Steve Wilson wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> piglet wrote: >>> On 06/06/2021 2:51 pm, Cursitor Doom wrote: >>>> On Sun, 6 Jun 2021 11:53:22 +0100, piglet <erichpwagner@hotmail.com> >>>> wrote: >>>> >>>>> On 05/06/2021 11:01 pm, Harry Dudley-Bestow wrote: >>>>>> On Saturday, June 5, 2021 at 2:54:46 PM UTC-7, Cursitor Doom wrote: >>>>>>> On Sat, 5 Jun 2021 11:36:25 -0700 (PDT), Harry Dudley-Bestow >>>>>>> <harry.dud...@gmail.com> wrote: >>>>>>> >>>>>>>> On Saturday, June 5, 2021 at 11:31:34 AM UTC-7, Ed Lee wrote: >>>>>>>>> On Saturday, June 5, 2021 at 11:14:09 AM UTC-7, >>>>>>>>> harry.dud...@gmail.com wrote: >>>>>>>>>> Looking to build a sensible FM radio transmit/receive pair, >>>>>>>>>> designed for short range (10s of meters) and excellent audio >>>>>>>>>> quality. Making for personal use, so very flexible on >>>>>>>>>> operating frequency, BOM cost etc. >>>>>>>>>> >>>>>>>>>> Had a look in my go-to Art of Electronics (not enough detail) >>>>>>>>>> and tried to find an app note or something on the subject, no >>>>>>>>>> dice. Designs on the internet have no explanations alongside >>>>>>>>>> them and are far too optimised for low BOM count at expense >>>>>>>>>> quality of output. >>>>>>>>>> >>>>>>>>>> Anyone got recommendations for design resources on the subject? >>>>>>>>> This should work: >>>>>>>>> >>>>>>>>> https://www.mouser.com/datasheet/2/268/microchip_technology_mchp-s >>>>>>>>> -a0001247358-1-1738104.pdf >>>>>>>>> >>>>>>>>> >>>>>>>>> But I haven't look too deeply into it. >>>>>>>> Sorry Ed I failed to specify that my project must be designed >>>>>>>> using only classic RF components like our forefathers used, no >>>>>>>> all-in-one IC's allowed. Finally a purist! Welcome aboard! >>>>>> Thanks for the recommendation amdx, I'll check it out. >>>>>> >>>>>>  From looking around it looks like my main choices are quadrature >>>>>> detector and PLL demodulation on the receive side. For my taste the >>>>>> quadrature detection is better since it is made out of discrete >>>>>> inductor + capacitor to get the phase delay rather than a newfangled >>>>>> IC, but I can't seem to find out if it's possible to get good >>>>>> quality audio out of it. >>>>>> Naively it seems to me that a large signal bandwidth compared to the >>>>>> IF would be the limiting factor, since that would push the phase >>>>>> shift out of the linear sin(x) = x region. Is my intuition correct >>>>>> here, and is it possible to get audio that is more or less >>>>>> indistinguishable to the kind you get over a copper cable? >>>>>> >>>>> >>>>> In the old days before quadrature and PLL detectors some descrete >>>>> "pulse count" discriminators were used at low-ish intermediate >>>>> frequencies for high quality audio reproduction with fewer parts. >>>>> Should be some 1960s designs online. >>>>> >>>>> I think you will find building the hifi micro-power transmitter will >>>>> be  much easier than building the matching receiver. >>>>> >>>>> piglet >>>> >>>> Warum sagst du das, klein Piglet? >>>> >>> >>> Because taking a few hundred millivolts AF and making a few micro-watts >>> VHF/UHF with pretty good to very good quality can be as a simple as a >>> one transistor "bug" like you probably messed about with as a teenager. >>> >>> However the converse, taking a few nano-watts VHF/UHF and converting >>> into hundreds milli-watts headphone level AF is going to require many >>> transistors. I am not counting a super-regenerative receiver as >>> although their linearity can be good enough to count as HiFi their high >>> noise level will rule them out. >>> >>> I reckon there is no need for crystal control if tx-rx are in the same >>> room environment experiencing similar temp drifts, a crude AFC in the >>> receiver could cope. >>> >>> piglet >> >> You can't usefully do FM with a crystal--the deviation is too narrow.> > Use PM. It is easy to do and very common. > > https://en.wikipedia.org/wiki/Phase_modulationSure, but it sucks compared with wideband FM. At that point the OP might as well use AM, which is a lot easier, and no worse than PM. Cheers -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
FM radio design resources
Started by ●June 5, 2021
Reply by ●June 6, 20212021-06-06
Reply by ●June 6, 20212021-06-06
