Problem with RFM83C / RFM85

This message was edited 2 times. Last update was at 23/07/2015 14:12:07

Hello All,

I have one pair of RFM85 TX and FRM83C RX modules both at 433MHZ, and I have a strange problem:

Transmitter off

Receiver connected to a 5V bench power supply, and dataout to a scope. I have the output switching always at random frequency.
I have no RF noise on the environment (other 433Mhz mudules operate ok).

I had try with several RMF83C (I have 5) and all have the same problem.
Antenna is a 16cm wire, but I had try WHIP also, with same problem.
Had try a 100K pull up resistor with no luck...

I'm doing something wrong here? Please help.

I dind't bought the modules here but I hope you can help anyway...

Thanks in advance

Joaquim

a quick glance.. those modules are on-off-keying (OOK) based on carrier on/off. Also called Amplitude Shift Keying, ASK, (digitally, 1 or 0).
These are too cheap. OOKs are a PITA.
What you see on your scope is normal. It's the absence of a received signal. Like an FM walkie talkie with the squelch open.
These OOKs rely on a transmitted signal of carrier on/off/on/off, etc, in Manchester coding so that no long strings of 1's or 0's are sent.
The OOK receiver has a "data slicer". This keeps the average carrier on/off as an integration - a low pass filter. An average of the 1's and 0's.
To detect 1's and 0's, the average in analog form is compared to the instantaneous voltage from the carrier signal, rectified to DC. (over simplifying).

The data slicer outputs the 1's and 0's based on comparing the average to the instant. If the data has too many 1's or 0's in a row, the average goes too high or too low and the data won't demodulate properly.
Often the data slicer is an op-amp.

So you trade off low cost modules for YOU having to find or develop the proper bit-by-bit encoding.

Rather than one of the FM modules. And these have a full packet engine aboard.

Avoid OOK/ASK radios, I say.