#qi-hardware IRC log for Sunday, 2014-04-27

solrizeare there still any ben nanonotes around?16:32
FrankBluesI have one... kinda gathering dust atm16:33
FrankBluesI've been thinking about running emulators on it - I used to have VICE working, and I thought someone had one of the older Mac emulators working16:35
solrizenice!  i didn't know about vice16:36
ysionneaunewbie question but, would it be very hard to make a simple 2.4 GHz GFSK demodulator?16:41
ysionneauI guess that the fact that some part of the circuit would need to handle high frequency (GHz) is what would be the big issue for DIY16:41
ysionneaubut if you buy some IC to do the PLL part for instance, maybe it's feasible, right?16:43
FrankBluesThere's a Dingux port of VICE that used to run on Ben, last time I tried it had strange dependency issues that I never got around to fixing.16:44
solrizeysionneau, i imagine something like an AM detector to get rid of the baseband, then a hardware or software PLL?  but i have no idea how stuff works at those frequencies16:49
ysionneauI mean, as long as I have a PLL to lock on the 2.4 GHz carrier16:53
ysionneauand then I can demodulate by mixing the input signal with the PLL locked output16:54
ysionneauall of this is the "hard part" because it runs at 2.4 GHz16:54
ysionneauafter that, it's baseband indeed16:54
solrizeyou have a 2.4 ghz pll?  that's what i didn't realize was possible, at least in the dinky equipment (e.g. bluetooth) that i imagine you're dealing with16:54
ysionneauhow can you demodulate a bluetooth signal without a 2.4 GHz pll ?16:55
ysionneauI thought this was mandatory16:55
larscsample _all_ the baseband ;)16:56
ysionneausample with 4.8 GHz ADC?16:56
ysionneauand then do everything as SDR16:56
ysionneaubut you need very expensive electronics16:56
solrize" i have no idea how stuff works at those frequencies" => pure speculation but i was imagining some crude LC filter made of pcb trace, then a diode detector16:57
larscysionneau: there are lots of SDR platforms which allow you to sample at 2.4GHz with a limited bandwidth16:58
solrizeyou should probably ask on #electronics where there might be some analog rf folks hanging out16:58
larscsay a few MHz16:58
solrizelarsc, is that stuff big and complicated?  i was presuming this is for cheap low performance gear like wifi or bluetooth16:59
ysionneaularsc: I am searching for very low price solutions16:59
larscsolrize: depends on your definition of big and complicated 16:59
larscysionneau: but you want the digital bt signal?17:00
wpwrak_ysionneau: what do you want to accomplish in the end ? build an SDR system ? receive BT ? something else17:00
ysionneaureceive BTLE17:00
solrizelarsc, big/complicated = you can't build it into a 5 dollar consumer gizmo17:00
larscthe cheapest solution is probably a off the shelf bt receiver17:01
ysionneauwell, in fact I want to learn RF17:01
ysionneauI think I will start with a simple FM demodulator17:01
larscthen it's not going to be cheap ;)17:01
wpwrak_ysionneau: have you been following what i'm doing on anelok ?17:01
ysionneauwpwrak_: yes :)17:01
wpwrak_and do you just want to receive or also send ?17:01
ysionneaupretty cool stuff17:01
wpwrak_then you already know some chips you can use :)17:01
ysionneaubut I would like to replace the chip with home made circuitery17:02
ysionneauwhen it's possible17:02
ysionneauto learn the RF part17:02
wpwrak_oh, i see. ah yes, there are components for that, too. and fpgas may come in handy, too17:02
solrizelarsc this is the copyleft hardware channel, using off the shelf bt doesn't count ;-).  i've been wondering about this myself because of the suspicious code blobs in all the bt processors17:02
larscsolrize: where do you draw the line?17:03
ysionneaularsc: I want to demodulate, then I get the baseband analog signal, then I can sample with cheap ADC and get the digital stream17:03
ysionneauit's just for fun, no big precise goal17:04
larscysionneau: pll + demodulator17:04
ysionneaularsc: does it sound like a very complex project?17:04
solrizeysionneau, this rf is kind of outside the computer zone... but there are some arrl publications?   arrl.org17:04
ysionneaufor sure I guess I need to buy the PLL since it will operate at high frequency17:05
ysionneaubut the rest of the board... I guess I can drop discrete components17:05
larscthere are also demoulators with a built-in pll17:05
solrizeysionneau, larsc does bt gear actually use plls?17:05
