For the past couple years, I have been attempting to determine what might be the best hardware platform for ham radio and other communications related applications, with a bias projected towards mobile platforms.
There are a slew of contenders, including those listed in figure 1. Some are more readily adaptable for SDR facilitation than others.
The graphic (above) shows a few options that I have seriously considered, and a couple for which I have built prototypes to aid in the testing process. The graphic is not intended to imply any capability or lack of capability with respect to the hardware, the SoC chip based SBC boards, or the operating systems. The selection only implies what I have made as a personal preference, for reasons that (shortly) – I’ll enumerate. In other words, the Raspberry Pi may use USB Audio, but I would prefer I2S if I can make it work. Many alternate combinations are possible, and I have refined a list of preferences for the audio configurations I’ll use.
Figure 1 shows the Cirrus audio board for the Raspberry Pi, and the Soft-66 Lite SDR hardware (built into the stand-alone project box I mentioned on other pages). The Cirrus audio board is slightly larger than the SDR hardware it services. It’s adaptable to the Raspberry Pi A+, B+, Pi2, and (so I have read) – Pi3. It works with the A+ and B+ out-of-the-box, and with the Pi2 and Pi3 after some friendly negotiation. A note about the hardware is in order here. The original pi models “A” and “B” had a 26 pin GPIO header, and the original Wolfson/Cirrus card matched this. See:
A subsequently released Cirrus adapter (Cirrus bought Wolfson) – uses the 40 pin GPIO header that is found on the pi models “A+” and “B+” and Pi2 and Pi3.
For a nice pinout of the newer Pi boards, see:
Apparently, the Cirrus card is capable of 24 bit 192KHz output (playback) and input (capture)! The latter specification is normally found only on more expensive audio adapters. The Cirrus may be purchased for a mere 30-40 dollars, US. See this page for more details:
Due to those specs, I have elected to use the Cirrus adapter on the Pi2 version of an SDR receiver that I’m currently building.
Figure 3: The Raspberry Pi/2 powered version of the SDR receiver I am building.
In figure 3, one can see the Raspberry version of the SDR receiver I am assembling. It’s in-process as regards the build, and details of that can be found at:
The Cirrus adapter uses (I2S) for its signal path to/from the Pi:
In figure 4, one can see the difference between the USB and I2S interfaces. It should be noted that some USB adapters use I2S internally. It should also be noted that I2S and I2C are two distinct interface specifications.
In figure 5, it can be seen that the little “homemade” tablet is getting quite cramped. Note that I did not mount the Cirrus card on the 40 pin IO connector of the Pi/2. This was to facilitate use of the GPIO pins for other things. There is also a vertical clearance problem to deal with. So, there will be individual wired female header lines coming from the audio board to the Pi/2 J8 connector (not yet in place).
Note: This author and site is not affiliated with the Raspberry Pi in any way. For information about those projects visit http://www.raspberrypi.org “Raspberry Pi” is a trademark of the Raspberry Pi Foundation.
Note: the author does not have a recent, applicable background in circuit building, or battery related issues, so this is presented as the work of a hobbyist, and is not meant for duplication by others. Readers should look elsewhere for design advice and info.
Soft66 Lite hardware can be found at:
Cirrus Logic (not afilliated with this site) is a fabless semiconductor company. More into may be found atwww.cirrus.com. Note: the soft66 Lite hardware is a product that is sold on a Japanese website (http://zao.jp/radio) – and is not affiliated with this site or author in any way.