling, that
    filters should be put near the boundaries of the enclosure, and that a
    comprehensive design to minimize the generation and transmission of
    EMI is best. However, you can learn a lot by sniffing, and it's not
    true that things change so much when you use a probe that it's
    useless.

    There are many books and publications on the subject, some of them on
    the web. You might start with a book by Montrose:
    http://www.amazon.com/exec/obidos/t...f=sr_1_1/102-3016581-3485711?v=glance&s=books

    Meanwhile, here are a couple of universal rules for EMI minimization:

    All signal paths have a corresponding 'return path', which is often
    thought of as flowing in a 'grounding system'. These return paths are
    just as important as the the signal conductors themselves. In
    general, signal and return paths should be close together at all
    points, thereby minimizing magnetic loop area. This is the single
    most important aspect of EMI minimization, both for susceptibility and
    radiation minimization.

    Shielding is generally effective to the extent that shielded circuitry
    is completely enclosed in a continuous conductive structure. Any
    openings or conductors exiting this structure require special
    consideration.

    Have fun.

    Paul Mathews
     
  7. Terry Given

    Terry Given Guest

    Hi Roland,

    In practice it is very difficult to measure such RF interference (be it self
    inflicted or otherwise). As soon as you attach a dangly wire (read as:
    aerial), you will change the behaviour of your circuit, often making it
    worse.

    In general you would be better off to read a book or two on EMI suppression
    techniques, then implement all the easy-to-do solutions. Henry Ott's book is
    excellent, but I prefer these two:

    Controlling radiated emissions by design, Michel Mardiguan, and
    Controlling conducted emissions by design, John Fluke jr (all this book
    really says is include all of the parasitics in your design, often easier
    said than done ;)

    Step 1 is generally to throw out your pcb layout, and do it again with a
    solid 0V plane - absolutely no slots! this is usually quite achievable on 2
    layers - I once had a 1500W PFC pcb re-designed in this way. we deleted the
    existing 0V traces, which freed up lots of room on the pcb. we then moved
    every bottom-layer trace to the top layer that we could - about 90% of them
    were simply layer changes! Then we poured an 0V plane, and spent about 4hrs
    reducing the size of the inevitable slots (where there are either traces on
    the bottom layer, OR adjacent vias which form a slot (these just get moved
    apart a bit, to keep the slots small, say 1cm max). The entire re-layout
    took our PADS guy 1 day, and the PFC worked beautifully afterwards (it used
    to explode), all without changing a single component!

    This is usually enough to solve your problem completely, all without
    measuring a damn thing.

    Step 2 is then to band-limit all input and output signals, right at the
    connectors - capacitors are the go here. If a signal is low-speed, then
    filter the shit out of it - dont provide more bandwidth than you have to.

    Then you can look at cables - twisted pair for H-field attenuation, an
    electrostatic screen for E-field attenuation, etc.

    Then start thinking about ferrites etc, which are usually used to try and
    compensate for lousy pcb layout - invariably it is far cheaper to just re-do
    the pcb layout.

    Hell, often a 4-layer PCB is a cheaper solution than wanky cables, ferrites
    etc - I did an EMI analysis on a bearing temperature monitor for a 400kW
    rock crusher (AC motor drives generate metric shitloads of EMI, they werent
    using $80/m screened cables etc) a while back. The controller was a 2-layer
    pcb with a long, skinny 0V trace, and it sucked majorly. The component
    routing was sufficiently complex that a 2-layer pcb was unlikely to work
    well, and would take a bloody long time to do. We deleted the 0V and Vcc
    traces, added 2 internal layers and made mid-layer1 = 0V plane, mid-layer2 =
    Vcc plane - again, only took an hour or two. It added about $10 to the cost
    of the controller (2x original pcb price) but instantly solved dozens of
    weird intermittent problems. Again, no measurements were taken. It was also
    about $10,000 cheaper than re-wiring the ac motor drives with appropriate
    cabling to reduce the noise source. Needless to say, the customer was happy.

    You are dead right about the cables - internal radiated noise invariably
    enters your cables, shoots out the box as conducted EMI, then radiates away
    again - this is why the filters etc. should be placed as close as possible
    to the edge of the box. Often when you place a ferrite on a cable, it turns
    conducted emi into radiated emi, which leaps around the "filter" and back
    into the cable - especially if your cable then runs across your pcb.....

    cheers
    Terry
     
  8. James Meyer

    James Meyer Guest

    What does potassium chloride have to do with conducted RF?

    Jim
     
  9. Terry Given

    Terry Given Guest

    Nicely put, Paul. Montrose's book is IMO pretty good - I have read it but
    didnt buy it, as I already have all the info elsewhere.

    How about a nice, terse summary:

    Current flows in loops. minimise them.

    Alas, the ubiquitous "ground" symbol in schematics, seems to be designed
    specifically to hide this fact from all and sundry, who seem to have
    forgotten KCL.

    Cheers
    Terry.
     
  10. Terry Given

    Terry Given Guest

    think dead white guy. I said KCL not KCl.

    Terry
     
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