Mike Richards G4WNC is taking a look at the LimeSDR Mini and 3D printing and showing how you can get away from the noise with an Airspy HF+.

Regular readers will recall I mentioned the LimeSDR development board a couple of months ago. This board offers a multi-channel transceiver with an operational range of 100kHz to 3.8GHz and a bandwidth of up to about 60MHz, all for $299 (~£230). Early buyers rapidly discovered that the HF performance was very poor due to the input matching networks and the board is best used with a decent up-converter for HF use. Lime have published some mods to improve the HF performance and you can request an HF modified board at order time. While this improves the performance, it’s still very deaf on HF and the open front-end makes it prone to out-of-band overload so an external up-converter is the best solution for LF-HF operation.

The LimeSDR now has a smaller and cheaper sibling in the form of the LimeSDR-Mini. The LimeSDR-Mini is the size of a large USB dongle, Fig. 1, and has a tuning range of 10MHz to 3.5GHz with a maximum bandwidth of 30MHz when using a USB 3. The Mini uses the same Lime transceiver chip as its bigger brother but has a smaller FPGA and a lower sample rate. The LimeSDR-Mini costs $159 plus $10 carriage, which equates to around £131. However, the units are liable for import duty, so you may well get caught for an extra £20 or so in UK taxes. As you can see from the photo, the LimeSDR-Mini uses a very high-density PCB with many components in the ‘fairy-dust’ category, so it’s wise to pop it in a case for protection. The suppliers have produced a very smart aluminium case but the cost is nearly as much as the LimeSDR-Mini at $140 so I skipped rapidly past that option. The alternative, acrylic, case looked suitable but was still way too expensive at $40 (£30)! With the help of Google, I discovered the design files for a 3D printed case that was available for free download under a Creative Commons licence:

www.thingiverse.com/thing:2816863

Because I haven’t managed to persuade Elaine to let me buy a 3D printer (yet!), I had to find someone to print the case for me. The answer was to use 3D Hubs (URL below). They operate as an agency selling the services of a large network of individuals and companies with 3D printing facilities.

www.3dhubs.com

Using 3D Hubs was very easy via their simple web interface. The first task was to upload the design files, which can be any of the common 3D printing formats. I used the .STL files for the Mini case. There were two files in the set, one for the top and the other for the bottom. Once the files had uploaded, the site displayed a scaled 3D view of the design that you could rotate to check that it looked as expected, Fig. 2. The next step was to choose the material and I selected ABS because this is a useful general-purpose material. However, it seems PETG would have been a slightly better choice so I’ve ordered a couple more in that material. With the design parameters complete, the site produced a quote for the work of £10.76 with a five-day delivery, which seemed very reasonable, especially as that was for two units. Once I’d placed the order, I received regular e-mail updates from the printer and the order was delivered a couple of days early. I’ve shown the finished unit in Fig. 3. I had to do a small amount of rework to make room for the PCB to fit but that only took a few minutes. The case lid has four tiny cone-shaped fixings that are a friction fit in the corresponding holes in the base. This was not a particularly good solution because the points were very fragile. The printer had warned me about this before the job was printed, which was helpful.

As you can see, the finished case looks the part and provides good physical protection for the LimeSDR-Mini. Performance-wise, the LimeSDR-Mini is very deaf at its low frequency range but worked well when paired with my SV1AFN upconverter. This upconverter translates the 0-55MHz frequency range up to 200-255MHz and includes good quality filtering to eliminate signals outside the 0-50MHz range. There is also a switchable HF LNA to provide some additional gain but this must be used with care or you will overload the front-end and cause more spurious signals. The SV1AFN upconverter is particularly well suited to the LimeSDR modules and its LF performance is excellent. You can see more details of the upconverter and purchase an assembled PCB for about €40 or a, very smart, boxed unit for €90 via Makis’s website at:

www.sv1afn.com/upcnv.html

For general amateur radio use, one of the attractive features of the LimeSDRs is their very wide bandwidths. This means you can observe (and record) large chunks of spectrum and I’ve shown an example of a 2-22MHz spectrum display in Fig. 4. This wide view is particularly useful for monitoring activity on the VHF/UHF bands. The wide bandwidth capabilities of the LimeSDR modules also mean that a USB3 connection is essential. The next Lime project for me is to get amateur digital TV running and I hoping to be able to do that using a Raspberry Pi.

