Lots to report this Month!
Mike Richards G4WNC has a few extra FT8 tips plus some inside details on the new Pi 400 and the RX888 32MHz 16-bit direct sampling receiver.
Mike Richards G4WNC
a £30 Integral SSD and it’s working well with the Pi, as has a much larger Sandisk 500GB unit, Fig. 2. The use of a fast drive only affects loading times, so you should notice that the Pi boot is a bit faster and programs will load quicker. Unless your software does a lot of disk interaction, the operating speed will be much the same as when using the SD card.
If you want to transfer an existing SD card to a new hard drive, the tools you need are already available in the Pi operating system. The ideal tool is Pi SD Card Copier that can be found in the Accessories menu. This tool makes a direct clone of the source disk. To safely transfer your system without affecting your SD card, follow these steps:
Begin by booting the Pi with the SD card you want to transfer
If you’ve not already done the upgrade, enter the following in a terminal session and reboot the Pi: sudo apt update && sudo dist-update -y
When the system is running, plug the SSD into a spare USB3 port (Blue)
Go to the Pi menu and select Accessories - SD Card Copier
In the SD Card Copier window set the Copy from Device to /dev/mmcblk0
Set the Copy to Device to /dev/sda, Fig. 3 Hit Start to begin the transfer
The transfer will take quite a while so just let it take its time. When the transfer is complete, you can remove the SD card and reboot the Pi, which will automatically use the external SSD as the boot drive. One particularly helpful feature of the SD Card Copier is that the file system on the copiedto drive is automatically expanded to use all the available space.
RX888 Direct Digital Receiver
I mentioned the new RX888 receiver a couple of months ago, but I now have my hands on one and have been playing, Fig. 4. Mine was ordered from eBay in early September and arrived at the end of October. The total cost was £148. The most exciting feature of the RX888 is the fact that it uses an LTC-2208 16-bit ADC to digitise the entire 100kHz-32MHz band, but doesn’t use the typical FPGA to downsample the data for the host PC. Instead, the RX888 uses a Cypress superspeed USB driver to send the raw ADC data to the PC for processing. Thanks to recent developments in USB technology, most modern PCs have high-speed USB ports that can handle the raw data rate. Of course, you will need a reasonably powerful PC and suitable software to manage the data.
Thanks to a healthy SDR developer community, SDR software is already available for the RX888, including Simon Brown G4ELI’s SDR-Console. I was able to run the RX888 with SDR Console on a 4th Gen i5 quad-core processor-based PC. This PC just about coped with the full 32MHz bandwidth, resulting in a processor usage of around 70% and the GPU (GTX660Ti) at 75%.
While it’s clear that you’re going to need a reasonably fast PC for this receiver, you can reduce the processing demands by selecting lower sample rates, and the following are currently available: 1, 2, 4, 8, 16 and 32MHz. These lower bandwidths are created in the PC by decimating or down-sampling the incoming raw data before application to the main SDR software. I’ve shown a simple diagram of the software processing in Fig. 5. Although the PC is still processing the full 32MHz bandwidth, this technique produces a much lighter overall load. For example, the i5 PC I mentioned before shows a processor and GPU load of around 35% when using the 16MHz bandwidth. This load reduces further as you select lower bandwidths.
As you can see from the photo in Fig. 4, the RX888 is supplied in a smart black metal box with lots of heatsinking. This is required because the ADC chip gets quite hot when working at full speed. While the metal box is excellent, I quickly discovered that the grounding is not so good.
The black powdered finish of the RX888 looks good but is also a good insulator! As supplied, there was no electrical connection between the ground line of the receiver and the case. I fixed this by scraping away the finish around the SMA
antenna socket and the contact points where screws secured the end panels to the body. I also found that the SMA sockets were set back from the edge of the PCB, Fig. 6a. As a result, tightening the SMA nuts would probably damage the PCB. The solution was to use the supplied nuts as spacers and wind them fully onto the SMA connectors, Fig. 6b.
I then used new star washers and nuts to secure the SMA sockets to the panel. In Fig. 7 I’ve shown two 32MHz wide spectrum displays from the RX888 while running SDR-Console. I terminated the SMA sockets with 50Ω loads, so the spectrum shows the no-signal spurious responses. The bottom display shows the RX888 as supplied, while the top display shows the results after fixing the ground bonding. As you can see, improving the bonding was worth the effort.
A key difference between the RX888 and the earlier Dragonfly RX666 is the inclusion of an RF preamp ahead of the ADC. This provides 20dB of gain and improves the sensitivity. The RX888 also has a Rafael 820T2 set-top tuner included to give access to the VHF bands as far as 1.2GHz. However, this is with reduced performance and limited to about 8MHz bandwidth. The software for this series of receivers is under active development by Oscar Steila IK1XPV and others. To keep upto date with developments, take a look at Oscar’s blog at: https://sdr-prototypes.blogspot.com
The NextGenSDRs group on groups.io is also worth joining.