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Rules of thumb for inter-stage RF shielding?

Discussion in 'Electronic Design' started by Andy Iakovlev, Sep 1, 2004.

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  1. Hello:

    I am looking for some rules of thumb for RF shielding between the
    different stages. In the device I am currently designing there will be:

    1) a DDS chip, 100kHz - 175 MHz output;
    2) a fully-differential elliptic low pass filter, with 175MHz passband
    and 70dB rejection after 225 MHz; applied to the output of 1)
    3) a VGA with 50dB control range, applied to the output of 2)
    4) a fixed 25dB gain stage, applied to the output of 3)
    5) output level detector stage, applied to the output of 4)
    6) level-control integrator closing the AGC loop, between 5) and 3),

    and I need to choose a minimum number of shielded enclosure cells, to
    put the above into.

    Thanks in advance for your advice.

    -- Andy
     
  2. Joerg

    Joerg Guest

    Hi Andy,

    Much can be achieved using a full ground plane, clever parts placement
    and the avoidance of parallel traces. You can also bury traces within
    power and ground planes. My guess would be that shielding may be
    necessary within the stages of your elliptic filter because of the 70dB
    rejection requirement. The 50dB amp might also need a little shield
    between input and output. Often this can be a simple metal piece that
    acts as a wall. Full enclosure may not be needed although you can buy
    the parts for that.

    If you opt for little enclosures make sure to close them with a few
    solder spots since the simple clamp-on lids tend to work themselves
    loose on transport.

    Regards, Joerg
     
  3. I read in sci.electronics.design that Andy Iakovlev <smdg_remove-
    [email protected]_worldonline_this-also.fr> wrote (in <41361d75$0$29664$636a
    >) about 'Rules of thumb for inter-stage RF
    shielding?', on Wed, 1 Sep 2004:
    Are you thinking of making the cell walls from 10 mm wide tinplate
    strip? One strip around the edge, on each side of the board, with
    internal partitions of the same material soldered across? Simple flanged
    tinplate covers, probably soldered on unless you really want to give
    easy access.

    Assuming you also have to meet EMC requirements, you almost certainly
    need six cells. You might be able to put 3 and 4 in one cell, but with
    maybe 75 dB gain (you don't actually say what the maximum gain of the
    VGA is), I wouldn't risk it for the sake of two bits of bent tin.

    But the physical arrangement matters a great deal, Forget square; go for
    a long, thin layout, so that the peripheral shielding acts as a
    waveguide below cut-off. That is a big help in preventing the output
    getting back into earlier stages.
     
  4. Do you mean that the partition walls would be necessary on both sides of
    the PCB (most if not all the components will be SMD)? How big could be
    the area of the opening in the walls?

    I was to potentially go as far as splitting the board into the pieces,
    and placing each in a cell of an enclosure like the one shown on the
    pictures I've posted in a.b.s.e (Message-ID:
    <4136e5ee$0$8333$> , subject: "Shielded
    enclosure"). The second picture shows the 2 available profiles of the
    adjustable internal walls, and of the corresponding enclosure.


    But then I would face the problem of properly interconnecting the
    sub-boards. If less extreme measures were enough, that would be of
    course better!
    The maximum gain of the VGA is 13dB only.

    Thank you!

    -- Andy
     
  5. Hi Joerg,


    For the filter, do you think that the PCB - level shielding (groung
    tracks between the stages) would be enough, or some tinplate walls would
    be required?

    Thank you for the advice!

    Regards,
    -- Andy
     
  6. I read in sci.electronics.design that Andy Iakovlev <smdg_remove-
    [email protected]_worldonline_this-also.fr> wrote (in <4136ebc0$0$8339$626a1
    >) about 'Rules of thumb for inter-stage RF shielding?',
    Start with the walls, and eliminate them later if you can. Then you get
    two credits - a 'works first time' one and a 'cost improvement' one. The
    other way, you get 'long development delay' and 'cost uplift'. It's a
    no-brain choice.

    I'm tempted to say ALWAYS start 'fireproof' and then cut back, but some
    'fireproof' solutions won't cut back much, and the guy who sees that one
    bipolar can replace four chips wins out.
     
  7. I read in sci.electronics.design that Andy Iakovlev <smdg_remove-
    [email protected]_worldonline_this-also.fr> wrote (in <4136eb0c$0$8333$626a1
    >) about 'Rules of thumb for inter-stage RF shielding?',
    Yes. The electric and magnetic fields spread out from the surfaces. 10
    mm depth is convenient if you have room for it, but you can cut it down.
    Make sure the lid doesn't short-circuit anything, even if someone
    presses it in.
    Slightly smaller than you need. (;-) Seriously, the openings act as slot
    antennas, so provided they are small compared with the wavelength of the
    highest likely harmonic of any signals, they won't have a significant
    effect.
    I stopped taking a.b.s.e, but I'm now getting it to look at your
    graphic. I don't like the idea of sub-boards: they result in everything
    being interconnected by variable inductors (the interconnections).
    So MAYBE you can combine 3 and 4, but is it worth it to save one strip
    of tinplate that you can leave out WHEN/IF you find it's not needed.
     
  8. Yes, the input impedance and the load impedane are 124 Ohm (or much so).
    The VGA's maximum gain is about +13dB, the minimum - about -37dB.
    Could you expand a bit why? I am planning to use LMH6502 (pushing it a
    bit on the high freq. end).
    I'll try to evaluate, analytically, to which point they will see each other.
    Yea... much sport!
    Thanks!

    Regards,
    -- Andy
     
  9. Joerg

    Joerg Guest

    Hi John,
    I'll second that. Very good advice indeed.