Harry Dudley-Bestow wrote:> On Sunday, June 6, 2021 at 10:47:28 AM UTC-7, Phil Hobbs wrote: >> Harry Dudley-Bestow wrote: You seen to be talking about a phase >> modulation (PM) system rather than FM. That'll work, but it doesn't >> have FM's SNR advantage. >> >> Armstrong's patent for FM makes a pretty good read actually--his >> big insight was that in the high-SNR limit, widening the bandwidth >> _improved_ SNR rather than degrading it as you'd expect. This is >> because in that limit the noise is linearly downconverted by mixing >> with the desired signal, so you only get one audio bandwidth's >> worth of noise regardless of how wide the deviation is. >> >> Once the SNR drops below about 15 dB, that stops being true--the >> noise intermodulates with itself, so you get contributions from the >> whole RF passband. >> >> The other thing that reduces noise in FM systems is _capture >> effect_ in limiters. (Some types of FM detectors have a limited >> amount of amplitude limiting, so it's sometimes erroneously claimed >> to be specifically an FM effect, but it happens in any system with >> a limiter.) >> >> Basically a limiter just preserves the zero crossings of its input >> signal, so it suppresses amplitude noise. That's good for a 3 dB >> SNR improvement right there, but less obviously it also suppresses >> noise and interference, even right on top of your signal. This is >> because the stronger signal sets the rate of zero crossings--the >> weaker ones can make them jiggle back and forth a bit, but can't >> change their frequency. You can convince yourself of this with a >> simple phasor construction. I'm too lazy to do an ASCII one, but >> you can see it by searching inside my book on Amazon for "capture >> effect". It's discussed on p. 547-548. >> >> If you figure out how to make a nice linear VCO, you can use one >> for TX and a matching one in a PLL for RX. You don't need a >> terribly linear phase detector for FM, because your output signal >> is the VCO control voltage and not the phase detector output.> Phil my intuition was that the reason FM got a SNR boost from going > to wider bandwidths was the information carrying capacity is > bandwidth * log(SNR) and SNR is linear with power, so if you spread > out your power across bandwidth you are basically taking it outside > the log function, making the information bandwidth linear with power > again (minus the small decrease in the log).That's why it's possible, but not why it works. There are a whole lot of ways of making the SNR worse by widening the bandwidth. ;)> I don't know how well this maps onto the proper explanation. I > hadn't thought to take a look in your book for help with this - when > I was considering the IR option I remembered you pointing out the > cascode trick you can do on solar panels to get audio frequencies > worth of bandwidth out of them, but like I said earlier I wanted to > do an RF project.In a pure analogue optical link, you also have to cope with the shot noise of the ambient light, which makes life difficult. (Sunlight is very bright.)> > For optimum audio quality, do you recommend going the PLL demodulator > route, or would a quadrature detector as described here: > > https://www.electronics-notes.com/articles/radio/modulation/fm-frequency-demodulation-quadrature-coincidence-detector-demodulator.php > > > work?Quadrature detectors are sort of OK, but depend very sensitively on the details of the 'quadrature coil'. A nice wideband PLL with a really linear VCO is the ticket. Pulse counting can be OK too if the IF is high, but you're going to need a linear VCO anyway, so you might as well just use two of them.> I can't really find much on the linearity of such detectors. > Intuitively it would seem to me that they would not work so well with > a large modulation index since it would push the phase difference > away from the linear region.Depends. With FM, you're using amplitude-limited waveforms, so the multiplier functions just like an XOR gate, which is nice and linear. A bilinear phase detector such as a Gilbert cell has a sinusoidal V(phi) characteristic if either of the waveforms is sinusoidal. If you overdrive both inputs so that it's not bilinear anymore, you get this nice triangular characteristic.> Discrete PLL's can apparently be a bit tricky to do in discrete > components so I think I'll stick with the simpler quadrature scheme > as described above unless it will seriously impact the quality of the > output.If you want to do a pure discrete design, a la 1966, you'll certainly learn a lot, but I think you're unlikely to get anything that a modern listener would describe as high fidelity without a great deal of effort and time. It's your project, of course, but I'd suggest allowing op amps and ceramic IF filters and such. (A 1966 design would also be a bit unwieldy for headphones--you'd wind up looking like Bullwinkle the Moose.) Mini-Circuits will sell you nice diode-bridge mixers for a few bucks, and those make good phase detectors. (You're going to need a phase detector for your quadrature detector anyway.) A quick IC comparator such as a FIN1002 LVDS line receiver could make a pretty good limiter--its propagation delay is only a few nanoseconds, and its output edges are a bit faster than 200 ps. Plus it costs 27 cents in reels. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●June 6, 20212021-06-06
