Avian’s Blog

Experimental radio equipment on VESNA can be used for more than spectrum sensing. Radio boards based on Texas Instruments CC2500 and CC1101 transceivers can also be used for packet transmission and reception and, perhaps somewhat surprisingly, also as flexible signal generators. These can be used in experiments when you want for instance to introduce a controlled interference in some system or to check if your spectrum sensor is working correctly.

Usually when I'm talking with people about what our hardware is capable of the conversation often starts with amazement at how small the radio part is and then inevitably turns to the question whether VESNA has a software defined radio. I have to answer no, it doesn't, and then there's usually an awkward silence because that seems like a dead-end for any serious research work these days.

True, these transceivers don't provide software access to the undemodulated baseband samples (like for instance USRP does). However they are still amazingly flexible. They offer plenty of reconfigurability and, after you get to know some of their quirks, can be adapted for a lot of weird use cases without hardware changes. If you skip the various high-level digital parts of the chip for proprietary packet format handling there's a flexible front-end in there that allows you to choose between a handful of frequency, amplitude and phase modulators and several options for channel filters and such.

The closest you can come to software defined radio is a transparent continuous transmit mode where you can feed the transceiver arbitrary binary data from the microcontroller and it will simply get modulated and fed into the antenna. There is a catch though, since the chips offer at most 2-bit quantization of the baseband signal before modulation (they were designed for simple digital transmissions after all). This means that you have to get creative if you want to approximate an analog modulation and be ready for plenty of quantization noise.

This is how it looks like on a spectrum analyzer when you run a direct digital synthesis algorithm on the microcontroller that generates a baseband sawtooth frequency sweep and use the amplitude modulator to up-convert it to 2.4 GHz:

(Click to watch RF signal synthesis using VESNA video)

Using delta-sigma modulation you can approximate arbitrary waveforms in this way. For instance, you can make a passable simulation of an (analog) wireless microphone transmission using a 4-FSK modulator in CC1101 tuned into the UHF band.

Of course, setting this up takes more work than popping a few blocks into GNU Radio Companion and part of my job is to make it more accessible to people using VESNA and our VESNA-based testbeds. If you're interested in such signal generation using Texas Instruments CC series, some platform-independent code capable of doing this should start hitting the vesna-spectrum-sensor repository on GitHub in the next few weeks.