The RF Space NetSDR

5 November 2011

© Bjarne Mjelde


RF Space have a number of SDRs in their portfolio - from the early SDR-14 via the cheap, USB-powered SDR-IQ (which was my first SDR), to the top of the line SDR-IP. The NetSDR could perhaps be described as a downscaled version of the SDR-IP, and costs USD 1450 + shipping from the USA.

This is not a review as such. It is a test of the NetSDR and relevant software from the perspective of a MW DX-er with a certain way of operating. Other DX-ers may have other interests or priorities.

NetSDR Overview

The NetSDR is an HF receiver with a frequency range up to 32 MHz. Its maximum sampling rate is 2 MHz at 16 bit, providing the user with a 1600 kHz bandwidth which can be recorded and played back for later analysis.

The NetSDR is different from most other SDRs in that it is connected to the PC with an ethernet connection rather than USB. It can be connected directly to the PC’s ethernet connection, or to a router/switch. It is hardware expandable, so you can install a 10 MHz OCXO Refclock for better accuracy, and various downconverters. In the making is also a second coherent channel board which could be used for dual channel reception/recording (albeit at 800 kHz, not 1600), and noise channel subtraction among other uses.

The NetSDR is equipped with 10 sub-octave filters improve selectivity. It is not the most sensitive SDR around - I measured a uniform -102 dBm (just below 2 uV) sensitivity on AM, 6 kHz bandwidth, 400 tone into 30 % modulation throughout MW and SW. In comparison the Winradio G33DDC is -110 dBm with its preamp on. Nevertheless it’s not at all bad for an SDR, and an external preamp will be helpful only in areas where the noise level is very low.

The name “NetSDR” suggests that it has a wider range of possibilities than being connected to one single PC. Indeed it has. According to RF Space, The NetSDR utilizes straight TCP/IP and ethernet for all communications. The receiver can be placed at remote locations, and the full 1.6 MHz waterfall and demodulation audio requires only a 5 kb/s link. In addition, several NetSDRs can be set up with different IP addresses.

I must admit part of the reason I bought the NetSDR was its potential for remote control. I must also admit that I haven’t manged to make it work yet, although I see that several NetSDRs are accessible worldwide. Part of the reason why I’ve failed so far is probably my own lack of knowledge about networks. Partly I think it’s because the documentation and guides I’ve seen so far take for granted that the user actually knows a thing or two about networks.

My NetSDR is connected to a router. Two PCs have access to the NetSDR via the router; a Windows 7 64-bit system and a Vista 32-bit system. Only one PC have access to the NetSDR at a time.

Graphical User Interface

The traditional software companion to RF Space SDRs has been Spectravue, now in version 3.22. In addition, SDR-Radio.Com GmbH is developing software for the NetSDR. In addition to these rather complex programs, you may want to test CuteSDR which is an open-source application for Windows, Linux and Mac. You really can’t judge an SDR without judging the GUI as well. So let’s assume that this is not about the NetSDR itself, but the hardware/software combination NetSDR and Spectravue/SDR-Radio.

SpectraVue 3.22

The history log of Spectravue dates back to 2003. My first experience with it was as an SDR-IQ owner in 2008 (version 2.34 if memory serves me right). Developed in “brute force code” as the author told me once, SV is a no-frills program with a simple layout and somewhat convoluted ergonomics. However, behind the menu choices one finds very powerful tools to configure the receiver. The spectrum and waterfall displays are excellent, and if you have enough processing power you can select 2.097152 Million point FFTs, giving a resolution of 0.95 Hz at 2 MHz bandwidth, and 15 milliHerz at 32 kHz. I’m doing fine with 2048 point FFTs.  Mode selection and bandwidth changes are easy, but if you want to go deeper there is always a menu or dialog box you need to access.

SV can make a recording of the selected DDC spectrum, as a manual choice (if enabled, just type “R” on your keyboard) or via five timers for scheduled recording. Files can be chained, so a long (say one hour) recording will be split into many smaller (700 MB, 2.1 GB or user defined) files.