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: [...]> You seen to be talking about a phase modulation (PM) system rather than > FM. That'll work, but it doesn't have FM's SNR advantage.I thought FM and PM were basically identical with the difference being the response to the modulation frequency. Audio equalization is used to compensate. https://lambdageeks.com/phase-modulation-frequency-modulations/ https://www.tutorialspoint.com/principles_of_communication/principles_of_commun ication_angle_modulation.htm https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part12/page1.html [...]> Cheers > > Phil Hobbs >-- The best designs occur in the theta state. - sw
Reply by ●June 6, 20212021-06-06
Steve Wilson wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > > [...] > >> You seen to be talking about a phase modulation (PM) system rather than >> FM. That'll work, but it doesn't have FM's SNR advantage. > > I thought FM and PM were basically identical with the difference being the > response to the modulation frequency. Audio equalization is used to compensate.They're both examples of _angle modulation_, but in practice are completely different. The key distinguishing feature of FM over PM is much wider deviation, leading to much better SNR for the same received power. An angle modulator driven by a pure tone produces v = sin(2 pi f_0 t + M sin(2 pi f_mod t)). The modulation index M is the peak phase deviation in radians, and is also the ratio of the RF and AF bandwidths. In the high-SNR limit, you win SNR like 20 log M, so wideband is a big win. It's a lot more difficult to make a very wide-range, very linear phase shifter than it is to make a good VCO. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●June 6, 20212021-06-06
piglet wrote:> If you haven't already then you might find ham radio publications > useful. Old ARRL handbooks are a good source of RF lore and I am a fan > of Wes Haywood's book Experimental Methods in RF Design. Hundreds of > milliwatts is non-trivial both legally and technically. Have fun!That book is $179 to $600 (from Abe Books) for a used copy. When a book gets that popular I never understand why they don't reprint it. -- Defund the Thought Police
Reply by ●June 6, 20212021-06-06
On Saturday, June 5, 2021 at 11:34:45 AM UTC-7, harry.dud...@gmail.com wrote:> On Saturday, June 5, 2021 at 11:23:41 AM UTC-7, Sjouke Burry wrote: > > On 05.06.21 20:14, Harry Dudley-Bestow wrote: > > > Looking to build a sensible FM radio transmit/receive pair, designed for short range (10s of meters) and excellent audio quality. Making for personal use, so very flexible on operating frequency, BOM cost etc.> > Why not use an infra-red diode pair to transmit / receive? > > No legal problems. > Yep I have considered that, was thinking of using a huge IR LED bounced off the ceiling, and receiver on my headphones.Another route that should be easy, is to use any broadcast FM receiver with one of the many available Walkman/iPod FM transmit boxes, usually automobile targeted. There's very low transmit power, and a selector switch for selecting an unused frequency for transmit. Third branch, is Bluetooth (but that takes some software or a prebuilt solution).
Reply by ●June 6, 20212021-06-06
On Sat, 5 Jun 2021 11:14:05 -0700 (PDT), Harry Dudley-Bestow <harry.dudleybestow@gmail.com> wrote:>Looking to build a sensible FM radio transmit/receive pair, designed for short range (10s of meters) and excellent audio quality. Making for personal use, so very flexible on operating frequency, BOM cost etc. > >Had a look in my go-to Art of Electronics (not enough detail) and tried to find an app note or something on the subject, no dice. Designs on the internet have no explanations alongside them and are far too optimised for low BOM count at expense quality of output. > >Anyone got recommendations for design resources on the subject?You should search one transistor FM (transmitter)
Reply by ●June 6, 20212021-06-06
