Teach you how to easily complete embedded wireless debugging

I recently had the pleasure of working on a project involving an Arduino Uno, an Atmel ATmega328 microcontroller, and an RFM12B module, an ISM band FSK transceiver, connected using the SPI bus. As usual, there were multiple hardware and software issues to address. As you will see, the MDO4000 Mixed Domain Oscilloscope helped us find the root cause of the noise problem. The setup I used was as follows:

The following screenshot shows what happens during the packet transmission:

The orange trace labeled A is the RF amplitude vs. time, captured with a simple antenna connected to the input of a spectrum analyzer. The SPI bus uses digital logic channel probing to trigger a single shot acquisition on a sent SPI bus command. Since this is FSK frequency shift keying, ideally the RF amplitude should be constant. However, we can see that this is not the case. Below, we have added the frequency vs. time trace, labeled F:

The noise burst in the upper left is the frequency variation over time, just before the transmission. I used the squelch function to clear the noise: [page]

OK, FSK looks fine, but the RF amplitude vs. time shows significant crosstalk from SPI bus activity to RF transmitter power. I suspected ground and/or power rail bounce, so I used a TDP1500 differential probe and measured Vcc to ground:

Pretty sure there is about 1 Vpp of noise when the transmitter is turned on. Use the Wave Inspector knob to zoom in, and turn on cursors:

With the RF transmitter turned on, you can see an increase in noise on the power supply. Using cursors, I measured the noise period to be about 64 ns. This is consistent with the crystal. This tells me that the power distribution needs better bypassing, especially around 16 MHz. Since this is just a prototype board and there are no power planes, I wasn't too worried. I added multiple ceramic capacitors in addition to the electrolytic capacitors after the regulator IC and that fixed the problem.

Now, all that’s left is software debugging. It’s that simple!