Computing stuff tied to the physical world


Picking up RF noise

In Hardware on Apr 14, 2010 at 00:01

In yesterday’s OOK scope, there were many pulses at around 880 µs and 1100 µs which I couldn’t explain:

Screen Shot 2010 04 11 at 152609

Here’s another graph from new readings, same sketch, same ELV 868 MHz receiver:

Screen Shot 2010 04 11 at 16.39.16

That’s clearly much better. The difference? It’s all about how you hook up the RF receiver!

This was used during the first set of readings:

Dsc 1337

And this was used for that second graph:

Dsc 1336

So the receiver is picking up a lot more noise from the Carrier Board!

It’s not that surprising, in hindsight – the Carrier Board has a lot of traces running all over it it to hook up the numerous different signals on the board.

Note also that it took 20% less time to pick up the roughly 1,000,000 pulses for that second graph. My hunch is that the receiver’s AGC is ending up being set more sensitive without the spurious noise picked up from the circuit next to the OOK receiver.

Anyway, that’s one mystery solved.

Is the Carrier Board a bad idea? Not so fast. Here’s the ELV 433 MHz receiver, plugged directly into a JeeNode:

Screen Shot 2010 04 11 at 17.19.58

And here it is again, with a Carrier Board between receiver and JeeNode:

Screen Shot 2010 04 11 at 17.28.43

So at 433 MHz, it’s now attenuating the noise from the JeeNode.

Unfortunately, it gets even more confusing: look how few pulses the 433 MHz Conrad receiver picks up when plugged directly into a JeeNode during the same amount of time:

Screen Shot 2010 04 11 at 17.38.21

I’m inclined to go with the Conrad receiver for testing BTW, since it seems to pick up all the important pulses. Much simpler to debug with less noise!

Looks like I’ll have to be real careful while comparing results and drawing conclusions. And use some sort of ground plane or shield in the final setup.