We are using the CC1101 Low-Power Sub-1 GHz RF Transceiver chip for telemetry in the 402-405 MHz frequency range (MedRadio band). We have noticed that the distances over which we are able to achieve a reliable communications link are far less than we had anticipated. Using a signal strength meter, we find that the minimum detectable signal (MDS) for the device seems to be about -85 dBm at 100 kbaud data rate, which is far greater than the MDS values given in the data sheet.
To confirm our observations, I set up an experiment using the CC1101DK433 development kit. Two units were set up about 6 meters apart in two different rooms of a residential dwelling. Using the SmartRF studio software, the Easy Mode 434 MHz 2.4 kbaud setting was selected. Measurements were taken at the receiver unit for different values of transmit power. Results are shown in the table below.
As can be seen the MDS in this experiment appears to be on the order of -85 dBm (about what we are seeing in our application). According to the data sheet, the MDS should be between -112 dBm (for 1.2 kbaud) and -104 dBm (for 38.4 kbaud).
Tx Power (dBm) Recv’d OK Avg RSSI (dBm) Packet Error Rate
-10 224 -76.6 2.6%
-15 223 -80.1 0.4%
-20 220 -85.5 30.4%
-30 - - 100%
I realize that the matching network/balun has some loss which may not be accounted for in the MDS value given in the data sheet. I also realize that in both the application and in the experiment I performed, the device is being used in a rich multipath environment which might be having some effect. However, it seems to me that I should be getting much better results than I am getting in both scenarios. I was wondering if anyone had an explanation for the discrepancy.
