Ufff. You aren't EE but you have give us a great article. All point that future is SDR DDC because issues about DDC will be covered with processing speed improvements.
Edu yy4gmj On Aug 20, 2013 12:58 AM, <[email protected]> wrote: > Warning: rather long reply follows. > If you don't care, or have your mind made up, please skip this email. > Fred asked for some education, so here goes... I'm not an expert, but > these are my personal ramblings from what I think that I have learned. If > you don't agree, and the moderator doesn’t like this discussion here, maybe > we can find another place to discuss? Or directly? > > Elecraft has done a WONDERFUL job taming the QSD/QSE "mixers" in the KX3. > I LOVE MINE. I regularly tout it at meetings, including tonight. I think > that I "sold" at least one KX3 and one K3 or another KX3. > > Having said that, the K3 and KX3 are subject to imperfections, just like > any other analog circuitry. Yes, the KX3 uses a QSD with "switches", but > they have analog characteristics for an analog signal as well (on-state > impedance, on/off switch times, off-state leakage, etc). The op amps that > follow them aren't perfect either. There will be a little imbalance > between the I and Q channels, either in amplitude or phase, due to > component variations. Kind of like the old phasing SSB days. All these > imperfections add up in a complex way, and need to be dealt with to reduce > image issues, both on receive and transmit. The KX3 does a remarkable job > handling this. That's just one set of problems with "analog" radio > circuitry. > > The "holy grail" of SDR is to put the ADC as CLOSE to the antenna as > possible. The reason is simple: the sooner you get the signal converted > into the digital domain (represented by numbers), the fewer imperfections > from analog world circuitry will affect that signal. A digital mixer just > multiplies two numbers, for example, a "LO" represented by a set of numbers > describing a sine wave, and an input signal, possibly from the ADC. Within > reason (and decent design), the numbers CANNOT change due to component > imperfections, therefore you will not see problems with a digital version > of a mixer, where the resulting signal is sort-of, close-to, but not what > you want. Two times two should always equal four, not 3.9 once, 4.1 the > next time, etc, due to component drift, or other issues. Filters are a > different issue, but still more consistent and reliably reproduced in DSP > than in the analog world. > > So, how can we get the HF signal right into the ADC? That is tough, > because the ADC itself has a limited dynamic range (and a generally poor > noise figure). Most SDRs use 12-bit to 16-bit ADCs, and sample from 66MHz > to 123MHz rates. Dr. Mitola suggests in a book that good HF reception > requires at least 130dB of dynamic range. IIRC 130dB works out to about > 22-bits of ADC conversion. 16-bit ADCs by themselves provide somewhat less > than 96dB, 12-bit ADCs provide slightly less than 72dB. So, traditionally > even the better SDRs needed some sort of filtering before the ADC - to > hopefully reduce any strong out-of-band signals. This may not work great > on some bands, like 40M, where strong MW broadcast stations often appear > just above or just below the ham band. > > There is a "trick" that is real-world, and reliable, that can improve the > dynamic range of an ADC, called oversampling, that provides "processing > gain". If you sample much faster than the signal of interest's frequency, > and decimate those samples (throw out every so many samples), you can > achieve additional dynamic range. I believe you gain roughly 6dB (or one > ADC bit) every time you decimate by a factor of four. > > Up until recently, extremely fast and deep ADCs were too expensive for ham > rigs, and it was hard to get more than about 20-24 dB processing gain. The > new Flex 6000 series has a 16-bit ADC that samples at 245Ms/s. This allows > them to have enough dynamic range to MAYBE not need filtering. Notice that > they do have filters built into the radio, that can be enabled or disabled. > Regardless, the main analog components in the 6000 series before the ADC > are only filters (which should contribute negligible distortion to the > signal of interest), and a preamp, also optionally in or out-of-circuit. I > think even Flex has their fingers crossed regarding true dynamic range in > the real world. > > Despite the comment on the Fun Cube Dongle Pro site, I doubt they have an > actual 32-bit ADC. Even lower-frequency ADCs, such as in sound cards, do > not achieve that level of success. In fact, I doubt that power supply > noise, PC board layout, and many other factors prevent audio-frequency > sound cards from achieving even the 24-bits that many claim. For RF at HF > frequencies, 16-bit ADCs are about as good as it gets - that we can afford > anyway. > > Don't get me wrong, a DDC/DUC-based radio CAN have serious issues as well. > Overload (poor dynamic range), bad noise figure of the ADC, not lining up > the sample values right (value overflow), and a host of other issues. > Filtering can be just as tricky (or more) in the digital domain as in the > analog world. And SDR hardware can be a pain to use and/or build. Anytime > a computer is involved, you are taking your chances. Hi Hi. Sometimes > knobs are just plain easier to use. > > Yes, there has been a lot of discussion about whether or not the K3 is an > "SDR". I personally believe that it is, because a significant part of the > SIGNAL PROCESSING is done in an executing software program, and that > program can be changed. Not as easily as loading a new EXE in Windows, but > it can be. Even though the RF front-end follows a traditional analog > superhet receiver design up to the second IF. In my mind, there's no > question that the KX3 is an SDR. It is similar in very basic layout as an > SDR Cube, Softrock/host computer, etc, in that it uses QSD/QSE "mixers", an > Si570, and a dsPIC back-end that is fully reconfigurable. > > I remember others here, including the author of much of the DSP code for > the K3 and KX3 has previously stated that they also believe both rigs are > SDRs. BTW, the Elecraft