I want to commence some work in developing a radio based on Software Defined Radio principles. I’ve been involved with SDR in various guises for a good 20 years, and I’ve never actually written the code to use the technology for radio communications. It’s time to have a go – but I’m not clear where to start.
I’ve been using direct sampling (as opposed to I/Q digitisation, as used in sound-card based SDRs) for years. It would be an alien concept to me to move (what I regard as) backwards. So I’m expecting direct sampling and spectrum generation; a high speed ADC covering the whole HF band, and either a DAC or DDS doing the same for replay. I’m attracted to the DDS route.
User Interface
One of my foibles is that I don’t want to be using a PC as the principal user interface. I’d like a software defined receiver/transceiver to have a user interface such as you associate with an HF radio – knobs, S meter and a spectrum display rather than a VGA screen. There’s no technology reason why SDR needs the PC; I want the PC to be an extension beyond the basic “box”. The “console” unit announced by Flex is a step in the right direction.
Use of Standards
I bought a secondhand board set for HPSDR. That could tick some of the boxes, but it’s already obsolete. Any amateur developed project of this sort will take several years. It's a fact omf modern DSP life that the ADC/DAC parts of such a project will have a lifetime of many years, but the processing will be obsolete in 18 months. I’d like to look at separating the ADC/DAC side of the project (that is specific to the amateur, HF operation) from the FPGA based processing (which is non application specific). The way to do this is through standards. there are already many relevant standards in existence; the Amateur community doesn't need to invent new ones, necessarily.
Hardware standards are relevant to separating the ADC/DAC from the processing. Perhaps having ADC/DAC etc on a mezzanine card connected to an FPGA processor board through a standard interface would allow the FPGA part to be updated by buying a newer commercial product, leaving the amateur community supplied hardware being just the data conversion. An ADC/DAC board with an FMC connector, plugged onto a Zedboard FPGA board, would be an interesting way forward. The processing side is commercially supplied, manufactured in huge quantities and supported by a much larger community. FMC is an open standard, and would allow the processing hardware to be updated to newer commercial technology without throwing away the amateur community investment. Following the Zedboard idea: suppose my ADC/DAC hardware were plugged onto a Zedboard. I can make a “commercial radio” like unit using the Zynq FPGA and two embedded 32 bit processors, giving me a “radio in a box” type capability. There’s a display driver, to connect possibly to a touch screen display. To connect out to a PC, there is an Ethernet connector.
Data interface standards are relevant to the PC connection. If everyone used the same approach, we'd be able to get to the point where any PC application would work with any radio. VITA49 is a standard for transferring I/Q samples: it would be relevant in this case to pass downconverted samples from the direct sampling receiver and FPGA processor into the PC processor. VITA49 is a "lightweight" protocol designed not to add much overhead. The soundcard interface standard from the PC world could also be used, but supports much lower bandwidth channels.
A Project concept
Features I’d like to aim for include:
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Direct sampling to cover the HF band
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Simple hooks to add transverters to low microwave bands
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A “user interface” with knobs, simple screen etc to the point of the overall unit “looking” like a conventional crystal filter based radio
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“Standard” signal processing on the front panel (e.g. filter width, IF shift, simple notch filter)
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The main tuning knob to be a high quality 1000 step per revolution unit, like you get with FT1000 class radios
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100W RF, or (preferably) very simple connection to external linear with ALC etc
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Resistant to obsolescence through use of standards
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Standards based interface to a PC for further processing beyond that in the box
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Standards based hardware implementation
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Ideally, the commercial suppliers would take the same standards approacvh at the edge of the box, for onward conneciton to a PC. The Elad FM Duo is a commercial product that has some of what I’m after; its connection to a PC is rather limited though, as far as I know with a sound card type interface. The Icom IC7300 has been announced since I started looking at this, and if it had an ethernet port with a standards based interface to sample data I'd get one. Unfortunately, they didn't provide that feature.
Does anyone have ideas/suggestions?