ysionneausolrize: http://www.ti.com/product/lmx316217:05
wpwrak_does anything not use a PLL ? :)17:05
larscysionneau: http://www.analog.com/en/rfif-components/modulatorsdemodulators/products/index.html#Demodulators17:05
wpwrak_PLLs are about as common in modern electronics as electrons :)17:06
ysionneauah nice , thanks larsc :)17:06
solrizewpwrak_, i dunno, we're talking about something with a design range of a few meters and in fact typical handsfree earpieces work a lot better if you hold the phone closer than keeping it in your pocket17:07
solrizeso they are willing to throw away a lot of the signal to keep the hw small and cheap17:07
solrizethat's why i was thinking of amplitude detection17:07
wpwrak_solrize: well, you can build an analog radio system, sure. old school :)17:09
wpwrak_not sure how quickly you'll bump into regulatory limit, though. but then if your sender is weak enough, maybe nobody will notice.17:10
ysionneaufor GFSK I guess I don't care about the I/Q output, right?17:10
ysionneauI just take one of the two17:10
ysionneauI/Q is useful for QAM, no?17:10
solrizewpwrak_, yeah i was thinking if you make it crappy enough, it doesn't need much hardware17:11
larschaving both makes the demodulation dead easy17:11
ysionneauI/Q is the output of the chip, so it's already baseband, right?17:11
ysionneauso already demodulated17:11
Action: ysionneau newbie, sorry17:11
ysionneauby trying to understand some analog circuits, I feel I'm putting a toe in an ocean of unknown stuff17:51
ysionneau"what's this ... ?" + wikipedia/google , then end up with explanation which raises another "what's this ?" etc17:52
Action: ysionneau affraid17:52
Action: whitequark is completely unable to grok analog circuits17:54
DocScrutinizer05>>SDR platforms which allow you to sample at 2.4GHz<< wut?17:55
solrizethose exist but not for bluetooth ;)17:57
solrizethey are for fancy pants mobile base stations17:57
whitequarkthere's some dongle which can sniff bluetooth, but it's absurdly expensive17:58
DocScrutinizer05I doubt *any* SDR is sampling with 4.8Gsamples/s17:58
DocScrutinizer05you always use a Local Oscillator to downbeat-mix the input to some Intermediate Frequency17:59
DocScrutinizer05for <100MHz you might do primary sampling18:00
larscI meant the baseband pll frequency, not the sampling frequency18:00
larscbut there are also systems that have such a sampling frequency18:00
DocScrutinizer05good luck with processing that data stream in realtime X-P18:02
wpwrak_larsc: every hobbyist should have one of those ;-)18:02
larscDocScrutinizer05: you'd typically not process it on a CPU18:03
larscin realtime18:03
DocScrutinizer05so where's the S in SDR then?18:03
larscin the FPGA18:03
DocScrutinizer05sure, a FPGA easily is processing a datastream of ~10GByte/s, probably via FFT18:03
DocScrutinizer05maybe you get away even with 5GB/s18:04
sb0yeah, and then some more. ask the radiotelescope folks :-)18:04
DocScrutinizer05when using 8bit samples18:04
DocScrutinizer05ysionneau: I/Q is basically the 90° phase skewed digitized samples of the IF. All (well, most) SDR is based on that18:09
DocScrutinizer05consider sampling a frequency of 2Pi with a A/D that samples (average in) windows of Pi. You'd either get a maximum output signal of +max -max +max... for frequency in phase with sampling, or a 0, 0, 0, when frequency is 90° off in phase18:12
DocScrutinizer05with I and Q sampling with a skew of 90° phase between the two A/D, you have supfficient info about your signal to do SDR on it, no matter what#s the modulation18:13
DocScrutinizer05basically for AM you would do I+Q, for FM you do I-Q (simplified picture)18:14
DocScrutinizer05since sampling at air frequency is simply impossible for all but the highest budget, at e.g. 2.4GHz, you first mix the input frequency with a Local Oscillator  in a ring modulator that creates f(in)+f(LO) and f(in)-f(LO) as output18:16
DocScrutinizer05then you usually filter out the f(in)-f(LO) band with a bandpass of a few MHz or kHz width, and then sample it or do analog decoding18:17
DocScrutinizer05you also want to have a bandpass at "antenna" to avoid another input frequency sneak in, f1(in)-f(LO) == f2(in)+f(LO)18:18
DocScrutinizer05that's called the mirror frequency18:19
larscthe trick with I/Q modulators acutally is, that there is no image18:20
DocScrutinizer05often SDR works with mixing down to zero, means LO frequency is (almost) equal to input frequency18:20