P3 IS A DDC RECEIVER, with a 14-bit ADC, sampling > at about 60Ms/s, and feeding an ASIC DDC chip, then a dsPIC chip for FFT > and other DSP work. Yes, it samples the 8.215MHz IF from the K3, but as > Elecraft states, it can be used on other radios as well. It does take > advantage of the K3's RF front end, including preamps, filtering, etc. I > would NOT hook an antenna directly to it! > > Spectrum analyzers are nice if you can afford one - many people can't. I > have both a homebrew W7ZOI one, and a commercial unit. They usually don't > have the sensitivity to look at typical weak HF signals, and they aren’t > usually designed to demodulate anything, or at least not CW or SSB. > Panadaptors/spectrum displays are really coming to the forefront of SDRs. > In fact, I would argue the opposite of your comment. Some hams are using > SDRs, or at least hardware originally designed as an SDR, for many bench > applications. Spectrum analyzers, network analyzers, antenna analyzers, > etc can be based on some of the better SDR hardware. HPSDR Hermes, HiQSDR, > even Softrocks, have all been used in those applications. > > I'm not a "digitalist", nor am I an "analogist". I have several radios: > RTL dongles/upconverter, K3/P3, KX3, SDR-Cube, tons of Softrocks, SDR-IQ, > Flex 1000, SDR-Widget, SDR2GO, HiQSDR, HPSDR boardset, Flex 6500. I know I > forgot a couple. I bought each for a reason, and don't disparage any of > them. I did finally sell my Icom IC-720a, a workhorse for over 20 years. > Except for the clack-clacking! > > I've just given talks on SDRs to two local Charleston, SC ham radio clubs > within the last week (one tonight). I've been asked whether someone should > buy a K3 or a Flex 6x00. To be honest, I told them if they wanted a radio > to "use and rely on" in the next year or so, get a K3, or possibly a KX3. > In about a year, the Flex 6000-series MAY be mature enough to change that > advice. As Jerry Pournelle used to say in Byte Magazine: RSN (Real Soon > Now). > > I want to see more than just tiny little slices at a time. For contests, > I want to not only see how the present band looks, but hopefully multiple > bands, so I can see how propagation is moving. No, I'm not a very active > contester, that's just one example. I LOVE the K3/P3, and have used them > on CW for Field day the last three years. The Flex (any Flex) just isn't > ready for that. > > I'm not an EE, just someone who has played with leading-edge amateur radio > for 45+ years. I was involved in the first licensed ATV repeater (WR4AAG), > packet radio development, spread spectrum, a little LF, and now SDR. I > know how to burn my fingers with an iron. > > In summary: learn about Direct Digital Conversion (Digital Down > Conversion) (DDC - whatever), look it up. Read about it. Don't stick you > head in the sand, or you too may be carrying around one of those ancient > Motorola 2M FM rigs with tubes in a Yaesu/Kenwood/Wouxun world. There are > definitely pitfalls to DDC/DUC radios. But, there are several reasons why > every "cell phone" has an SDR inside. > > Sorry for the length. To each his/her own. > 73, Terry, WB4JFI > > -----Original Message----- From: Fred Jensen > Sent: Monday, August 19, 2013 3:33 PM > To: [email protected] > Subject: Re: [Elecraft] K3 and whats next from Elecraft > > I could use some education here please: > > What is the advantage of sampling directly off the antenna? I'm > surmising that's what is meant by "real SDR" vs "hybrid SDR". I don't > know if 32-bit ADC's existed when the K3 was being designed, but > Elecraft settled on the hybrid superhet analog 1st IF/digital 2nd IF for > at least one reason and probably many. > > The dynamic range of signal amplitudes at my coax connector across the 0 > to 30MHz spectrum has got to be huge, even after LORAN-C shut down. Is > there really any benefit to having that broad a spectrum with that large > an amplitude range in the number soup? > > If you want to observe that spectrum, wouldn't it be better to use a > spectrum analyzer and save your communications receiver to be optimized > to deal with the tiny little slices we want 99.999% of the time? > > If the K3 sampled at the 8MHz 1st IF, after the analog roofing filters > [i.e. RF amp and 1st mixer only], would it make the K3 a "real SDR" in > the eyes of those who don't see it that way now? > > I'm serious about the "education" part, this "Is the K3 an SDR" thread > has been going, on and off, for more than a year. It seems that we have > two ideologies going, "Digitalists" vs "Analogists." Unfortunately, > debating ideologies qualitatively is rarely productive. > > My education is in math, my understanding of EE is the result of 60 > years in ham radio colored by a math background. > > 73, > > Fred K6DGW > - Northern California Contest Club > - CU in the 2013 Cal QSO Party 5-6 Oct 2013 > - www.cqp.org > > On 8/19/2013 11:38 AM, Arie Kleingeld PA3A wrote: > > We don't have to worry about processorpower. The hardware is already >> there and still developping. And Yes, sampling up to 6m is possible. >> Apart from that, the interfacing from the machine to us humans can >> certainly do better with some innovation (new ideas) as well. >> > > Op 19-8-2013 18:07, Lynn W. Taylor, WB6UUT schreef: >> >>> >>> Consider for a moment an SDR with no analog filtering at all. >>> >>> How wide would the passband be? Are we going to cover everything from >>> MF to 6m? >>> >> > > ______________________________**______________________________**__ > Elecraft mailing list > Home: > http://mailman.qth.net/**mailman/listinfo/elecraft<http://mailman.qth.net/mailman/listinfo/elecraft> > Help: http://mailman.qth.net/mmfaq.**htm<http://mailman.qth.net/mmfaq.htm> > Post: mailto:[email protected].**net <[email protected]> > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________**______________________________**__ > Elecraft mailing list > Home: > http://mailman.qth.net/**mailman/listinfo/elecraft<http://mailman.qth.net/mailman/listinfo/elecraft> > Help: http://mailman.qth.net/mmfaq.**htm<http://mailman.qth.net/mmfaq.htm> > Post: mailto:[email protected].**net <[email protected]> > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
