As mentioned in a previous post last week, UUGear have recently released their VU GPSDR expansion board for their Vivid Unit single board computer with touchscreen. Together, this combination results in a handheld Linux system, with built-in RTL-SDR and upconverter.

The VU GPSDR has some interesting features, including:

• GPS-assisted 24 MHz clock for improved frequency accuracy and stability
• An integrated 108 MHz up-converter for HF (under 30 MHz) reception
• Dual programmable rotary encoders for tactile control
• A software-controlled frequency output port for experiments
• Software features, including OpenStreetMap integration and ADS-B aircraft tracking 

Assembly

We won't repeat the assembly steps as the instructions show everything clearly, but we can say that the assembly steps were clear, and the assembly itself was easy. It was simply a case of plugging in a few jumper wires between the Vivid Unit and VU Extender board, screwing down the extender board, and then slotting in the VU GPSDR into the Extender boards mini-PCIe slot, before finally screwing down the GPSDR. Assembly took less than 10 minutes.

Physical Design Review

The system is put together like a sandwich. You have the screen and Vivid Unit on the top, then the Extender board, and finally the VU GPSDR on the bottom.

The Vivid Unit and GPSDR are essentially bare PCBs that connect to one another via the PCIe slot on the Vivid Extender board. This means that there is no enclosure, and you are essentially handling PCB parts in their raw form. In the future, we would like to see an optional enclosure to protect the unit better.

The exposed design results in some flaws that we have to point out. The shielding cans on the VU GPSSDR unit sit on the rear of the system, and during operation, they get very hot to the touch. So much so that handling the unit requires a bit of care to avoid the hot spots. Most of the heat appears to be coming from the AMS1117 LDO on the rear, which gets up to 80 °C, so be careful not to touch it accidentally. From the photos you can see that the RTL2832U and R860 are heatsunk to the shield. This is a good idea to keep the chips cool, but it also means that the metal gets quite hot to the touch. So handling the unit only from the edges is recommended. 

Secondly, because the Vivid Unit does not have a built-in battery, you need to power it separately via its USB-C port on the side. This makes the ergonomics of handling the unit a little trickier as you also have a cable sticking out. UUGear has noted that they are working on an 18650 battery pack, so this issue may be resolved in the future.

Finally, the "GPS" in the GPSDR comes from the fact that there is a GPSDO with a built-in GPS patch antenna on board. When active, a GPSDO provides excellent frequency stability, meaning that signals will be on frequency and will not drift.

Indoors, of course, no GPS fix is possible. But the uBlox NEO-M8N GPS module used in the GPSDR also has a fallback TCXO, so even without any GPS fix, the frequency accuracy of the system is good. UUGear also noted that the GPSDO automatically activates once a GPS fix is achieved, so no action is needed when you take the unit outdoors.

Realistically, the design issue with the GPS patch doesn't really matter anyway. For most use cases in handheld operation, the built-in TCXO will be sufficient. Any use case requiring extreme GPSDO precision will probably involve the device being mounted upside down and used remotely. 

The screen is clear and bright, the two encoder wheels are non-indented and are in a good spot, and so is the SMA antenna port, although the VU Extender's USB-C plug can block the antenna SMA port if a really fat plug is used (normal-sized USB-C plugs fit OK). The screen is large and has a high resolution, making it possible to use the onscreen keyboard. However, it is still a little fiddly for typing and clicking, so we ended up plugging in a small wireless keyboard.

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