Author Topic: Direct Digital Sampled RF Transceiver  (Read 19716 times)

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Offline Kalvin

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Re: Direct Digital Sampled RF Transceiver
« Reply #50 on: February 22, 2016, 10:49:00 pm »
Whoa! Impressive piece on coding  :-+ I have done some real-time multi-rate programming for the Motorola 56K (56300 series) processors in assembly 10+ years ago. It was quite a feat to keep the pipeline busy without too many stalls. There were somewhat less registers available, but I managed to place the data structures in the memory so that accessing and processing the data didn't require too much effort to move the data back and forth. Actually I bought two of those same boards some time ago as I happened to see these videos: and I haven't had time to do anything yet, though. I guess I did see your video too, and got impressed about the capabilities of that small microcontroller and the 80Ms/s 12-bit ADC.

Online Howardlong

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Re: Direct Digital Sampled RF Transceiver
« Reply #51 on: February 23, 2016, 04:32:27 am »
The FM implementation has always been especially interesting to me. From a bit of searching and Google translating, it is an undersample followed by a downconversion using the same methods. The beauty is that the downsampling isn't as bad as it might first seem because of the undersampling and because FM channels are about 20 times wider than AM channels, so the filtering is of around the same order. For AM, we are looking at a ~10kHz channel at about 1MHz. For FM, it's a 200kHz channel at 100MHz, but as it's undersampled it's more like 200kHz in 40MHz, so it's a similar ballpark, but I couldn't find his decimation rate, the main protagonist in CPU terms. What's interesting is that the demodulation in general is generally rather petty in CPU terms compared to the downsampling and downconversion. I note from the translations that the author was running at about 97% CPU, in the same ballpark as my implementation.

For me, the implementation was an academic exercise. If you wanted to do narrow band HF, the decimation rate is just too high: at 7MHz you're looking for a 3kHz SSB, so a 2000:1 frequency ratio compared to the 100:1 or so for AM or undersampled FM. You could undersample the HF signal, but then you'd need some pretty decent filtering in the analogue domain to get the passband narrow enough.

Offline kanuk

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Re: Direct Digital Sampled RF Transceiver
« Reply #52 on: February 27, 2016, 03:28:37 pm »

Hi  Look here:
« Last Edit: February 27, 2016, 03:35:25 pm by kanuk »

Offline AF6LJ

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Re: Direct Digital Sampled RF Transceiver
« Reply #53 on: February 27, 2016, 03:41:21 pm »
That's Very nice.

Offline piranha32

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Re: Direct Digital Sampled RF Transceiver
« Reply #54 on: March 01, 2016, 05:32:47 am »
Anyone here ever tried something like this?

This is a project in the making, that has been going for a while now (mostly conceptual/planning/reality checks up till now).  This is possible with current technology, but bloody expensive to pull off.....that is if you are "allowed" to buy the components (ever tried to buy radiation hardened FPGAs before?).

My initial requirements were:
- HF right up to UHF (0 - 500MHz) - all HAM modulations (including squeezing Codec2 into there)
- No decimation/mixing in hardware at all
- DSP for everything between the ADC and DAC (16-bit)
- Initial implementation HF only, but solution had to be able to go to a 500MHz modulated signal.
- and lastly, a simple digital LCD screen/touch/knobs/speaker solution to fill in the rest of the radio (preferably classy - technology without aesthetics is just not on).

The whole idea behind this is to filter and amplify your RF signal, and then directly sample the RF signal via the ADC, and everything leaving the DSP would exit through a DAC. All further processing (IF mixing, filtering, modulation, demodulation) is done digitally. Although it might look like a pointless exercise, it can be done....and I think one reason might be to separate the RF standard(s) from the hardware. If anything changes, you just update your firmware. Another reason would be to start implementing the open source Codec2 into hardware (I'm a big fan)

If you lower a bit your expectations, it won't be difficult to find. FDM-DUO from Italian company Elad would be one example. Receiver is implemented on a DAC feeding an FPGA, which does filtering and downconversion. Transmitter is implemented in a bit more traditional way, by upconverting digitally modulated baseband signal. Supports all major modulations (no Codec2 unfortunately, but not all is lost. Elad releases firmware updates quite regularly, so maybe one of the future ones will support it). The ADC is clocked at 122.88MHz, so the upper frequency is a bit lower than your initial requirement, but coverage from 100kHz up to 54MHz is still quite impressive. By adding a few external components (filter, amplifier) and exploiting higher order Nyquist zone, you can work in 2m band as well. All of this in a knobbed box, which can be used as a standalone QRP trx.
Aside from that they also have direct sampling radio receivers.

The Big Names of the ham radio industry are just waking up. Icom is going to release IC-7300 with direct sampling receiver, however it is still spottable (on exhibit stands) promiseware.

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