GPS External antenna detect issue / test recipes

Andy Green andy at openmoko.com
Wed Apr 30 19:15:22 CEST 2008


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Somebody in the thread at some point said:

| I tried to get an external antenna today but nobody sells them around
| here, so I ordered one for tomorrow.

It turned up, but it had the wrong connector.  I cut it off and soldered
it in the external connector for testing.

With this external antenna (TomTom 5A00.206) the current detector
outputs 2.15V into the antenna source mux, vs "Recommended operating
range" of 2.7V "min" on this source select signal.  So the select line
is out of this "recommended operating range" both ways.

They don't show the "on impedence" against Vcont level in the datasheet,
but half a dozen times, whenever they talk about isolation or insertion
loss they define Vcont as being "2.8V / 0V".

I also created a couple of canned UBX packets to control antenna
short/open detect, which are attached.  I used them like this:

echo 0 > /sys/class/i2c-adapter/i2c-0/0-0073/neo1973-pm-gps.0/pwron &&
sleep 1s && echo 1 >
/sys/class/i2c-adapter/i2c-0/0-0073/neo1973-pm-gps.0/pwron && stty -F
/dev/ttySAC1 -echo && cat ubx-noant-0 >/dev/ttySAC1 && cat -u
/dev/ttySAC1 | grep -v ^$

ubx-noant turns off short and open detect but leaves "Antenna supply
voltage control signal", ubx-noant-0 turns even that off.  I saw the
corresponding changes in antenna status report so I know these commands
were understood, but it made no difference to the very bad GPS
performance I see here.

To my regret forcing Vcont to 3.0V or 0V when on external or internal
antenna did not make any difference either.

But when I left the thing for 20 minutes, it did find one satellite and
the time.  Otherwise it finds nothing even on a nice external antenna
here at the moment.

- -Andy
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