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802.15.4 RF issues

Discussion in 'Electronic Design' started by linnix, Oct 27, 2012.

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  1. linnix

    linnix Guest

    We are using the Microchip MRF24JA40 802.15.4 RF transceiver,
    might look into Atmel's AT86RF231 and Freescale's MC13202 as well. We
    have one working board talking to a Microchip pre-made module, but two
    non-working boards. The only difference is the crystal. The working
    one is 5 mm SMD and the non-working one is 11 mm 2 pins. Could it be
    so sensitive to crystal selection? Do we need to pre-screen them for
    productions?

    The Freescale chip allows crystal calibration, but won't solve drift
    problem. My non-working board detects signals occasionally, but not
    good enough to pull in any packet. It seems to be drifting in and out
    of the channel.

    Also, the 1/4 length for 2.5GHz is 3cm. Most WiFI antenna seems to be
    bigger. Are they 3/4 length? Would 1-1/4 be better? We are just
    using a wire coiled on top of the PCB.
     
  2. Guest

    a different crystal will probably need different capacitors to be
    spot
    on frequency. Try measuring the frequency, 802.15.4 requires
    something
    like +/-40ppm
    I believe if you get the caps right wor the crystal you should able
    to
    stay with in +/40ppm over temperature and aging
    all kinds of different antennas as long as it is matched I'd assume
    what
    ever their put on give an acceptable performance

    but if devices are close I don't think it matters much

    I worked on developing the freescale 802.15.4 stuff but that was many
    years ago

    -Lasse
     
  3. linnix

    linnix Guest

    The non-working one says 20ppm, not sure if it's stability or
    tolerence. However, the cheap stuff (10 for $1.95 including shipping
    from Hong Kong) might be fake. The working one (80 cents + $3
    shipping from digikey) is actually 30ppm. Will try to order more test
    crystals later.
    I have a 9cm wire talking to the Microchip's PCB E antenna. My signal
    (on the devices) is much weaker than the Microchip module
    (coordinator), but seems to be OK.
     
  4. Guest

    my point is that for each different type of xtal you need to measure
    the resulting frequency and adjust the caps, it can be much more than
    40ppm
    off
    -Lasse
     
  5. linnix

    linnix Guest

    Yes, will have better layout for next version.
    They suggested 4 layers with 4 ground plane (Digital, Analog, Crystal
    and RF). But that would be too expensive. I am cutting corners with
    2 layers. Perhaps i am just lucky with the one working board.
     
  6. linnix

    linnix Guest

    Until i get the 2.5GHz frequency counter, i just have to keep trying
    different crystals and caps. I wonder if it's because of the ceramic
    SMD vs. metal case HC-59 crystal, in terms of para. caps.
     
  7. Guest

    you can just measure the xtal frequency

    -Lasse
     
  8. linnix

    linnix Guest

    Would it be possible to temperature compensate the crystal with the
    MC13202? That might force us to switch part.
     
  9. Guest


    http://cache.freescale.com/files/rf_if/doc/app_note/AN3251.pdf

    seems to indicated that is wouldn't be necessary but with the
    switchable load caps and temperature measurement I guess you could

    -Lasse
     
  10. Joerg

    Joerg Guest


    Four (!) different ground planes? That is usually a recipe for disaster.


    As Lasse wrote, check for correct burden caps. They may very well have
    to be different between crystal versions. If the burden caps are wrong
    oscillation start may be recalcitrant and your frequency can we off.
    Easy way to check if you don't have a good frequency counter: Program a
    timer to spit out 1MHz. Then borrow a shortwave receiver, pull in WWV or
    WWVB or 5, 10 or 15MHz, listen to the beat. If the beat is in the kHz
    range you won't be a happy camper with channel lock up at 2.45GHz.
     
  11. Make sure the board is cleaned properly. The No clean flux can cause
    problems for crystals, especially under the crystal.

    Cheers
     
  12. Oliver Betz

    Oliver Betz Guest

    where "just measure" means using a high impedance (low capacitance)
    probe or an output not connected to the crystal.

    Oliver
     
  13. Oliver Betz

    Oliver Betz Guest

    :

    [...]
    could you explain this further, what kind of influence are you talking
    about?

    Oliver
     
  14. Oliver Betz

    Oliver Betz Guest

    Jon Elson wrote:

    [...]
    hundreds of ohms between nets will kill most circuits, won't they?

    If they don't kill them immediately, they will do slowly by
    electrochemical migration.
    Cleaning boards is a demanding task, especially if they are soldered
    with "no clean" flux. No clean often means "no chance to clean".

    Oliver (still curious what Martin meant)
     
  15. Oliver Betz

    Oliver Betz Guest

    Jamie wrote:

    [...]
    I think the air gap between the case and the board will contribute
    much more uncertainty.

    Oliver
     
  16. Guest

    I have not been followed this discussion with great attention, but
    there are several issues related to PCB material humidity and
    temperature dependencies in oscillators.

    When building free running HF oscillators (e.g. VFOs) never use two
    (or multi) layer constructions near the LC resonant circuit. The stray
    capacitance between the resonant LC components and the PCB ground
    plane affects the frequency. Unfortunately, this stray capacitance
    varies with the air humidity and temperature, which affects the
    dielectric constant of the PCB material and hence affect the
    frequency.

    For simple oscillators in the UHF/microwave range, a free running
    oscillator made of 1/4 wavelength PCB traces are extremely sensitive
    to these issues.

    Anyway, one should remember that a frequency drift of 1 Hz at 25 MHz
    is 100 Hz at 2.45 GHz. I was once debugging a GHz signal source based
    on some VHF overtone crystals and wondered, why the frequency was
    shifting every few seconds. I finally discovered that I was breathing
    on the crystal, that cased the frequency drift :).

    When using HF fundamental or VHF overtone crystals as a reference to
    frequency multipliers or PLLs, you really need to pay attention to the
    PCB material and layout around the crystal oscillator.
     
  17. Oliver Betz

    Oliver Betz Guest

    Hello Jon,

    not announcing a Followup-To: is bad.

    So again for c.a.e:

    hundreds of ohms between nets will kill most circuits, anyhow.

    If they don't kill them immediately, they will do slowly by
    electrochemical migration.

    Cleaning boards is not easy. No clean often means "no chance to
    clean".

    Oliver
     
  18. linnix

    linnix Guest

    Regarding 2.5GHz test. If i can test the 16M/20M clock, why brother with 2..5G?

    Not Zigbee. Just native 802.15.4. Zigbee requires at least 20K code, which would be difficult to fit in a 16K µC.
    Most 802.15.4 chip set docs are available without NDA. I read the spec forMRF24J40, AT86RF231 and MC13202 several times. Unfortunately, only the MC13202 provides a Clock Out interface based on the crystal.
     
  19. linnix

    linnix Guest

    There is a separate Clock Out (16M, 8M, 4M or 23K) pin, in addition to the two crystal pins. I can just feed this as the µC clock input, and calibrate it again an external low-freq source. I guess Motorola Corp (sorry, Freescale) has more experiences in making RF chips.
     
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