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Dual Coherent SDR Receivers – Current State

coherent sdr receivers

This article describes the current state of hardware and software solutions for coherent SDR receivers within reach of hams and hobbyists. Take a look. 

Previously, I have described technical requirements for minimum Spatial Interference Filtering Techniques. We need to start with a good selection of dual channel coherent SDR receivers, particularly in the mid-price range. These need to be supported by software that can create digital beamforming.

Dual coherent SDR receivers must have synchronized frequency translation and sampling clocks across two independent receivers. Each must provide synchronized baseband data for further processing, either in the radio or supporting software. This can be accomplished over USB but more likely over a high speed Ethernet connection.

In short, it’s slim pickings at present. But there are a few hardware and software solutions that meet or almost meet the SIFT requirements. Let’s take a look.

The Apache Labs ANAN-7000DLE does the job right now. We have already described how the ANAN-100D together with PowerSDR software provides spatial filtering. However, the Apache Labs products are fairly expensive.

Meanwhile, over at FlexRadio, the 6700 and new 6600 transceivers are equipped with dual channel coherent receivers, mainly for amplitude diversity reception. Unfortunately, the proprietary firmware and SmartSDR software does not presently support spatial filtering. Perhaps in the future. The 6700 is also expensive. The now discontinued FlexRadio 5000, with its second receiver option, did support beamforming with the PowerSDR software.

Currently, there are two dual channel coherent SDR receivers on the market. One comes from Israel – the Afedri SDR-Net Dual Channel. The other solution is the LinkRF IQ+. The Afedri covers 100 kHz to 30 MHz and provides its data over Ethernet or USB. The IQ+ is specified for VHF and UHF, but is said to receive down to 3.5 MHz.

I would consider the Afedri as the first choice in the mid-range, with its 12 bit ADC and a reasonable US$360 price tag. You can do spatial filtering with Linrad, SdrDx and PowerSDR_mrx software using this radio.

Coherent SDR Receivers – More Choices

SDRPlay recently released its RSP2 spectrum capture unit. Interestingly, RSP2 has connections for external clocking. You can tie two RSP2 units together in a master-slave arrangement, or drive both with an external clock. From my reading of the block diagrams, this common clock should provide synchronized local oscillator and sampling clock across multiple units.

Unfortunately, you need to connect two instances of a control program to run these two receivers. This might work for diversity reception. But we still need some software that can control two RSP2 together and enable spatial filtering. Another issue might be that the fractional-N synthesizers will only lock to average frequency rather than phase.

I will be watching closely, and may experiment with RSP2 and Afedri solutions later in the year. Here is an example of Linrad and the Afedri dual channel spatial filtering.

Also, many people are experimenting with creating coherent SDR receivers with a pair of cheap RTL-SDR dongles. In some cases, they are hacking the units to tie the clocks together. In other cases, they are making recordings and then synchronizing during post processing using GPS signals. Lastly, there are now GNURadio processing blocks that control two RTL sticks concurrently. A recent experiment in Europe unearthed some of the challenges to achieving coherence with these devices.

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