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Separation of RF board and microcontroller board caused problems

Discussion in 'Electronic Basics' started by Dummy, Sep 1, 2004.

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  1. Dummy

    Dummy Guest

    As an attempt to shrink the size of radio, RF and microcontroller
    board are separated with a flex PCB, instead of residing on the same
    PCB. RF ground is connected to microcontroller ground through the flex
    PCB. Besides ground, the are still a number of pins connected from RF
    to microcontroller board. Both PCBs share the same 7.5V supply.

    Separation of RF board and microcontroller board has caused a problem
    which certain amount of distortion was measured from the transmitted
    DTMF tone. A check on the radio without RF and microcontroller
    separation showed that the distortion level was far lesser, 13% vs 5%.

    The distortion disappeared when separate power supply was used for RF
    and controller board. Can somebody shed some light on this?
     
  2. Welcome to the world of mixed signal design where GND is not GND,
    but a few mV above plus some noise on it depending on the spatial location.
    To start with, you first have to know what exactly you're measuring
    where and how.

    Rene
     
  3. peterken

    peterken Guest

    What thickness of copper was used on the flex board ?
    What's the power consumption of the radio-part ?

    Possible solutions (one or all):
    Use a stranded connection for ground connection between both boards, better
    ground connection between both boards
    Use extra caps on the power lines on the boards (say 1nF for RF in parallel
    with say 47uF for power stability)

    Ultimate solution
    Revise the layout, putting power-consuming parts closer to power supply
    (or power the boards from the side of the most power-consuming end)



    As an attempt to shrink the size of radio, RF and microcontroller
    board are separated with a flex PCB, instead of residing on the same
    PCB. RF ground is connected to microcontroller ground through the flex
    PCB. Besides ground, the are still a number of pins connected from RF
    to microcontroller board. Both PCBs share the same 7.5V supply.

    Separation of RF board and microcontroller board has caused a problem
    which certain amount of distortion was measured from the transmitted
    DTMF tone. A check on the radio without RF and microcontroller
    separation showed that the distortion level was far lesser, 13% vs 5%.

    The distortion disappeared when separate power supply was used for RF
    and controller board. Can somebody shed some light on this?
     
  4. Joerg

    Joerg Guest

    Probably the ground trace on the flex isn't wide enough and now the
    power trace acts as a ground connection as well. This can introduce
    noise if the power supply isn't very clean or becomes modulated by heavy
    load changes. Such load changes can happen when, for example, memory
    banks on the controller are read or written to.

    Bottomline you may have to add ground connections, ideally a plane
    although that isn't easy on a flex. Also, look at your power supply with
    a scope set to AC. Check it down to the millivolt level.

    Regards, Joerg
     
  5. Is it FM? What is the transmitter output power, and where is the
    antenna? Where is the circuit generating the RF output? Is this a
    transceiver ... receiver? Please give more information. Maybe you can
    get more specific help.

    You may be dealing with issues like injection locking. Changing layout
    may not be enough.

    Regards,

    Frank Raffaeli

    http://www.aomwireless.com/
     
  6. Dummy

    Dummy Guest

    I've checked the power supply with scope before. There were some
    noises on the line, with Vp-p ranged from 150mV to 200mV. These noises
    were found on 'normal radio' (without separation of RF and
    microcontroller) as well. But amazingly, there was no DTMF distortion
    seen in 'normal radio'.

    A tweaking on audio PA to reduce its gain helped, but the audio volume
    was slightly low from rated audio. Distortion was mostly seen when
    audio knob was turned to max.

    Audio PA resides in microcontroller board.
    When audio PA supply (7.5V) was connected with separate external
    supply, no problem was observed. By disabling the speaker from audio
    PA, there was almost no distortion seen at DTMF tone.
    The crux is, audio PA supply. Additional info is the audio PA circuit
    was similar with 'normal radio'. Can the problem occurs when RF and
    controller share similar supply? In 'normal radio', they are sharing
    same supply as well, but no problem was seen.
     
  7. Joerg

    Joerg Guest

    Hi,

    150mV is a whole lot of noise. When you split the portions with a flex
    and there isn't too much ground in the flex then part of the ground
    return will inevitable be via the supply. That's when some of this 150mV
    noise can be modulated back onto RF signals, clocks, PLL loops and so on.

    Can you try to regulate the voltage going to the DTMF circuitry, or to
    all the circuitry except for the PA? The idea would be to feed the PA
    raw power and then the rest of the unit gets clean regulated power. You
    can use a low drop-out regulator so you don't lose much in voltage. It
    looks like there is some noise caused by the PA stage that ends up on
    the supply rail and then enters the rest of the circuit. Alternatively,
    you could try to decouple the PA but that's harder. There you either
    need lots of capacitance and there may not be enough space for that, or
    some kind of nifty current regulator that evens out the spikes being
    sent from the PA.

    One test I'd do for sure and it's easy: Lay a short and very wide copper
    path, for example 3M copper foil, from one board to the other and see
    whether the DTMF distortion drops to acceptable values.

    Regards, Joerg
     
  8. Dummy

    Dummy Guest

    Finally, the root cause of the distortion had been found.
    There were four screws on the radios. The screws serves the purpose of
    holding radio PCB and chassis together. When one of the screws was
    removed, somehow, no distortion was seen at DTMF tone! That screw
    (contacted to ground) resided very near to 7.5V supply line in third
    layer of PCB layout. But I was still a bit bewildered on what happened
    actually. Could you please shed some lights?
     
  9. Rich Grise

    Rich Grise Guest

    The four mounting posts aren't coplanar, so when you tighten the fourth
    screw, it warps the board.

    Cheers!
    Rich
     
  10. Dummy

    Dummy Guest

    I don't think that's the problem. When washer or insulator was mounted
    on the screw ground pad, no DTMF distortion was seen after the fourth
    screw been tightened.
     
  11. Joerg

    Joerg Guest

    Hi,
    That sounds like either the screw may be touching some traces down in
    the PCB or, more likely, you have a serious case of a ground loop. Do
    both boards have a full ground plane?

    Regards, Joerg
     
  12. Dummy

    Dummy Guest

    The screw only touches the ground pad at first layer.
    No full ground plane is used. Insulation of screw from ground pad or
    removal of screw can solve distortion problem. It's still a mystery to
    me.
     
  13. (in <>) about 'Separation
    of RF board and microcontroller board caused problems', on Sun, 5 Sep
    2004:
    It's almost certainly a ground loop. But exactly what is happening
    depends on the details of the schematic and PCB layout, which is not
    practicable on the net.
     
  14. Joerg

    Joerg Guest

    Hi,
    I agree with John, this sure sounds like a ground loop. It can be very
    hard to make a RF/controller combination work reliably without a full
    ground plane. I would probably spring for a relayout with a full ground
    plane even if that means one or two more layers on the boards. The other
    issue with this unit might be EMC, especially with a plastic enclosure.

    Regards, Joerg
     
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