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#11
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An SDR or DDS question?
"gareth" wrote in message
... Presumably those SDR rigs which do not work on the IF but directly from antennae must have, separately from the DSP processor, some semblance of a DDS generator (but without the final DAC) to act as the equivalent of the VFO, for I cannot perceive that a fractional-Hz tuning rate could be achieved with machine code running in the DSP processor? Thanks to all who responded, even he whose post was riddled throughout with gratuitous rudeness, and it seems that I was almost correct, but with the exception of Brian Morrison's URL where the heterodyning was done with a PLL and fractional-N synthesizer. I wonder if Mr.Gratuitously Rude actually read through his URL reference for therein is a clear reference to the phase accumulator, a necessary part of the DDS process? |
#12
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An SDR or DDS question?
On Sun, 7 Feb 2016 12:40:57 -0000
"gareth" wrote: I wonder if Mr.Gratuitously Rude actually read through his URL reference for therein is a clear reference to the phase accumulator, a necessary part of the DDS process? When did you drop the "I never follow URLs" rule, Gareth? |
#14
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An SDR or DDS question?
"gareth" wrote in message
... "Brian Howie" wrote in message ... In message , gareth writes Presumably those SDR rigs which do not work on the IF but directly from antennae must have, separately from the DSP processor, some semblance of a DDS generator (but without the final DAC) to act as the equivalent of the VFO, for I cannot perceive that a fractional-Hz tuning rate could be achieved with machine code running in the DSP processor? I'm not an expert ,but I think what you're asking is " how is the local oscillator generated" in a direct conversion SDR and "what determines its resolution" There is an example here, :- http://www.radioelementi.it/public/saqrx.pdf The "c" source code is here,which I can just about understand ( My software background is FORTRAN and Matlab) :- https://sites.google.com/site/sm6lkm/saqrx/ Softies shouldn't have a problem with it although I was able to mess about with it and recompile it successfully In this case the spectrum is dc to 22050Hz in 512 steps. It's not the LO precision ( it's floating point in this one) that limits it but the size of the FFT , the sample rate and thus the record length, that sets the minimum FFT bin width . This one tunes in lumps of about 43Hz Thank-you Brian, but what you have URLed is already at baseband, being VLF. Multitudinous apologies, Brian, I tell a lie, so thank-you. I've downloaded all (and even run the prog although nothing is attached to the input of the sound card) and will proceed to study the code, although, obviously in this case, at a max freq of 22kHz, creating a VFO is quite within the capabilities of PC-based machine code. Also, a study of the code should answer a previous question of mine about how to use the sound card in applications such as this. As I indicated elsewhere, the nub of my question was correct, there is a VFO-equivalent in the front end of wide-range DSP SDR rigs, that is separate from the DSprocessor (except in those cases where a DFFT is applied to the whole input range) Thanks again for your effort. Otherwise, for light relief and to ease the aggressive tensions that my innocent enquiry seems to have fomented, I append an article that I wrote nearly 20 years ago .... -----ooooo----- Alimentary Fourier Anal-ysis ---------------------------- (C) Copyright 1997 Gareth Alun Evans G4SDW The Fourier Transform is rather like the Human Alimentary canal.... The Alimentary Canal takes food, and then transforms it into something that is a function of the rear end, and hence may be referred to as, "The Food-Rear Transform". The Fourier Transform causes integration over a one second period, and results in an infinite series of sines. The Food-Rear Transform causes disintegration over a 12-hour period, and results in an infinite series of signs...."Gents", "Ladies", "Rest Room", etc. etc. The Fourier Transform (Tukey's reasoning?) takes Time and transforms it to a frequency spectrum. The Food-Rear Transform takes Parsley, Sage and Thyme (Turkey's seasoning?) and transforms it to a frequently active rectum. "Laplace", literally, "The Place", is how you deal with the transform. "Walsh" is something you do to your hands afterwards. "JPeg" is what you hang your jacket on to. It is a form of compression such that you take up less space and spend less time there (although the output may be more granular). The Fast Fourier Transform must be done through an appropriate window, otherwise an effect known as the "Picket Fence" problem, also known as "Spectral Leakage", can occur. Diarrhoea is known as the FAST Food-Rear Transform, and if done through the wrong window can result in a problem (also known as Rectal Leakage) all over the Picket Fence. The Picket Fence problem is also known as, "Scalloping", something which is normally done to pie crusts. In the old days, pies used to be made with "Four And Twenty Blackbirds"; but nowadays, members of the Jay family are used. When you go to someone's house for tea, you politely eat everything that is put in front of you, but your reaction might be, "Eat the Jay (Oh my God!) Tea???". Similarly, the Fourier Transform is a great consumer of e^(jwT)... "E to the j, omega, t". The Fast Fourier Transform is famous for its Butterflies. The Fast Food-Rear Transform is famous for its Butterflies-in-the-stomach. Dirichlet's are the Fourier Transform's conditions, without which the result is divergent. Diarrhoea ****s are the Food-Rear's conditions, with which the result is VERY divergent. Having experienced the Fast Fourier Transform, you are then faced with the Inverse, which can be very daunting. It is a paper exercise but it can be quite messy. Having experienced the Fast Food-Rear Transform, you are then faced with the "In Verde", literally "In The Grass", which can be very daunting indeed. (The paper exercise can be quite messy.) |
#15
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An SDR or DDS question?