Beating the Noise

Unless you live out in the sticks, you are probably struggling with man-made noise compromising the RF noise floor. While the WSJT-X modes are a big help for data modes operators, it can still be a battle. There are opportunities for using noise cancelling techniques with some of today’s more advanced SDR transceivers but many of us are stuck with the local conditions. One way forward, that’s getting easier to achieve, is remote operation where your rig is located in an RF quiet location and accessed using an internet link. However, that’s a pretty drastic and often expensive solution. A more economical alternative could be to mimic commercial practice and employ separate transmit and receive sites, the simplest option being to keep your main rig at home but use a remotely located receiver. One modern receiver that’s ideal for this application is the Airspy HF+ SDR, Fig. 5. This is a very capable receiver that’s enjoying continuous firmware development to fine-tune the performance and the result is an excellent SDR receiver for amateur radio use. Coverage is 9kHz to 31MHz and 60-260MHz, thus covering all the LF/HF bands plus the 4m and 2m VHF bands.

To complement the receiver, Airspy have developed the Spy Server software that can run on many different computer systems, including the Raspberry Pi. One of the problems associated with accessing SDRs over the internet is the poor and variable bandwidths offered by typical internet connections. The bandwidth requirement for an Airspy HF+ sending full IQ samples is around 13Mb/s but this needs to be a sustained data-rate or the receiver will stutter. This moderate data rate is often difficult to sustain, thus making remote SDR receivers a bit more of a challenge. The Airspy team have developed an ingenious solution to the bandwidth problem by moving some of the signal processing from the SDR software to Spy Server. Instead of just passing the raw samples to the network, Spy Server first decimates or under-samples the incoming IQ data to reduce the sample rate to a few tens of kilohertz, just wide enough to contain the selected modulation type. That immediately provides a significant network saving but what about the spectrum display?

The spectrum display is produced by Spy Server software running on the Pi instead of the SDR software on the PC. The resultant video frames are then sent to the host computer. The video frames only need to be sent at around 15-25 frames per second and therefore have a much lower bandwidth requirement. In addition to handling the signal processing, the Pi also runs the server that combines the decimated IQ samples, FFT video frames and receiver control signals and makes that combined data stream available to multiple users. I’ve shown a block diagram of the Spy Server’s operation in Fig. 6. Spy Server manages to reduce the bandwidth requirement to just 0.35Mb/s for common narrow-band modes such as SSB, AM and NBFM. That’s a bandwidth reduction of well over 90% and enough to make Spy Server workable over most internet connections.

When Spy Server is operating with a single connection, the operator has complete control over the receiver and can tune anywhere within the receiver’s tuning range. However, if another operator connects, the central tuning point is locked. This is because it’s not practical to have more than one operator controlling the main tuning point. However, each connected operator can still tune anywhere in the receiver’s bandwidth, which is around 660kHz for the HF+ and plenty wide enough for most amateur bands. By changing a few values in the Spy Server configuration file, the receiver’s tuning range can be restricted or locked. There is also the facility to automatically disconnect users after a pre-set period.

The HF+ and Raspberry Pi combination is very compact and only requires a single 5V 2A DC supply because the HF+ takes its power from the Pi’s USB port. Because this is such a compact setup, you might find it easier to persuade a friend or relative living in a good location to host your remote receiver. You can even use Wi-Fi for the network connection, thus limiting the cabling to the 5V supply and an antenna, Fig. 7! This could also be a project for your radio club. If you could find someone with a good radio location who would be prepared to run Spy Server, the IP address could be shared with club members. In addition to the potential to reduce noise using a remote receiver, the same receiver can be used as a useful signal indicator for checking the quality of your transmissions.

Setting-up the Pi

As I’ve run out of space here, you can refer to my blog for details of how to load Spy Server on the Pi. I’ve also shown how to run Spy Server as a Linux service. This is particularly useful for remote operation because the server will automatically restart in the event of a failure and the Pi will automatically reboot after a power failure. For those that don’t want to mess with Linux, I have programmed microSD cards available with Spy Server installed along with several useful update scripts. These also include seven pages of printed instructions. You’ll find these in my Web shop at:

g4wnc.com/shop

This article was featured in the October 2018 issue of Practical Wireless