On Sunday, June 6, 2021 at 6:58:35 AM UTC-7, 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: > >> >> ========================== > >> >> > Looking to build a sensible FM radio transmit/receive pair, designed for short range (10s of meters) and excellent audio quality. > >> >> > >> >> ** Just one system ? Mono or stereo ? AC or battery powered ? > >> >> > Making for personal use, so very flexible on operating frequency, BOM cost etc. > >> >> ** You are are aware of wireless mic systems for musical performances ? > >> >> Most are variable frequency, wide band FM with "twin diversity" receivers on UHF. > >> >> The latter is very important as single receivers work poorly indoors due to standing wave self cancellations. > >> >> > >> >> Building your own one sounds absurd > >> >> > >> >Phil: > >> >I just want to build one system. I know some FM use a difference signal for stereo but that sounds too complicated for my first project, so I'll just build two systems and operate them of different bands for L/R. The system will be USB powered on the transmit side, and battery powered on the receive side (I'll drill a hole in my headphones and tap off the battery). > >> > > >> >You are of course correct that it is absurd to build my own setup if the goal is to just get a working setup. But this is a learning project and so the journey is the important thing, not the destination! I will look into 'twin diversity' receivers for FM radio. Do you think I could avoid standing wave problems if I just occupied a huge bandwidth (say the whole 902-928MHz band)? > >> > >> Alison knows sweet FA about RF and is one of the biggest trolls on this group - as yu'll find out if you continue your exchange with him. Just thought I'd give you a heads-up on that little fact. > > > >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. 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. > -- > > Britain: do the right thing and hand Australia back to China.Thanks for the advice about pulse counting receivers piglet. I had heard of them, but they sounded like a newfangled digital thing rather than a ye olde device. I will look into that too.
FM radio design resources
Started by ●June 5, 2021
Reply by ●June 6, 20212021-06-06
Reply by ●June 6, 20212021-06-06
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. Peter Dutton is another creep from the Deep North. https://en.wikipedia.org/wiki/Peter_Dutton -- Bill Sloman, Sydney
Reply by ●June 6, 20212021-06-06
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
Reply by ●June 6, 20212021-06-06
On Sunday, June 6, 2021 at 8:28:02 AM UTC-7, piglet wrote:> On 06/06/2021 2:51 pm, Cursitor Doom wrote: > > On Sun, 6 Jun 2021 11:53:22 +0100, piglet <erichp...@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. > > pigletpiglet I am planning off powering the transmit from a PC usb port, so I should have 5V/1A to play with. I don't know what the efficiency of a FM radio transmitter is, but even if it's pretty bad I should be able to push a couple hundred milliwatts into the air so I anticipate the sensitivity of this system will not be a big problem. If there's an easy way to multiply up a crystal to the required frequency then I think I'll do that, else I will go with an LC based frequency generator. The transmitter consuming 5 watts will probably make it heat up though, so I will be wary of temperature-dependant frequency shifts in my design.
Reply by ●June 6, 20212021-06-06
On 6/6/2021 8:58 AM, Cursitor Doom wrote:> On Sun, 6 Jun 2021 04:26:51 -0700 (PDT), Bill Sloman > <bill.sloman@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: >>>>> ========================== >>>>>> Looking to build a sensible FM radio transmit/receive pair, designed for short range (10s of meters) and excellent audio quality. >>>>> ** Just one system ? Mono or stereo ? AC or battery powered ? >>>>>> Making for personal use, so very flexible on operating frequency, BOM cost etc. >>>>> ** You are are aware of wireless mic systems for musical performances ? >>>>> Most are variable frequency, wide band FM with "twin diversity" receivers on UHF. >>>>> The latter is very important as single receivers work poorly indoors due to standing wave self cancellations. >>>>> >>>>> Building your own one sounds absurd >>>>> >>>> Phil: >>>> I just want to build one system. I know some FM use a difference signal for stereo but that sounds too complicated for my first project, so I'll just build two systems and operate them of different bands for L/R. The system will be USB powered on the transmit side, and battery powered on the receive side (I'll drill a hole in my headphones and tap off the battery). >>>> >>>> You are of course correct that it is absurd to build my own setup if the goal is to just get a working setup. But this is a learning project and so the journey is the important thing, not the destination! I will look into 'twin diversity' receivers for FM radio. Do you think I could avoid standing wave problems if I just occupied a huge bandwidth (say the whole 902-928MHz band)? >>> Alison knows sweet FA about RF and is one of the biggest trolls on this group - as yu'll find out if you continue your exchange with him. Just thought I'd give you a heads-up on that little fact. >> 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. 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. This place is so much fun! In the famous words of Rodney King<https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwic5rHcxIPxAhWXVc0KHVG7CaEQFjAIegQICBAD&url=https%3A%2F%2Fwww.ocregister.com%2F2017%2F04%2F30%2Fla-riots-25-years-later-rodney-kings-can-we-all-get-along-still-matters%2F&usg=AOvVaw3gOYzfzXG4n8vvRk90KQkO> Can we all get along ... <https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwic5rHcxIPxAhWXVc0KHVG7CaEQFjAIegQICBAD&url=https%3A%2F%2Fwww.ocregister.com%2F2017%2F04%2F30%2Fla-riots-25-years-later-rodney-kings-can-we-all-get-along-still-matters%2F&usg=AOvVaw3gOYzfzXG4n8vvRk90KQkO> <https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwic5rHcxIPxAhWXVc0KHVG7CaEQFjAIegQICBAD&url=https%3A%2F%2Fwww.ocregister.com%2F2017%2F04%2F30%2Fla-riots-25-years-later-rodney-kings-can-we-all-get-along-still-matters%2F&usg=AOvVaw3gOYzfzXG4n8vvRk90KQkO> Mikek -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●June 6, 20212021-06-06
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? >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. 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:> 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. > > pigletYou can't usefully do FM with a crystal--the deviation is too narrow. 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
On 06/06/2021 5:29 pm, Harry Dudley-Bestow wrote:> On Sunday, June 6, 2021 at 8:28:02 AM UTC-7, piglet wrote: >> On 06/06/2021 2:51 pm, Cursitor Doom wrote: >>> On Sun, 6 Jun 2021 11:53:22 +0100, piglet <erichp...@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 > piglet I am planning off powering the transmit from a PC usb port, so I should have 5V/1A to play with. I don't know what the efficiency of a FM radio transmitter is, but even if it's pretty bad I should be able to push a couple hundred milliwatts into the air so I anticipate the sensitivity of this system will not be a big problem. > If there's an easy way to multiply up a crystal to the required frequency then I think I'll do that, else I will go with an LC based frequency generator. The transmitter consuming 5 watts will probably make it heat up though, so I will be wary of temperature-dependant frequency shifts in my design. >Ham radio folk have indeed made FM transmitters by frequency pulling or phase shifting a crystal and harmonic multiplying up many, many times to transmission frequency. In the 1970s I worked on all-descrete theatrical/Film/TV VHF radio microphones that used that method so it was considered HiFi capable. 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! piglet
Reply by ●June 6, 20212021-06-06
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. > > 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.comPhil 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). 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. 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? 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. 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.
Reply by ●June 6, 20212021-06-06
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. > > Cheers > > Phil HobbsUse PM. It is easy to do and very common. https://en.wikipedia.org/wiki/Phase_modulation -- The best designs occur in the theta state. - sw
