Dennis Frie

- a couple of projects and pictures


(2012) Spectrum analyzer with OpenLRS (RFM22B)

I have been using/testing OpenLRS for a while, an Opensource UHF system with the popular RF module RFM22B.
The module supports a descent frequency range, an excellent RSSI (0.5 dB per step) and a good sensitivity. While it's an excellent module for communication, it can also be used as a simple spectrum-analyzer.

By changing the frequency rapidly and reading the RSSI-value multiple times, one can get a good idea of the noise and communication at the scanned frequencies.
I started with a simple sketch, and got pretty good results. Below is shown a graph with the first tests:

Top: OpenLRS with 5 channel frequency-hopping Bottom: TSLRS

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I also tested the noise from a GoPro camera. The noise-peaks seemed to be identicial to a more "professional" scan I have ssen.

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As I got pretty good result with my first test, I decided to make a nice little GUI.
Here shown a scan with OpenLRS turned on, 3 different plotting-styles.

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The RF-chip at the RFM22B module supports 240 - 960 MHz. Unfortunatly the analog filter is tuned for 430 MHz, making the sensitivity at higher and lower frequencies a lot worse. To test this, I did a quick test yesterday. OpenLRS (a RFM22B module) configured to frequency-hopping between 23 channels, and a receiver as frequency-analyzer. The Y-value is RSSI. 1 step = 0.5 dB, meaning quite a lot of attenuation from 430 MHz to 240 MHz.

First test with the modules connected directly with a cable and 60 dB attenauter:

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Next test was with antennas on both transmitter and receiver:

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The GUI and code is freeware and can be downloaded from flytrons forum or rcgroups.
The code and GUI was used to determine, that the UK version of the OpenLRS firmware was in fact running at 439 MHz and not 459 MHz as expected. Here a picture of the firmware that was suppose to be using 459 MHz - not quite ;).

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