Signal received only when transmitter is probed - RF Cafe Forums
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Post subject: Signal received only when transmitter is probed
Unread postPosted: Fri Oct 15,
2004 7:40 pm
I am using a Melexis transmitter (72031 - 915 MHz, FSK) in a remote sensing device. The
problem I have encountered is that the receiver will not receive the signal (so I assume the transmitter is not
transmitting) unless a wire of some sort is placed on the ROI pin of the transmitter. This wire need not be
grounded and has to be a couple inches long for the 'wire fix' to work. I found this 'solution' while using the
o-scope on that pin. My only experience with RF comes from the LINX line of transmitters and receivers so
designing with a non-'Plug-n-Play' device is giving me some problems.
Could this be a ground loop problem
or is it something more complicated. Any advice would be greatly appreciated.
Unread postPosted: Sat Oct 16, 2004 10:18 am
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
Without knowing what your baseline configuration is, it will be difficult to provide
much help. The wire on the ROI pin causing the Tx to work could be due to many things from a bad solder joint to a
faulty crystal or other component. Even the IC could be damaged from ESD. Could you please take a look at the
couple questions below to provide a little more insight into your situation to others and me?
I took a
quick look around the Melexis website and found that they offer a demo board for the TH72031.
Is that what you are using for experimentation? If not, does your PCB layout generally
follow the outline given on page 8 of the user's guide that comes with it?
You have probably already seen these, but there is a page on the Melexis website with application notes.
Here is the datasheet for the part if anyone else cares to offer
What are you using for a receiver? If it
is one of the Melexis devices, are you using the recommended PCB layout for it?
Finally, do you have
aspectrum analyzer available for troubleshooting?
Kirt Blattenberger :smt024
Unread postPosted: Tue Oct 19, 2004 2:35 pm
transmitter I am using the circuit shown on the Melexis data sheet for the EVB with the same values for all
components although the physical layout is quite different to fit on my PCB. On the receiver side I am using the
Melexis EVB (TH71111) just to ensure the transmitter is operating properly.
As far as the spectrum analyzer
goes, I don't access to one.
Thanks for the reply,
subject: Problem Resolved
Unread postPosted: Wed Oct 20, 2004 3:36 pm
For anyone interested...
The problem was indeed a simple one. The 'wire fix' was simply adding a certain amount of capacitance to the
circuit and thus tuning the frequency to a value within the bandwidth of the receiver. In the end, with a little
trial and error, it took changing a few capacitors to different values (quite different from the data sheets) and
now, all is well.