Since the options for scheduled recordings are rather limited, I tried to set up System Scheduler to work with Spectravue, which I did with success with the Winradio G31DDC last year. Unfortunately, for reasons unknown to me, I could not make System Scheduler send keypress commands to Spectravue.

Playback of recordings is done by first stopping the receiver, then selecting a new input device (wav file), and then selecting the file. If you want to go back to listen live, you have to do the same procedure over again; select new input device (NetSDR) followed by start. Changing DDC bandwidth also needs the receiver to stop, select bandwidth, and start. Compared to other SDRs I have (Perseus and Winradio G31DDC and G33DDC) this is a slow process especially for an active MW DX-er who fights against the clock all the time.

Bandwidths can be controlled by way of a slide control. Both USB, LSB, AM and SAM (AM sync) provide very good audio quality, although the AGC decay time is limited to 2 seconds.

My experience of version 3.22 is that it’s rock solid, and if the spectrum resolution is kept reasonably low, quite light on the CPU.

SDR-Radio cooperates with RF Space and offers full support for their receiver range. In addition, SDR-Radio can play back files created with the Perseus SDR and the Winradio G31DDC SDR (and probably the G33DDC too in the near future). For this test, I used version 1.4 (beta) build 774, build 814 and build 815.The latest official release as per October 2011 is version 1.3 (May 2011)

SDR-Radio has a resizeable main window with an MS Office-like ribbon bar on top. You can select a 10 Hz to 2000 kHz spectrum display, a waterfall display, or any combination of both. You can even select a Zoom tool which covers 10 kHz by default, but can be varied from 10 Hz to 300 kHz.

Like Spectravue, you will need to select the input source (such as the NetSDR), and then press “Start”. Although it is possible instruct the software to autostart the SDR when the software launches, it does seem somewhat cumbersome. If you want to switch from live listening to playback of a recorded file, you first need to press “Stop”, then go to Input source, select “Play IQ Data File” and then select the appropriate file. If you want to go back to listening, you have to do the whole process over again.

In comparison, going from live to playback in Perseus involves selecting “wav”, then select the file you want. The Excalibur G31DDC and G33DDC are easier yet; they can play back the last recorded session right away; for other files select “Browse” and pick your choice. As soon as you’re done with the playback, the software switches back to monitoring.

SDR-Radio is highly configurable, probably to an extent where it is difficult to learn to use all options effectively. That may not have been the purpose either, one will have to dress it up to meet the demands of the user. Still, I feel that SDR-Radio could benefit from a more simplistic GUI. It is sort of overwhelming.

Tuning is done in one of three ways: By entering the frequency in a dialog box, by mouse control, or by the up/down and left/right keyboard arrows. The tuning step size is chosen from the “Scroll” button and applies to the mouse’s scroll wheel and keyboard up/down. Keyboard left/right moves the frequency by 10 times the tuning step size.

Otherwise, there is not much keyboard control over the software. I find operation much quicker if I can use keyboard shortcuts, at least for the most common functions, like selecting mode, starting and stopping a recording etc.

The bandwidths are associated with the modes, but they are highly configurable. Generally I like the audio in the LSB and USB modes, especially after some adjustments. The “AM” button has five choices: AM, ECSS:DSB, ECSS:LSB, ECSS:USB, Sync.AM. I find the AM choices less suitable for DX - the audio is not stable as in the USB and LSB modes but much more fluctuant, as if the AGC was set too fast. It appears that the longest AGC decay time is 2 seconds, which in my opinion is too short in many instances. Another anomaly is that if you’re in LSB or USB and want to switch to AM, the audio mutes for almost two seconds. This means you don’t want to go back and forth between SSB modes and AM modes while DX-ing.

Generally, I find the audio quality of Spectravue better than SDR-Radio.

Recording a frequency spectrum is important to me, and probably to many SDR users. SDR-Radio lets the user choose maximum file size, if it is a time-restricted or continuous recording, and if a prebuffer (named pre-record cache) is selected. An impressive 30 second prebuffer can be selected. Starting a recording is swift - press Start in the Record menu. The file will be date, time and frequency stamped. Stopping it is just as easy. Files larger than a predefined size will be linked. The function would be even more effective if there was a keyboard shortcut defined to it, like “R” or “Ctrl-R” to start a recording and “S” or “Ctrl-S” to stop it.

