FDIM 2012: The YADI micropower digital interface

Next in our tour of projects from the Four Days In May event at the Dayton Hamvention is an innovative radio-to-computer interface.

The YADI rig-to-computer interface prototype

The unique feature of YADI, Yet Another Digital Interface, is a micropower VOX circuit. This lets it sip power while waiting for a signal, where other interfaces are less battery-friendly. The secret is that the VOX amplifier is biased in “class E”, according to its designer, Dana Browne AD5VC. (It is possible he meant class C, since class E amplifiers are tuned and a VOX amp is broadband.)  That means that the amp uses no bias current when there is no signal. Its biasing also causes it to rectify the input, eliminating the need for an additional rectifier. The quiescent current is less than the self-discharge current of a 9V battery. In other words, the battery loses more energy sitting on a shelf than this interface needs to keep idle.

Dana, with Jim Giammanco N5IB, designed it with an eye towards emergency communications, particularly after hurricanes in their home state of Louisiana. It is designed to interface any radio and to any computer sound card, to support the many digital modes available today.

A kit version of the interface is being produced by the Baton Rouge Amateur Radio Club. The kit version was also on display at FDIM, but unfortunately, my photos of it did not come out. Suffice it to say that the kit includes a good-looking PCB, features easy through-hole assembly, and fits in a mint tin. The price is $35.

Unfortunately, I can’t find a link for the kit, other than some copies of the manual on a file sharing site. The BRARC web site is a vacant placeholder, and Google turns up no other leads. I hope this worthy kit becomes available soon and gets the publicity it deserves.

FDIM 2012: N8WE’s CW transceiver

A few weeks ago, I went to the Four Days In May event at the Dayton Hamvention, and brought back some pictures of the cool projects I saw.

N8WE's 200 mW CW transceiver

Glenn Hazen, N8WE, brought his 20 meter transceiver project to show-and-tell night. The radio’s receiver uses a Softrock Lite II downconverter with a laptop running a software-defined-radio (SDR) application. The transmitter is a 200 mW Morse code transmitter. That’s the code key on the right.


Choosing a high-performance audio ADC

As I discussed two weeks ago, it is time to refocus my efforts on the DSP-based ham radio project that started this blog. Let’s take a look at the architecture I had in mind originally:

Block diagram of a near-zero IF receiver with I and Q paths

This is a popular topology for radios that put their intermediate frequency at or near 0 Hz. It is also very similar to the most popular ham software-defined radio topology. In fact, those ham SDRs simply substitute a PC sound card for the ADC and the PC’s CPU for the FPGA. Here, though, I don’t want to bring a PC into the picture yet. Radios that require a PC don’t feel like “real radios” to me. I want to end up with a self-contained box, though I won’t mind if it optionally integrates with a PC.

For simplicity, I want to implement the receiver’s automatic gain control (AGC) functional digitally, after the ADCs. This means that with a well-designed front end, the ADC dynamic range will become the radio’s dynamic range.

With that in mind, I went looking for the best dynamic range audio ADC I could find, or at least the best one I could afford. I have identified seven manufacturers of 24-bit audio ADCs. Here is the best that they have:

Part SNR, A-weighted THD+N Package Price
TI PCM4222 123 dB -108 dB TQFP-48 $29.99 D*
TI PCM4220 123 dB -108 dB TQFP-48 $19.10 M
AKM AK5388 123 dB (Note 1) -110 dB LQFP-44 $10.93 D
AKM AK5394A 123 dB -110 dB SOP-28 $22.00 D
Cirrus CS5381 120 dB -110 dB SOIC-24, TSSOP-24 $32.22 D/M
Wolfson WM8786 111 dB -102 dB SSOP-20 $3.48 M
ADI AD1974 105 dB -96 dB LQFP-48 $10.03 D
NXP UDA1361 100 dB -88 dB SSOP-16 $1.37 M

Figures are typical values as shown in the manufacturer’s data sheet. All are for two channels active and 24-bit PCM output. Prices are quantity 1 from the cheaper of Digi-Key or Mouser.

1. The AK5388 is a four-channel ADC with 120 dB SNR. The 123 dB SNR requires the use of “mono mode”, which connects each stereo pair in parallel with a single input, effectively creating a two-channel ADC with 3 dB better SNR.

* Non-stock item, but a limited number of units are currently in stock.

The AK5388 looks like the price/performance winner, at $10.93 for 123 dB SNR and the best THD+N. One catch is figuring out the mono mode, which didn’t have crystal-clear documentation in the data sheet. On the other hand, even in four-channel mode it has 120 dB SNR, which still puts it at an excellent price/performance point.

Lurking in the SNR specification is A-weighting, which is a specification used for audio that isn’t much seen in the measurement world. The idea behind A-weighting is to reflect the human ear’s varying perception of noise at different frequencies by doing a weighted sum of the noise in the SNR measurement. Thus, frequencies where noise is more audible count worse than frequencies where it isn’t. A-weighting is not quite the right measurement for a near-zero IF ADC, first because a communications receiver’s bandwidth is quite a bit smaller than the 20 kHz used for the A-weighted measurement, and second because the communications signal being digitized is not necessarily at its final audible frequency yet.

I used A-weighting for the comparison because all of the ADCs were specified that way. Some of them also had an SNR in 20 kHz bandwidth specification, which does not weight the noise spectrum. For those ADCs, the 20 kHz SNR was 3 dB lower than the A-weighted SNR. , I decided to compare the A-weighted SNR to put the ADCs on equal ground, even though the 20 kHz bandwidth SNR is closer to what I would like to know for a communications receiver.

It’s worth mentioning that I looked at precision ADCs as well. I couldn’t find any 24-bit precision ADCs from Analog Devices or Linear Technologies that had a high-enough sample rate to be usable. In contrast, TI offers the single-channel ADS1281 and ADS1282, which offer a stunning 130 dB SNR (unweighted) at 250 samples per second. These might be reasonable for a Morse code receiver with a 100 Hz passband. When used for sideband, these ADCs would have to be operated in their 4000 SPS mode, at which their SNR drops to 118 dB. When one considers that filtering the output of the audio ADCs down to 100 Hz would provide an extra 6+ dB of SNR (because less bandwidth means less noise power), the ADS1281/1282 no longer has such an advantage. Worse, a pair of ADS1281 (two are needed because they are single-channel) will set you back $108 at Digi-Key.

In the end, I’ve found a combination of excellent performance and a good price. The AK5388 it is. The next step is to build or buy a board for it so I can start experimenting.

The Plan

I’ve been thinking for some time about a DSP-based ham radio.  After
considering and discarding more grandiose schemes, I was inspired by my
Norcal 40A.  It and the original Norcal 40 are fairly simple and highly
reproducible (thousands were built).  However, performance was not
sacrificed in the name of simplicity.  Instead, the rig was carefully
designed to make the most of its NE602 mixers and crystal filter.

Why not try for the same goals in a DSP-based rig?  In theory, one should
be able to subsume all of the IF and most of the RF and baseband into the
DSP, leaving little but filtering components and a few amplifiers outboard.
The result would have a small number of components and would be fairly easy
to build.

I set a goal to build a self-contained radio, not a PC-based software defined radio.  It will be narrow band for simplicity.  As much functionality as practical will be done digitally.  Finally, the design should be reproducible by others.  That, in turn, means that it should be documented, it should use a low-cost DSP toolchain, and it should be insensitive to component variation.

Finally, I have an interest in delta-sigma techniques and multirate DSP, and the radio will be an excellent platform to explore and experiment with those technologies.

Continue reading The Plan