    Regards, Joerg
     
  10. Joerg

    Joerg Guest

    Hi Andy,
    Take John's advice to heart, in previous post.

    The enclosure you posted as a photo on a.b.s.e. looks very good but you
    may be able to use tin walls on the board itself. You can, or at least
    used to be able to, obtain walls with a row of pins that are soldered
    into grounded vias. When adhering to a certain geometric pattern you
    could even get matching lids. But it has been a long time since I used
    these.

    With the enclosure shown it seems you would have to split the circuit
    board into section to fit into the individual compartments. That becomes
    very tedious. Also, if you do use an enclosure like the one shown don't
    rely on the snap fingers alone to hold the lid. Sometimes it is best to
    provide a few solder spots after the board is tested and ready to go.

    Last but not least: If you ever cut this tin stuff with metal shears be
    careful. I have some scars to show from the deep cuts this can cause.
    Also, be careful with beryllium copper fingers that some boxes may come
    with. Beryllium oxyde can be poisonous and it is usually not necessary.

    Regards, Joerg
     
  11. john jardine

    john jardine Guest

    Assuming your filter is a 'matched' LC design I'd have thought there's no
    way you'll keep a 70Db rejection if it's followed by 50Db of VGA. The VGA
    design would need to be very sophisticated to retain a firm resistive (say)
    50ohm input impedance over that amount of gain and frequency variation. The
    filter itself should also be a problem. If the inductors can 'see' each
    other then it's curtains. Sound like you'll need lots of tin plate to
    seperate the filter elements, which in turn will act with the tinplate to
    generate yet another filter response, which then can't be simulated and can
    only be prototyped on the bench.
    Essentially, lots of screening, lots of buffers and *lots* of bench work are
    called for.
    regards
    john
     
  12. john jardine

    john jardine Guest


    I was assuming discretes. My comment may not apply to the IC. (I'll grab a
    pdf).
     
  13. What I think I will do: I will plan for the walls, and provide for the
    necessary space & material, but won't install them in the first pass.

    At some point the effort starts to seem incommensurate, if not backed by
    the past experience! That's why I was looking for the advice on a
    reasonable starting point. There are 6 inductors in the filter (2 in
    each stage, it's a fully differential filter). Create 6 cells for a 7th
    order lowpass filter would seem to me too much... :)

    Thank you.
     
  14. Hi Joerg,

    I have looked up such walls (tin plated steel with pins) across a number
    of distributors - have not found yet. I would prefer to use some
    standard product rather than cut the walls myself. Would you have any
    pointer to such product?

    I foresee this as a set of inside-board walls, together with the
    perimeter walls, with a common cover. Would the contact of the inside
    walls with the cover be a serious concern? I.e. if there are no fingers
    there, the contact would be in a couple of points for each wall, with
    long narrow holes between the wall's border and the cover - elsewhere.


    Thank you.

    Regards,
    -- Andy
     
  15. I see. I have abandonned the idea of sub-boards for the moment.
    I agree. This would be a really very small overdesign.
     
  16. I read in sci.electronics.design that Andy Iakovlev <smdg_remove-
    [email protected]_worldonline_this-also.fr> wrote (in <4139a01c$0$31383$636a
    >) about 'Rules of thumb for inter-stage RF
    shielding?', on Sat, 4 Sep 2004:
    I didn't suggest cells within each building block; just one cell per
    block. but for the filter it would be wise to provide for one cell per
    stage, not one cell per inductor.
     

  17. I have done some computation. The source inductor is 1mm long, 0.8mm
    diameter. I have evaluated the ratio of the magnetic field strengths at
    a given measurement point and that at the center of the source inductor.

    The coupling coefficient between the source inductor and a similar
    inductor placed at the measurement point would be equivalent to that
    ratio (not bigger than 5 times that? I would need to do more computation
    to get more precision).

    For the measurement point at 3mm from the inductor's center axis and in
    the plane orthogonal to that axis going through the center of the
    inductor - the ratio is about 0.003. So looks like I should be able to
    place the 2 inductors of each stage of the filter (it's a fully
    differential version of a 7th order low-pass elliptic filter) as close
    to each other as 3mm without significant harm to the filter's
    characteristic.

    For the distance between the inductors of the successive stages, 5mm
    seems ok, as the ratio is then about 0.00083

    Does this sound correct?

    I will need to get the inductors and do some real measurement, of
    course, to confirm/deny this forecast.


    Thanks,
    -- Andy
     
  18. No, you did not suggest cells within each bulding block. I came from the
    exchange with J.Jardine that I might need walls within the filter -
    which provides kind of an extreme example (as I feel it). Sorry for any
    misunderstanding.
     
  19. No, you did not suggest cells within each bulding block. I came from the
    exchange with J.Jardine that I might need walls within the filter -
    which provides kind of an extreme example (as I feel it). Sorry for any
    misunderstanding.

    Regards,
    -- Andy
     
  20. Joerg

    Joerg Guest

    Hi Andy,
    Sorry but I don't since these were made in-house for my designs.
    Basically they are stamped for mass production and the vias need to be a
    bit larger than usual to accomodate the wider "pins", which really are
    just extensions of the metal.

    One source might be a ham radio supplier. Another solution that is a bit
    cumbersome in production would be wire loops in the board that connect
    to the ground plane and the shield pieces are then soldered to these.
    But that's not a nice way to do it.
    Fingers are fine, personally I'd just stay away from copper beryllium
    (the cheap versions won't likely have that anyway). But I'd add a few
    solder spots just to hold that cover in place. You'd be amazed what can
    work itself loose even from a tight fit while riding on a propeller
    aircraft or on a rough truck journey over washboard roads. You never
    know where your products will be shipped some day.

    Regards, Joerg
     
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