whitequarkthe cake is a lie18:23
DocScrutinizer05for the LO usually PLL is used (actually an analog oscillator, a reference XTAL oscillator, and two programmable dividers feeding their output to a PLL that adjusts the analog oscillator), which you program to create a LO frequency of f(in)-$some-offset. This $some-offset is the Intermediate frequency you want to use. In shortwave receivers this IF is 455kHz usually, on FM receivers (80-200MHz) it's usually 10.7MHz18:32
DocScrutinizer05so for FM the mirror freq is 2 * 10,7MHz18:32
DocScrutinizer05...off from your intended receiving frequency18:32
DocScrutinizer05you want a bandpass at antenna that gives a sufficient dB attenuation for frequencies that are this far from your center freq, so even strong local transmitters at mirror freq don't interfere with your signal18:35
DocScrutinizer05and you definitely don't want to build all that from discrete components for BTLE ;-)18:37
DocScrutinizer05when you want to learn about that stuff, maybe RTLSDR is a good starting point18:39
DocScrutinizer05the dongles are still available for a 10 to 20 bucks18:39
DocScrutinizer05and some of them should even be able to reach 2.4GHz18:40
ysionneauthanks for the big sum up :)18:45
DocScrutinizer05NB that RTL dongles lack most of the filters/bandpasses I mentioned. The cheat on it18:46
ysionneauso I/Q output of the demodulator are still signals at the carrier frequency?18:48
DocScrutinizer05they don't use proper antenna bandpass so mirror frequencies may be a problem, and they don't use IF filter bandpass so the A/D for I/Q is prone to aliasing18:48
ysionneauthey are not located after the multiplication of the local oscillator ?18:48
DocScrutinizer05no, they use a downbeat mixer tuner as described above18:48
DocScrutinizer05the tuner output is some Intermediate Frequency of a few MHz which then gets digitized by the RTL2832 chip18:49
DocScrutinizer05the tuner has the PLL aka LO and the mixer, and some amps18:50
DocScrutinizer05the RTL2832 sends I/Q data to PC via USB18:51
ysionneauah ok so it lowers the frequency a bit, but not until baseband18:51
ysionneauok :o18:51
DocScrutinizer05depends on how you program it18:51
DocScrutinizer05"baseband" is not demodulated signal though. What you call "baseband" would be IF of zero18:52
DocScrutinizer05and it's actually usually doing that18:52
ysionneauIF = input frequency?18:52
DocScrutinizer05well, some setups use a very low IF of some kHz or so, to get rid of the DC that sneaks in to I/Q when you mix down to exactl zero18:53
DocScrutinizer05Intermediate Frequency18:53
ysionneauwhat I call baseband is when you mix the input signal with exactly the carrier frequency (by using a PLL to obtain it, in phase)18:53
DocScrutinizer05the input to the RTL2832, coming from tuner output18:53
ysionneauthen you should get your demodulated tone, but centered at 0 Hz18:54
ysionneauinstead of centered on the carrier freq18:54
DocScrutinizer05well, modulation not working like this18:54
DocScrutinizer05for AM you might get away with it, yes18:54
ysionneauyes I think most of my "modulation" thinking is centered on QAM stuf18:55
DocScrutinizer05but then it's pretty hard to filter away the LO signal from your input signal when you mix down to exact 0 to demodulate18:55
ysionneauwould you have some litterature to educate me on those topics ?18:55
DocScrutinizer05hmm, maybe something comes to mind when I tthink a bit about it18:56
ysionneauon several examples: FSK, QAM all sort of different modulation scheme18:56
DocScrutinizer05right off top of my head I'm short of suggestions18:56
DocScrutinizer05well, those are all already digital modulations18:57
ysionneaubook or website (pdf..) whatever 18:57
DocScrutinizer05FSK is digital FM18:57
ysionneauI think I can start by digging up my old school book18:57
DocScrutinizer05QAM is digital AM (more or less)18:57
ysionneauyep I got that18:57
DocScrutinizer05sorry, afk18:58
ysionneauI'm mostly interested in digital communications18:58
ysionneaunp, thanks :)18:58
Action: wpwrak hates those *_19:06
pcercueilarsc, hey, do you know an internet provider in Germany that offers contracts without a minimal period?20:09
larscthat's what I use20:14
pcercueiI didn't know you were into K-pop20:16
pcercueiunless mnet.com is not the right website20:16
pcercueiah, .de20:17
pcercueiof course20:17
larscI think I can refer you20:20
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