Powering a combination circuit, hacking a GPS antenna

Discussion in 'Embedded Systems and Microcontrollers' started by clathe, Mar 4, 2007.

  1. clathe

    Thread Starter New Member

    Mar 4, 2007
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    I am creating a GPS datalogger following this circuit, but I have a standalone GPS board and a 5v lithium battery from a backup GameBoy charger. Question is: can I power both the GPS (with active antenna) and the PIC circuit with the same source? Would I need some sort of filtering capacitor? Another question: I have an active antenna that I took out of the housing and would like to cut down to about a foot in length. Do you think I could take the SMB connector off of the GPS board and just solder the antenna cable directly to the board? I can provide pictures if needed... thanks in advance!
     
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Why remove the SMB connector? What power does your circuits need and what can that battery deliver? Filtering for what?
     
  3. kender

    Senior Member

    Jan 17, 2007
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    Are you concerned about the EMI that might come from the PIC to the GPS unit?

    Taking the antenna out of the housing is not alwaysa good idea. The antenna might have been tuned with the housing. Removing the housing could move the center of the band of an antenna by some 5-20MHz. If the band is sufficiently wide though, the receiver would still work, but the signal availability might degrade.

    It can be done. I've soldered the coax cable from the GPS antenna to the board before. Here's a picture of the internal board of a Marconi "Smart Antenna GPS". However, soldering the cable instead of using an rf connector (such as SMB) will degrade the signal availability somewhat.
     
  4. clathe

    Thread Starter New Member

    Mar 4, 2007
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    I didn't know if a common ground would usually be an issue, or if there would be a rippling problem... I planned on using a moderately sized capacitor (20uf or so) to help with this.

    I don't know much about it, but the housing was plastic, without any sort of connection to anything... it was just glued in.


    Does the shielding have to be soldered to the board at all? To the ground or anything? Sorry if these questions are kind of off-base, I'm learning all this stuff as I'm going along. :p
     
  5. clathe

    Thread Starter New Member

    Mar 4, 2007
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    I want to remove as much from the board as possible, as the connector sticks out quite a bit, and I would ultimately like to make this small enough to fit in a pocket. The battery pack provides a steady regulated 5 volts, as it is designed to replace a charger for a Game Boy, so it has to be a steady current... it can provide 5v at 800mAh, and is a 8000mAh battery pack (should last awhile) :p. I just wanted to make sure there wasn't an issue with common ground or rippling or anything of that sort.
     
  6. kender

    Senior Member

    Jan 17, 2007
    263
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    If you star-ground the PIC and the GPS (as oposed to bus-ground), it's very likely that PIC and GPS module on the same ground will work. Add an LC filter to the power supply of the GPS module. Also, try to avoid switch-mode power supplies - they generate noise that can prevent the GPS from working.

    In short, the antenna depends on a surrounding dielectric. Plastic has a different dielectric constant that the air.

    Shield of the coax cable must be soldered to the shield pads of the removed SMB connector.
     
  7. clathe

    Thread Starter New Member

    Mar 4, 2007
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    0
    At the risk of sounding simple, what does star-ground and bus ground mean? and an LC filter? :p I am learning as I go along.



    Makes sense... if the antenna signal is too degraded, I can replace the housing.


    Makes sense... thanks for the help!
     
  8. kender

    Senior Member

    Jan 17, 2007
    263
    0
    See attached drawing. Top is a bus-ground (which would be OK for digital up to a certain speed). Bottom is a star-ground. The key point of a star-ground is to connect the grounds of all the subsystem (I call them sub-grounds) at exactly one point. For example, circuit boards in the high-end instruments have separate analog and digital ground planes. Sometimes star-ground is called "totem pole ground". Star-ground is really a circuit construction feature, and many schematics don't indicate that there should be a star-ground.

    LC filter is a passive filter consisting of an inductor (L) and capacitor (C). Low-pass LC filters are often used for power filtering. Sometimes the inductor is replaced with a ferrite chip. You can find more than I can tell you by googling "LC filter". Be prepared to tweak the L and C values during the testing stage of your project after the circuit has been constructed.
     
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