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UHF-Satcom.com - A brief review of the RF-Space Net-SDR software defined radio Thursday, 24. February 2011 08:45
One of the very first Net-SDR's was sent to UHF-Satcom.com for evaluation, particularly focusing on its use for satellite and weak signal work. The initial impressions when unpacking the unit are that it is solidly built, having a nice steel enclosure. A quick waft around the powered up unit with an RF sniffer probe attached to a spectrum analyser showed no significant RF emissions. The pictures below show the two views of the radio, they may be clicked for larger versions of the images. (Note, ignore the 'shack dust' that has accumulated on top of the unit)
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On the rear panel of the Net-SDR, 3 BNC connectors provide the various RF I/O, with the RF input being internally switched to the the optional down converter board. The 'REF' input allows the input of an external 10MHz reference signal, typically from a GPS locked oscillator, or a Rubidium standard, in order to provide enhanced frequency stability, such as may be required when working with extremely narrow bandwidth signals, or very weak signals requiring lots of integration. An external radio may be connected to the Serial port, this then allows control of that radio, via Spectravue. Typically this would be used where the Net-SDR is attached to the IF output of a wide band communications receiver. The Ethernet port is for attachment to a 100Mbps minimum network, whilst the USB Setup port can be used for initially configuring the unit. A linear 5V PSU was used in all testing. On the front of the unit, there is a push power switch, and 3 LED's indicating ADC Clip, (Red) Run (Yellow), and Power (Green).
The specifications of the Net-SDR can be found below;
| Frequency Range: 10KHz to 34 MHz Digital Down Converter: Xilinx FPGA PC Interface: Ethernet 100 base-T (UDP/TCP/IP) Filters: 115+ dB 80% Alias Free BW Decimation Rate: Variable 40-6400 (in 4 increment) Output I/Q Bandwidth: 10 KHz to 1.60 MHz DDC Flatness: <0.5 dB Dynamic Range: >104 dB MDS Preamp Off (500Hz): -129dBm 1.8 - 30 MHz MDS Preamp On (500Hz): -130dBm 1.8 - 30 MHz Analog to Digital Converter: 16 bit w/ Dither Pre-selectors: 10 Bandpass Filters Attenuators: 0, -10dB, -20dB, -30dB Sample Rate: 80.000 000 MHz (lockable to external 10MHz reference with optional module) Memory: 65536 x 16 bit samples External Radio Control: Built-in RS-232 port Power: 5 Volts DC at 1.5 Amps |
Ethernet Bandwidth Requirements:
2.00 MHz Output Sample Rate = 64.0Mb/s*16bit
IQ 1.00 MHz Output Sample Rate = 48.0Mb/s*24bit IQ
0.50 MHz Output Sample Rate = 24.0Mb/s*24bit IQ
0.01 MHz Output Sample Rate = 0.60Mb/s*24bit IQ
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On connection of the Net-SDR to an Ethernet network with a DHCP server, the unit rapidly acquires an address;
Feb 24 08:32:24 dhcpd: DHCPDISCOVER from 00:50:c2:bb:88:01 (netsdr) via eth0
Feb 24 08:32:25 dhcpd: DHCPOFFER on 123.45.67.200 to 00:50:c2:bb:88:01 (netsdr) via eth0
Feb 24 08:32:29 dhcpd: DHCPREQUEST for 123.45.67.200 (123.45.67.1) from 00:50:c2:bb:88:01 (netsdr) via eth0
Feb 24 08:32:29 dhcpd: DHCPACK on 123.45.67.200 to 00:50:c2:bb:88:01 (netsdr) via eth0
When streaming 500KHz of RF input bandwidth, the Ethernet output speed is 2133.0 kB/s. The nice feature of the Net-SDR is that the network commands appear to be the same, or at least very similar to that of the SDR-IP, so software written for the SDR-IP should work - at least when testing 'other' software, this seemed to be the case.
A test was performed where the RF input was terminated with a 50ohm load, to check for internal spurs or noise. Using the 35MHz 'real mode' with 1.2KHz bin bandwidth, and an FFT averaging factor of 50, the test showed no signs of internally generated carriers or other noise. The noise floor was in fact remarkably flat with less than 1dB of ripple from DC to 31.5MHz. In using the radio, it is clear that it does indeed have a low noise floor, and therefore would be perfectly suited to environments where weak signal 'DX' might be received. The following screenshot shows the internal spur check;
Below are a couple of screenshots from SDR-Radio and Spectravue centred on
0.007650GHz (click for larger versions of the pictures);
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A quick test of the Minimum Discernable Signal was carried out at 30MHz using a 3.8Hz FFT bin size;
The -130dBm signal was certainly easy to detect, making this receiver ideal for weak signal work, such as Amateur Deep Space reception. The screenshot below shows a 2MHz wide display of Inmarsat AOR-E satellite, as can be seen, the display is very clean and free from any artefacts;
You can visit the RF-Space website for further information on the NetSDR.