RTL-SDR Rx Power Calibration – Telecommunications, Navigation & Electronics

Introduction

The RTL-SDR is an incredibly versatile SDR receiver. In the previous post (Ref.1), I looked at the LoRa protocol and its performance over a long distance. It is useful to have an idea of the absolute Rx level in dBm of a received signal. Using the RTL-SDR with SDR#, the absolute level depends on several parameters including the the Rx gain setting.

Measurement Setup

Fig.2 GNU Radio Schematic for PlutoSDR SigGen 915MHz

Fig.3 PlutoSDR rf_gain = 36 = -50dBm at 915MHz

GNURC Tx Level	Signal Hound Rx Level	RTL Rx Level	RTL Rx Level
0	-85dBm	-67dBFS	-47dBFS
10	-75dBm	-58dBFS	-37dBFS
20	-65dBm	-49dBFS	-27dBFS
30	-55dBm	-40dBFS	-16dBFS
40	-45dBm	-30dBFS
50	-35dBm	-20dBFS
60	-25dBm	-10dBFS (overload)
	Fc=915MHz BW = 2.4MHz	Gain = 0dB Fs = 2.4Msps	Gain = 19.7dB Fs = 2.4Msps

Fig.4 RTL-SDR Rx Power Calibration Table

Fig.5 LoRa Packet Transmission Adjacent to Spectrum Analyzer Level = -45dBm

Fig.6 LoRa Packet Spectrum on SDR# Peaking -23dBFS (RTL gain=0dB)

Figure 1 shows the measurement setup. A PlutoSDR is used with GNURadio as a signal generator operating at the LoRa frequency of 915MHz as in Figure 2. The transmit gain variable is adjusted and the result recorded on a calibrated Signal Hound spectrum analyzer as in Figure 3. Results are recorded in Figure 4. Once this is done, the output of the PlutoSDR is connected to the RTL-SDR and results measured on SDR#.

Figure 5 shows a LoRa packet captured on the Signal Hound at approx -45dBm. Figure 6 shows a LoRa packet captured on SDR# at approx -25dBFS. From the table this equals approx -40dBm roughly the same result.