Serial Data Transmission with RF - Range Testing

To recap, I now have a receiver with very little noise on the data out pin between actual packets. I have a transmitter alternatively sending two characters 500ms apart. And I modified the receiver board to toggle a LED depending on which character was received.

My thinking was that I could walk away from the transmitter and watch for the LED to start blinking erratically or cease blinking at all. Then I could make some changes and see if the range got better or worse.

I had done something similar with nRF24L01 modules some years ago and that had convinced me 2.4GHz wasn't suitable because in real life the range was too short to be useful.

My initial test showed a range of only 11m across the workshop. The decoding was reliable right up to the doorway. The following day I put the TX unit on the ground near the door and the RX unit back on the bench to look at the data stream. Rock solid. So I iteratively moved the transmitter out of the workshop. At the property boundary it was now, according to Google Maps, 27m from the receiver and down a decline. But still in a direct line through the doorway.

The blinking was steady indicating no errors. 

 

What is noticeable is that as the signal strength drops with increasing range the pulses get narrower. 

The edge of range was when the transmitter was about 32m from the receiver and now screened by the workshop wall. At this point the light began blinking erratically. Looking at the received data stream it was obvious why this was. 

The pulses were getting so narrow that either the start pulse is outside the allowed range weak or, as seen from the second last pulse above only just hitting 2V, the pulses are not hitting a logic high level.

A quick check: Assume Vcc=3.7V. Logic high for the micro is say 70% of Vcc or 2.59V. The LM358 can swing to perhaps 1.2V of Vcc, or 2.5V. So logic high is not being met. 

At 5v the problem goes away. 

Three possible fixes: 

1. Replace the LM358 with a R2R output opamp.
2. Add a pull-up resistor to the data line.
3. Use two LiPo batteries and the onboard regulator.

The third option is expedient for range testing purposes but adding a pull up resistor is no harder and maybe relevant to a battery powered receiver.

I added a 10k resistor from the data line to Vcc and the high logic level is now being met. I didn't persist with this because the following day when I went to do the range testing the noise was again present. Given the hardware issues with this receiver model I decided to suspend work since I now have a better receiver board.

 

 

Footnote for those landing here via Google Search:

My suspicion is that a lot of people trying to get these receivers to work on a LiPo battery had issues with this. This problem is exacerbated when both the ATMega and Rx unit are running at 3.3V. So if you landed here trying to work out why your Arduino project isn't working now you have a hardware issue to consider.