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Signal Received Only When Transmitter is Probed - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

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.

Thank you,
Mike Walters


Kirt Blattenberger
Post subject:
Unread postPosted: Sat Oct 16, 2004 10:18 am
Site Admin
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings Mike:

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. ... E_EVB72031

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 an opinion.

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


Post subject:
Unread postPosted: Tue Oct 19, 2004 2:35 pm

For the 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,


Post 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.


Posted  11/12/2012

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