GPS External antenna detect issue

Andy Green andy at
Mon Apr 28 20:53:34 CEST 2008

Hash: SHA1

Hi folks -

While trying to make DM2 GPS work good, I became annoyed at the range of
behaviours the GPS unit and finally spent the whole latter part of the
day meddling.  I don't think I found where the problems came from, but I
found something that is less good than it should be.  GPS stopped
working altogether on my boards at the moment (unrelated to ripping the
cans off the GPS section I believe) so I don't know where I got to.

U7605 seems to be planned as an "automatic external antenna detect"
thing, when the external antenna starts drawing current I guess the idea
is to switch VCONT to select "OUTPUT" 1, the external antenna RF.

However nobody cleaned up the output of the current sensor, eg, with a
schmitt trigger to stop oscillatory behaviours for example.  The output
driving net VCONT shows 810mV with no external antenna connected.  Also
the output is not so low impedence itself yet drives PNP base Q7602 by
10K, affecting the source.

RF mux U7606 (UPG2012TB) that actually selects the GPS antenna:

specifies low level input selecting "OUTPUT" 2 is min -200mV, typ 0 and
max 200mV.  The other active level is specified min 2.7V, typ 2.8V, max
3.0V.  So you have to drive that chip hard to 0V or the power rail to
select either input.

But at our normal situation with internal antenna and no load on
external antenna, we drive VCONT to 810mV (26% Vdd).

- -Andy
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