Small Platform Digital Radio
Part 2 - The Hardware

So now we have a theoretical basis to begin our project. The first order of business is to to look at the questions and goals and try to answer each question so we can set out to satisfy our first goal.

Can it be reduced in size and weight and still be useful?

Despite the small size and weight of the Small Format computer, it can be powerful enough to do most common tasks demanded by portable or mobile digital radio. Let’s look at a very popular hardware platform and see what the specs and capabilities are of this credit card sized computer.

We will use the Raspberry Pi 3 as an example in this project. The latest Pi 3 model is the Pi 3 B+. Not that any other small format computer module would not do as well. But, the Pi has demonstrated popularity, adaptability, and usefulness in the last few years that lends this application to this platform exceedingly well. More than 19 million Pis have been sold to date.

Specifications

pi3specs

The Pi 3 Model B+ is based on the same quad–core, 64–bit processor, as the Pi 3 Model B. Like the Model B, the B+’s is based on a Arm Cortex A53 architecture.

However, the B+ ups the speed of the CPU to 1.4GHz from 1.2GHz in the original Model B, an increase of 16.7%. While the original Model 3 B only supported 2.4GHz WiFi, the Model B+ has a dual–band wireless antenna, supporting 2.4GHz and 5GHz 802.11 b/g/n/ac WiFi. This is a big update to the existing Pi 3, including a re-spin of the BCM2837 CPU (running faster at 1.4GHz), new dual–band wireless connectivity (so you can connect to both 2.4 and 5 GHz WiFi networks), better thermal control, and more. As always, you’ll need an up–to–date Raspbian SD card, but the new model is backwards–compatible with all previous models and runs the same operating systems and programs. The Raspberry Pi 3 Model B+ is available now at the usual price of $35 from most sources.

The 5GHz 802.11ac WiFi has been found to be capable of about 100 Mb/s throughput in testing, more than double that of the 2.4GHz 802.11n WiFi found on the Pi 3 Model B. That said, most will be unable to achieve a significant improvement in noticable throughput.

The B+ also offers support for Bluetooth 4.2, an improvement over the 4.1 support found in the Pi 3 Model B. On top of the WiFi upgrade, the wired internet also has a speed bump, courtesy of the board’s new Gigabit Ethernet over USB 2.0 bridge, which ups the maximum throughput to about 300Mbps. The new 3 B+ replaces the LAN9514 USB hub with a with a LAN7515 adding Gigabit Ethernet. It’s still limited by USB 2.0 but gives a 3x LAN throughput increase, which will please a lot of previous Pi users with network hungry applications in the home and in ham radio applications.

Another plus for those using the Pi with a wired Ethernet connection is the inclusion of support for a Power Over Ethernet (POE) and Hardware Attached On Top (HAT) boards, which will add the ability for the Ethernet cable to power the board.

The following is a comparison of earlier models of the Raspberry Pi with the latest model the 3 B+:

pi3sysbench

Perhaps the most exciting aspect of the Raspberry Pi is the use of a microSD card to load the operating software (Raspian OS). The microSD card holds the operating system and user data in a solid state disk configuration. This is removable media, which means the user can change basic storage content at will. You could conceivably have multiple SD cards with different software on each.

For instance, one could be set up for FLDIGI/NBEMS, one for WinLINK or BPQ (or LinBPQ in this case). Many possibilities exist for removable media as the base operating environment. With the use of an external USB drive (mechanical or SSD) even more storage is available. Or one could conceivably use a larger SD card (up to 256 Gb) and install the OS on it to provide the maximum base storage on the SD card alone without an external drive. SD cards are available for under $20 each. The “thumb” drive is also an option for more storage and flexibility.

For our project we will use the supplied 32 Gb SD drive and installed Raspian operating system supplied with the starter kit. We will also discuss the display options for your mobile or portable system. They are many and you have to choose which fits your situation and budget. The choise for this project will be a 7 inch TFT LCD color display that is small enough to take anywhere and light enough to fit in luggage or backpack. Only you know what size screen will fit your application best. A search on the Internet will show many options from the small 5–inch 640x400 TFT screen to 21–inch 1265x1024 desk sized displays. You could even hook it to your 4k display if you wish. But who would be able to lug that kind of display on a trip. It has limited use outside the shack.

Here are the adapter cables available for the Raspberry Pi:

hdmi-dvi  hdmi-vga

How does the Pi 3 B+ stack up against your desktop or laptop? Don’t expect it to be as fast, even though it is a 64–bit computer. After all, trade–offs have been made for size and power consumption. However, you will find it to be quite capable and surprising for such a small package.

Project hardware configuration

For this project, we will make some specific choices based on complexity and costs. As you learn more, you will be able to add to the project to fit your specific need and application. The list below outlines were we will start and what the initial costs will be:

  Raspberry Pi 3 B+ Starter Kit
  Various versions apply ranging in content and price from Amazon and others for $65 to 89$. One kit includes a display for $149.

  Video adapter cable for HDMI to VGA or DVI (if needed) $7-$14
  Compact TFT LCD color display 3” HDMI $19.99-$36.99
  Compact TFT LCD color display 5” HDMI $29.99-$79.99
  Portable TFT LCD color display 7” HDMI $52.99
  Desktop 11” 1920x1030 LCD color display $149.99
  could be too bulky for luggage or backpack and at 2x the cost of smaller displays.

A word about keyboard and mouse.

The Raspberry Pi 3 B+ has two methods to accommodate keyboard and mouse. The first is a wired solution – USB keyboard and wired mouse directly to the supplied USB ports on the Pi. This may or may not be a good option for you depending on your situation. The Pi 3 B+ comes with 4 USB ports. Obviously, the wired mouse and keyboard would take up two of those slots.

keyboard

The second solution is to use a Bluetooth enabled keyboard and mouse. This is a viable option because it is wireless and leaves all supplied USB ports available for external equipment (like a SignaLink or other computer interface and/or a PACKET TNC). Here we narrow our options in that most Bluetooth keyboards are rigid. If packing options are limited this might not fit the packing requirements.

A more flexible solution might be to use a flexible keyboard and add a USB hub. The flexible keyboard can roll up and occupy a fraction of the space of a rigid one. Flexible keyboard and mouse could occupy less than half the space requirements of a rigid version.

Both keyboard types can be found from Internet sources for prices from $12.95 for the rigid wired combo to $23.99 for a rigid compact all–in–one wireless version. The flexible keyboard ranges in price from $8.99 for keyboard alone to $28 for the keyboard / mouse combo. As with the display options, you must decide what is best for your application and budget.

So, for simplicity sake, we will use the basic starter kit and a wired USB keyboard and mouse. This would put our startup costs somewhere well under the $200 goal for startup costs.

Now we have a well capable small format computer that will serve us well during this learning experience and form the basis for a flexible and portable system for “go–kit” use.

Rev. 1.00 2018-04-01 AD5XJ