SDR-Radio also offers scheduled recordings, in the form of an Outlook-inspired calendar. You can choose a recording length that suits you, and the process of adding new tasks is quite straightforward. A copy function (implemented from build 815) allows hourly tasks, such as top of the hour recordings to be set up very quickly.

Like I said before, going from live to playback is a bit of a hassle for those used to Winradio and partly Perseus. I hope there will be a way to make this smoother.

However, the playback function itself is quite good. It not only allows you to play back a file, it allows you to jump back and forth within the linked files in the folder, while keeping the frequency you listen to. You can maneuver within the recording several ways. 1) You can use predefined time steps, which makes it easy to monitor a frequency over a large period of time, independent of how many files the recording consists of. 2) You can go to any specific time within the recording session, and you can choose to start and/or stop listening at the start and/or end of the file, or at a specific time. And 4) you can make a loop from second x to y so that you can listen to that period in the file or linked files you specify.

This resembles the brilliant Perseus-Station List combo, but files need to be linked to be shown in the “Play IQ Data File” window above. Station List makes all files in the directory available for playback, and such an implementation in SDR-Radio would make the playback function truly excellent.

I briefly tested files recorded with Perseus and Winradio G31DDC too, and they behaved well.

SDR-Radio has an audio recorder which lets you record interesting DX. You can choose between MP3 (36 kb/s, 8000 Hz, mono) and WAV (configurable but default is 48 kHz, 16 bits, mono). Recordings can be automatically limited in length or size. It also allows you to start a new recording on the fly while saving the old one, or you can make a new recording which replaces the old one. Each file is automatically named with date and time. Some very nice features, but unless I’ve missed something it appears that playback is not possible. Which should be no problem for most since we have audio players on our PC. I personally favour Total Recorder. I find the MP3 file properties unnecessary tight; 64 kb/s and 22000 Hz would enable better audio quality.

SDR-Radio is not easy on the PC’s CPU. My Vista-32 Intel dual-core, 3 GHz E8400 processor on the test PC is reasonably powerful, at least by last year’s standard, and I have a dedicated graphics card too, but the program uses only one core which is up around 90 % all the time.

Rather often, command responses are somewhat slow, there is often half a second or more lag when I change frequencies, audio level, drop-down menus or bandwidths. And sometimes, the S-meter seems to more or less freeze for minutes. Spectravue is much less demanding, although at full FFT Spectravue does have the power to all but shut down the PC.

Testing the software on a Windows 7-64 PC with an i5-2310 processor at 2.9 GHz meant much smoother operation, but it’s still a bit of a resource hog.

To end where we started, with the hardware, it should be mentioned that the NetSDR is supplied with a 5VDC switching power supply. I am no fan of switching (and potentially noisy) PSUs, and I’m a bit disappointed that an expensive radio like the NetSDR isn’t sold with a properly, regulated PSU. Personally I would also have preferred that the receiver had a 12VDC power source, like Winradio’s receivers have. I haven’t tested the 5VDC supply, as I have a large linear 5V supply which powers both the NetSDR and a Perseus.


OK, so this was more about SDR-Radio and Spectravue, than about NetSDR, but of course the interface  is a vital part of the experience. On the other hand, the hardware can be as good as it gets, if the software doesn’t follow suit results will be disappointing. In my view, both Spectravue and SDR-Radio have some “quirks” related to monitoring, recording and playback that should be resolved. The NetSDR is quite sensitive, appears to have very good ultimate selectivity and can sample a bandwidth of 2 MHz (effectively 1600 kHz). In addition it will work as a remote receiver with a very small bandwidth, eliminating the need for remote control like LogMeIn and others. Not least, there is room for more hardware which have the potential of expanding the capabilities of the receiver far beyond that of current amateur grade SDRs. And once it’s been set up correctly, you don’t have to worry about USB ports or drivers that suddenly stop working. It’s bullet proof.