Signal Capture Differences and Modulation

[postman]

Hi there,

Attached are captures of an identical signal taken by two different devices. Device 1 is a bladeRF with a large monopole antenna that I do not know the exact specs of, and Device 2 is a RTL-SDR with the stock antenna.

Device 1 Spectrum Image: http://imgur.com/a/b7HkG#0
Device 2 Spectrum Image: http://imgur.com/a/b7HkG#1

One can see how they are related, but their characteristics are quite different. Can you think of why the spectrum looks so different, especially as it concerns the multiple frequency artifacts for the RTL-SDR and none for the bladeRF?

Additionally, do you have any thoughts as to what modulation may be in use here? The RTL-SDR suggests FSK, but I don't know what to make of the bladeRF capture...

Thanks!

[postman]

Hey, thanks for the response. Sure, here is a raw capture: https://docs.google.com/file/d/0B9UP0pU ... VYazQ/edit. I don't have access to my BladeRF right now so this is with an RTL-SDR dongle and its generic antenna. The file parameters are:

samplerate 2.4e6
frequency 345e6
format: 8-bit unsigned quadrature/2 channels

I can get some better captures with the BladeRF tomorrow morning. Let me know if there are any details you want me to focus on and I can get you what you need. My end goal is to be able to replay these signals, and I figure knowing the modulation and the data in question is a good first step.

[sigblips]

See the spectrogram image below. The signal in your rtl_close_2.u8quand file is FSK with a 7300 baud rate. It's a carrier detect tone, followed by a preamble, and then the mark/space data bits.

rtl_close_2.u8quand_FSK.png

I noticed that the gain is too high and your signal is clipped. This causes distortion. The I and Q channels are the green and purple in the Waveform image below:

rtl_close_2.u8quand_waveform.png

It is a amazing that the FSK spectrogram image is so clear considering the clipping distortion.

Attached are captures of an identical signal taken by two different devices. Device 1 is a bladeRF with a large monopole antenna that I do not know the exact specs of, and Device 2 is a RTL-SDR with the stock antenna.

Device 1 Spectrum Image: http://imgur.com/a/b7HkG#0
Device 2 Spectrum Image: http://imgur.com/a/b7HkG#1

One can see how they are related, but their characteristics are quite different. Can you think of why the spectrum looks so different, especially as it concerns the multiple frequency artifacts for the RTL-SDR and none for the bladeRF?

You are comparing a 24 MSPS rate capture (bladeRF) with a 1 MSPS rate capture (RTL-SDR). That's a huge difference. Things will look very different at those scales.