The Electrosense network is an open source project aiming to deploy radio spectrum sensors worldwide. The idea is to help analyze and understand radio spectrum usage across the globe. Each sensor consists of an RTL-SDR, Raspberry Pi and an optional downconverter to receive the higher bands. If you're interested we wrote an overview of the project in a previous post.
Recently we received a sample of their Up/Downconverter expansion board which is used to expand the frequency range of the RTL-SDR to 0 MHz to 6 GHz. The converter board is entirely open source with the design files available on GitHub. The team note that they are also working on a V2 version which will be cheaper and smaller. The schematic and Firmware for the V2 is also available right now, but it is still under early testing and may change.
The board is not for sale, however you can apply to be considered for a free unit if you want to host your own Electrosense node and meet their criteria. If you do not you can still produce the board yourself. The team mention that the design is easily hand soldered, but there are a few difficult LGA components like the PLL, crystals and mixer which require a heat gun to solder. A the same time they also note that it is possible to get PCB manufacture and SMT assembly done for you for dirt cheap by PCB prototype companies like JLC PCB.
The Expansion Up/Downconverter Board
The converter board has 4-input SMA ports (only 3 are used) and one output port which connects to the RTL-SDR. The first input port is for the HF antenna input. This input connects to the circuit which converts 0 - 30 MHz into a higher frequency which can be received by the RTL-SDR. The second port is simply a pass through for the standard 24 MHz - 1.766 GHz range of a normal SDR. The third port is unused, and the fourth port connects the antenna to the downconverter circuit which allows us to receive from 1.766 GHz to 6 GHz.
Frequency Control
There is also a USB port which is used to power and control the mixer on the converter. In this mixer controlled design the RTL-SDR stays tuned to the output IF frequency, which in the case of the downconverter is 1.576 GHz.
Tuning to different frequencies then is a matter of using the converters serial UART connection which can be connected to via USB and any terminal emulator software such as PuTTy for Windows. After plugging the unit into a Windows PC it will automatically install the drivers, and then you can check device manager for the COM port number.
Then for example, to receive the 2.4 GHz band, you would simply type the terminal command "convert setup 2400000" which would result in the 2.4 GHz spectrum being output at an IF frequency of 1.576 GHz. Note that the bandwidth of the IF output is limited to 50 MHz due to the SAW filter, and also we need to remember that the downconverted spectrum is inverted.
If you're interested in more information the extension board manual is available here, and information about the design here.
Testing
With Electrosense the converter board is used with RTL-SDR dongles as their own software sweeps through the bands logging signal levels. However in our tests we decided to use an Airspy Mini to create a wideband sweep via the Spectrum Spy software so that we had some clear spectrum images.
First we set the converter to receive the 2.4 GHz WiFi spectrum as shown below. We can clearly see the WiFi channels in use here.
In the next test we tuned to the much quieter 5.8 GHz WiFi spectrum. In the top recent section of the waterfall you can see where we began an internet speed test over our Google WiFi node which increased spectrum activity.
We also tested the 1.8 GHz and 2.1 GHz cellphone bands as shown below. Note that at the edges of the signals we can see the rolloff effect of the 50 MHz SAW filter.
Finally we also tested the upconverter circuit with an RTL-SDR. This allowed us to easily receive broadcast AM with a chosen conversion upconvert frequency of 72 MHz.
Conclusion
The electrosense converter board is a very useful tool that can be used to expand the frequency range of any SDR like an RTL-SDR, Airspy or SDRplay device. The controllable mixer + SAW filtered IF design results in a clean signal, and almost zero signal imaging like what would occur on a wideband downconverter. However with the controllable mixer it requires an extra tuning step via the terminal.
Based on the BOM, building the device would cost roughly $80 in parts (cheaper in bulk) with the most expensive parts being the LTC5549 mixer and the MAX2870 PLL. Partial assembly for a few PCBs might cost $50. However the team note that once finished the V2 board will reportedly be about ~$25 cheaper in components.
With the introduction of $70 HackRF's being available on Aliexpress which can naturally tune from 0 MHz to 6 GHz with 20 MHz bandwidth, a converter board may not make that much sense for basic experimentation purposes in terms of price and as signals in the 2 GHz+ frequency bands are typically quite wideband. However, the converter board when combined with an RTL-SDR or higher end SDRs should offer better overall RX performance.
If you wish to try and obtain a free unit for hosting an Electrosense node, you can try and apply for an Electrosense sensor.