March | 2018 | netchosis

I set out wanting to capture the signal transmitted from my gate opener (pictured at the very bottom) but the results are confusing so I am attempting to document what I did.

The first thing I I did was build a basic receiver in gnu radio to do the capturing shown here.

osmocom source: This an interface to our software defined radio (hardware)

sample rate: 16M ( our hardware can effectively sample between 8 million IQ samples per second and 20M because of my initially confusing results I upped it to 16M hoping it was a resolution problem.
Ch0 : frequency: ( center frequency ): 300M 300mhz
Ch0: RF gain: 0 this this controls a rx amp built into our hardware)

QT GUI sink: ( just a way to out put our signal in the frequency domain )

the brown line is peak hold using this I estimated the transmit frequency to be ~ 300.2 mhz the blue spike at 300 is a gnu radio side effect which occurs at the center frequency.

The next step I assumed would be to attempt to “tune” and demodulate the signal so I built a another flowgraph for that:

File source: ( just our IQ samples stored in a file)
Throttle: This prevents gnu radio from sending data faster than 16Mps
QT GUI sink : outputs the previous graph
Frequency Xlating FIR FILTER ( was supposed to accomplish the fallowing)
- tune to our signal by downshifting captured spectrum by the difference between the center frequency and the tuning frequency
- low pass filter the shifted signal with a cutoff of 50kHz and a transition width of 1Khz
- decimate the outgoing data stream down to our working_samp_rate of 400kHz
- XFF details ( how we do the above with this block)
  - Decimation: int(samp_rate/working_samp_rate)
  - Taps: firdes.low_pass(1, samp_rate, filter_cutoff , filter_transition)
  - Center Frequency : freq-center_freq

Demodulation:( maybe this is where I went wrong?) : I am only really familiar with on off keying so I use the so called “complex to magic” block to provide a high constant value when the carrier is present and a low ~constant value when the carrier is absent.

The fallowing shows but the input provided to complex to mag and the output afterwards:

So Looking at things in the time dimension demodulated or not I don’t see anything I would recognize as digital maybe a nice square wav? So I suspect that at least 1 of the fallowing things are true.