"gareth" wrote in message
... "Brian Howie" wrote in message ... http://www.radioelementi.it/public/saqrx.pdf https://sites.google.com/site/sm6lkm/saqrx/ Wow! Wow! Wow! Dev-C++ a free C++ compiler for Windows thrown in, and much smaller than MicroBloat's products! Watch this space!!!!! |
#16
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An SDR or DDS question?
"gareth" wrote in message
... "gareth" wrote in message ... "Brian Howie" wrote in message ... http://www.radioelementi.it/public/saqrx.pdf https://sites.google.com/site/sm6lkm/saqrx/ Wow! Wow! Wow! Dev-C++ a free C++ compiler for Windows thrown in, and much smaller than MicroBloat's products! Having now had time to persuse the C code, I have the answers to all my questions! I'd like to recommend to ALL readers of these NGs that they download the source code and the free C compiler whilst they are available, even if not immediately using them. Within the AudioCallBack routine is where it all happens, including the phase accumulation making the DDS for the local VFO, and the FFT. It's some years aince I've seen a Windows program done at the knife-and-fork level and so that takes me back before the times of Visual Basic, and TCL. As Brian said, it is easy to recompile and build the program again, thus giving the opportunity to dabble with the code by trying out minor edits, even if limited to the colours (colors for the yanks) of the Windows. The program clearly relies on the speed of a PC, for example, the DDS calculates sin and cos on the fly from the phase accumulator rather than using a look-up table, and I'm afraid the dinosaur in me keeps harking back to my days of assembler programming on a PDP11/20 where the slowest instruction (save for RESET at 50 mSecs) was BICB @(R1)+, @(R2)+, taking about 17 uSecs. All-in-all, this revelation by Brian is to be recommended to those who wish to see an example of DSP programming to run on a PC. Well done, Brian, and many thanks! |
#17
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An SDR or DDS question?
Bernie wrote:
On Sun, 7 Feb 2016 12:40:57 -0000 "gareth" wrote: I wonder if Mr.Gratuitously Rude actually read through his URL reference for therein is a clear reference to the phase accumulator, a necessary part of the DDS process? When did you drop the "I never follow URLs" rule, Gareth? Principles have always been fluid for our Gareth. -- STC // M0TEY // twitter.com/ukradioamateur |
#18
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An SDR or DDS question?
"Stephen Thomas Cole" wrote in message
... Bernie wrote: On Sun, 7 Feb 2016 12:40:57 -0000 "gareth" wrote: I wonder if Mr.Gratuitously Rude actually read through his URL reference for therein is a clear reference to the phase accumulator, a necessary part of the DDS process? When did you drop the "I never follow URLs" rule, Gareth? Principles have always been fluid for our Gareth. Eh? I thought he was teetotal. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.uk |
#19
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An SDR or DDS question?
In message , gareth
writes All-in-all, this revelation by Brian is to be recommended to those who wish to see an example of DSP programming to run on a PC. Well done, Brian, and many thanks! Yw Someone has modified the code to use increased sound card sample rates , and add other features . https://sites.google.com/site/swljo30tb/home I can receive MSF amongst other stations on 60KHz with the 192KHz version on an untuned 5ft loop antenna I use on MF. Brian -- Brian Howie |
#20
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An SDR or DDS question?
"Brian Howie" wrote in message
... In message , gareth writes All-in-all, this revelation by Brian is to be recommended to those who wish to see an example of DSP programming to run on a PC. Well done, Brian, and many thanks! Yw Someone has modified the code to use increased sound card sample rates , and add other features . https://sites.google.com/site/swljo30tb/home I can receive MSF amongst other stations on 60KHz with the 192KHz version on an untuned 5ft loop antenna I use on MF. The accompanying PPT is in foreign, presumably Swedish, but there is an interesting page showing the VLF antenna, and the interest to me comes from numerous tappings along the antenna with a coil down to ground. An early form of loop antenna, perhaps? Is there scope here for experiments on similar such tappings to bring a Top Band antenna down to a size to fit a suburban garden? |
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