On Sunday, June 6, 2021 at 3:01:16 PM UTC-7, whit3rd wrote:> On Saturday, June 5, 2021 at 11:34:45 AM UTC-7, harry.dud...@gmail.com wrote: > > On Saturday, June 5, 2021 at 11:23:41 AM UTC-7, Sjouke Burry wrote: > > > On 05.06.21 20:14, Harry Dudley-Bestow wrote: > > > > Looking to build a sensible FM radio transmit/receive pair, designed for short range (10s of meters) and excellent audio quality. Making for personal use, so very flexible on operating frequency, BOM cost etc. > > > Why not use an infra-red diode pair to transmit / receive? > > > No legal problems. > > Yep I have considered that, was thinking of using a huge IR LED bounced off the ceiling, and receiver on my headphones. > > Another route that should be easy, is to use any broadcast FM receiver with one of the many available > Walkman/iPod FM transmit boxes, usually automobile targeted. There's very low > transmit power, and a selector switch for selecting an unused frequency for transmit. > > Third branch, is Bluetooth (but that takes some software or a prebuilt solution).whit3rd the idea of this project was a fun and educational thing, not necessarily a practical one. So an off-the-shelf thing that works is not what I want. wrt bluetooth actually this entire project came about due to my disgust about the absolute trainwreck that is bluetooth, with its ridiculously high latency and poor reliability. You can literally send a signal around the entire planet in less time than it takes a signal to get from my laptop to my headphones. Hobbs yeah I have no problem using single-purpose block parts, I'm well aware that building something out of bjt's and twisted wire is beyond my current abilities. I was thinking of using something like this for the phase detector as if my thinking is correct I would also be able to use it for the 933MHz->IF downconversion too: https://www.minicircuits.com/pdfs/ADE-2ASK+.pdf
Reply by ●June 6, 20212021-06-06
@LM I know about those, but I'm pretty sure they will have way too much distortion for what I want. I would like to have a high quality audio signal. I'm not an audiophile or anything but I don't want to be able to tell the difference between this and an the same signal sent over an audio cable.
Reply by ●June 6, 20212021-06-06
On Sun, 6 Jun 2021 08:14:43 -0700 (PDT), Bill Sloman <bill.sloman@ieee.org> wrote:>On Sunday, June 6, 2021 at 11:58:35 PM UTC+10, Cursitor Doom wrote: >> On Sun, 6 Jun 2021 04:26:51 -0700 (PDT), Bill Sloman >> <bill....@ieee.org> wrote: >> >> >On Sunday, June 6, 2021 at 6:47:00 PM UTC+10, Cursitor Doom wrote: >> >> On Sat, 5 Jun 2021 17:33:52 -0700 (PDT), Harry Dudley-Bestow >> >> <harry.dud...@gmail.com> wrote: >> >> >> >> >On Saturday, June 5, 2021 at 5:26:32 PM UTC-7, palli...@gmail.com wrote: >> >> >> harry.dud...@gmail.com wrote: > ><snip> > >> >Cursitor Doom knows very little about anything, but he's just as ill-informed about the depth of his ignorance as he is about everything else. >> > >> >He's also an even worse troll than Phil Alison - mainly because Phil Alison knows quite a bit audio (which does include FM radio) and devotes some of his tine to telling people about it. >> > >> >Cursitor Doom's "heads up" is just one more reminder that Cursitor Doom's head is firmly up his own backside. >> >> The three worst trolls on this newsgroup are Bill Sloman (by some margin) followed by Phil Alison and Destitutedrugabuser. I believe they all come from Sydney, which no doubt goes a long way to accounting for this phenomenon; Sydney's lingua franca being outstanding in the English-speaking world for it's unashamed >> courseness. > >Cursitor Doom is wrong a lot of the time. He's probably not the worst troll here - John Doe is even worse, and Flyguy isn't far behind. Nobody else is in the hunt. > >I'm not from Sydney - I might live in Sydney now, but I was born northern Tasmania, went to university in Melbourne, then spent 22 years in the UK and 19 more in the Netherlands. Decadent Linux User Numero Uno is an American - as is obvious to everybody except Cursitor Doom. > >I can't say that I've noticed that Sydney's lingua franca is particularly coarse - I do hang around with university professors and the like, so I might not hear the same sort of language that Cursitor Doom would if he visited here to further his criminal interests. > >> There's not a lot to be said in favour of the rest of the country, either, if the truth be told, but the Sydneyites are a special breed indeed when it comes to getting attention by *any* means. > >Queensland is worse. Look at Clive Palmer. > >https://en.wikipedia.org/wiki/Clive_Palmer > >Admittedly he was born in Victoria (in 1954) and wasn't moved to Queensland until 1963.Never heard of him. However, even your Wikipedia states: "In December 2012, on Christmas Day, Palmer hosted a buffet lunch for 650 disadvantaged people, mostly children and their families." Among a list of similar philanthropic activities, so he clearly doesn't deserve your denigration. I suspect you only despise him for being a conservative politician. As a Communist, you would naturally detest anyone you percieve as 'counter-revolutionary' no matter how kind hearted and charitable they may be. -- Britain: do the right thing and hand Australia back to China.
