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Re: [Discuss-gnuradio] Car alarms and garage door openers

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Car alarms and garage door openers
Date: Tue, 06 Jun 2006 11:34:52 -0700
User-agent: Mozilla Thunderbird 1.0.8-1.1.fc4 (X11/20060501)

Michael Milner wrote:

After the Wired article today, I've received a couple of email from
people who are concerned that the USRP could be used to clone their
keyfob transmitters for car alarms and garage doors.  I'm not concerned,
since there are already many ways to do this (just check the back of
pupular science magazine).  However, I am curious about it.  I know that
we can capture and play back any rf signal.  The question is whether
that replayed signal would result in the door being unlocked.  I was
under the impression that most of those systems allow an unlock code to
only be used once, but does anyone out there know for sure?


I just took a look at my car's keyfob with the USRP RFX400 board (it seems
to transmit around 433.923MHz.  It is using FSK modulation, a few tens of
kilohertz deviation.  It demodulates pretty well, but the centre frequency
isn't very stable (Is there any way to automatically detect the centre
frequency within a range?)

Just eyeballing the data with the scope, it seems to change with every
keypress.  It's hard to really see what the data is.  Is there anything in
GnuRadio that will let me measure the pulse widths?
Thanks for looking into this. I think that most keyfobs use SAW oscillators instead of crystals to save money. This results in very bad frequency drift, which the receiver will need to compensate for.


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