The Afedri SDR-net - Second Looks

I had some time this weekend to check into the Afedri SDR-net further. My beverages and the QDFA are all down, so I used one of the QDFA loops (7 m high and 20 m wide) as a temporary ALA-100 loop. It has quite a bit of gain especially on the SW bands.

The Afedri SDR-net sports a 12-bit, 80 MHz ADC, in contrast to the Perseus and SDR-IQ which use 14-bit ADCs, and the QS1R, NetSDR and Excalibur series which use 16-bit ADCs. Apart from a 30 MHz low pass filter, there is no band pass filtering. All the SDRs above except the Excalibur G31DDC have band pass filtering. If you buy an Afedri SDR-net you will accept some compromises, and you will need to do some mitigation measures. The question is of course: Is it worth it.

The Afedri SDR-net is only the second amateur-grade SDR I know which can be connected to the PC via Ethernet, the other being the RF Space NetSDR which sells for USD 1400 (I consider the SDR-IP a commercial-grade SDR). Unlike the NetSDR though, the Afedri SDR-net can also use the more common USB connection. However, the USB connection limits the spectrum to 230 kHz (250 ksamples/s) while the Ethernet connection gives access to 1225 kHz (1333.33 ksamples/s). If you opt for the Ethernet connection (highly recommended), be sure to use a crossover Ethernet cable when you connect the SDR to the PC. A standard Ethernet cable should be used if you go via a router or a switch.

There are two ways to power the SDR. There is a 12.6 VDC input in front (and a DC jack is included), however the DC voltage can be anywhere from 7.5 to 15 V. The problem with the DC input is that another compromise – a switching DC/DC converter working on 930 kHz – will reduce MW performance. My recommendation is to use the micro-USB input for DC power. If so, you can choose to use one of the PC’s USB ports, or you can modify the USB cable to power the SDR from an external, linear 5VDC supply. The USB connection draws 470 mA. This is close to the 500 mA limit of a USB port, so you may want to keep the cable as short as possible. Some laptops are reported to deliver less than 500 mA.

As mentioned by Alex in a comment to this post, a root hub connector will deliver 500 mA, while other hub connectors may be limited in current. Hence, if you're unable to power up the Afedri SDR-net from one USB connector, it may help to try another.

Sensitivity is an important issue for MW DX-ers in quiet areas. I measured around -91 dBm on AM, 6 kHz bandwidth with a 400 Hz tone modulated by 30 %. Internal gain from its Variable Gain Amplifier was set at 14 dB (see below). This is around 10 dB lower sensitivity than the NetSDR. Although sufficient for many purposes, the Afedri SDR-net will not resolve the weakest signals. A Norton push-pull preamplifier, from Kiwa or other manufacturers, will be helpful in many instances if signal levels are low.

Another vital question related to sensitivity is: When does the ADC go into clipping? You can adjust the VGA gain of the Afedri from -10 dB to 35 dB. It is possible to tailor-suit the gain with the use of a small app inside the HDSDR program (which comes with the Afedri). Pushing it all the way up to 35 dB may not be a good idea unless you are certain that there are no strong signals around. In this setting, I got ADC overload by feeding one signal generator signal at -28 dBm signal outside of the received spectrum. Reducing the gain to 26 dB raised the overload threshold to -23 dBm, while at 14 dB the threshold came within a more acceptable -12 dBm. The G31DDC, also without bandpass filters, did not overload within the -8 dBm limit of my signal generator. Of course, one signal does not reflect "real life".

The -91 dBm sensitivity was measured at 14 dB VGA gain. In theory, sensitivity could be higher, but rarely in practical terms (at 35 dB VGA, sensitivity was measured to -106 dBm, at 26 dB it was -104 dBm).

Hence, external filtering will be necessary in most instances. A SW listener near strong MW stations will want a high pass filter to eliminate overload from MW. As for me, in many instances I can probably live with a 26 dB VGA setting, but I will invest in a low pass filter to eliminate the strongest SW station around here, Murmansk-5930. It is sometimes heard with signals above -10 dBm.

It’s a bit too early to make conclusions about the Afedri SDR-net. The Arctic nights are light, and signal levels low, making it difficult to judge how it will perform in real DX situations. My impression however, is that for the price it’s a very capable SDR, able to run with a variety of software. You should however buy or make high pass and/or low pass filters to avoid overloading the ADC. And you may need to use an external preamplifier if sensitivity becomes an issue. So, if you are willing to compromise: Yes, it's worth it.