Tom Del Rosso wrote:> Phil Hobbs wrote: >> Harry Dudley-Bestow wrote: >>> Right, so this is the block diagram that I have come up with so far: >>> https://imgur.com/a/ZIbZz4y >>> >>> Not sure about the transmitter side - didn't find much in the way of >>> clear explanations on the internet but the art of electronics says >>> on the topic of FM transmission "it is often best to modulate at low >>> deviation, then use frequency multiplication to increase the >>> modulation index." (page 899, 2nd ed) so the block diagram does >>> that. I don't know how I would go about multiplying all the way up to >>> 915MHz, since a VCO based on op amps obviously won't go that high. I >>> suppose I'll have to find and buy an off the shelf part for that >>> step. I'm also no sure about the sidebands. I suppose I want a single >>> sideband (SSB), but I have not quite figured out yet how to go about >>> putting that in. >>> >> >> Ain't no such animal as SSB FM. >> >> For an educational project, it's worth going through the derivation of >> the FM spectrum. You'll need Wolfram Alpha or a copy of Abramowitz & >> Stegun to get the Bessel function expansion, but it's about a >> third-year undergraduate problem. >> >> When you do it, you find that the FM spectrum is made up op of a >> forest of sidebands. The nth-order sideband amplitude is >> proportional to J_n(m), where m is the modulation index (see one of >> my earlier posts). >> The math is not difficult, and it's really quite pretty. > > A million different FM receivers have been made and sold. Were many of > them designed by someone who derived the FM spectrum first?Did you count them? ;)> It doesn't make sense that something that's been done for 100 years > should be less approachable than other hobby projects. There are several > methods, so the answer should be how to do one of each.I look forward to reading your explanations of each one, and their respective pros and cons for the OP's application.> The transmitter > can come later, being easier but more dangerous for interference.The OP said it was an educational hobby project, so what's wrong with a bit of education? Knowing the functional form of the coefficients makes it easy to estimate the effects of sideband cutting and phase nonlinearity, which is pretty useful in the detailed design of the IF filters, for instance. Also the math is just really pretty, in a small way. I have no idea how far the OP will get with all this, but I'm all for encouraging folks who show an interest. 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
FM radio design resources
Started by ●June 5, 2021
Reply by ●June 10, 20212021-06-10
Reply by ●June 11, 20212021-06-11
On 10.6.2021 17:18 PM, Tom Del Rosso wrote:> Phil Hobbs wrote: >> Harry Dudley-Bestow wrote: >>> Right, so this is the block diagram that I have come up with so far: >>> https://imgur.com/a/ZIbZz4y >>> >>> Not sure about the transmitter side - didn't find much in the way of >>> clear explanations on the internet but the art of electronics says >>> on the topic of FM transmission "it is often best to modulate at low >>> deviation, then use frequency multiplication to increase the >>> modulation index." (page 899, 2nd ed) so the block diagram does >>> that. I don't know how I would go about multiplying all the way up to >>> 915MHz, since a VCO based on op amps obviously won't go that high. I >>> suppose I'll have to find and buy an off the shelf part for that >>> step. I'm also no sure about the sidebands. I suppose I want a single >>> sideband (SSB), but I have not quite figured out yet how to go about >>> putting that in. >>> >> >> Ain't no such animal as SSB FM. >> >> For an educational project, it's worth going through the derivation of >> the FM spectrum. You'll need Wolfram Alpha or a copy of Abramowitz & >> Stegun to get the Bessel function expansion, but it's about a >> third-year undergraduate problem. >> >> When you do it, you find that the FM spectrum is made up op of a >> forest of sidebands. The nth-order sideband amplitude is >> proportional to J_n(m), where m is the modulation index (see one of >> my earlier posts). >> The math is not difficult, and it's really quite pretty. > > A million different FM receivers have been made and sold. Were many of > them designed by someone who derived the FM spectrum first? > > It doesn't make sense that something that's been done for 100 years > should be less approachable than other hobby projects. There are several > methods, so the answer should be how to do one of each. The transmitter > can come later, being easier but more dangerous for interference.What was done nearly 100 years ago did not have HiFi quality, as the original post wants. For broadcast quality FM reception, a bandwidth around 300 kHz is needed, with as linear phase as possible. I doubt that this is achievable without either ready-made modules or good measuring equipment. The suggested 900 MHz band poses a larger challenge to the hobbyist, as the stray inductances and capacitances together with transmission- line effects play a much larger role than on e.g. 100 MHz band. -- -TV
Reply by ●June 13, 20212021-06-13
On Wed, 9 Jun 2021 03:50:06 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Cursitor Doom wrote: >> On Mon, 7 Jun 2021 09:29:30 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>> Depends for what. The OP is probably not going to be using direct >>> digitization at 900 MHz for a battery-powerd hobby project, and I'm not >>> going to be using $20k worth of lithium niobate modulators to put FM >>> sidebands on my diode laser, even if they worked at my wavelength. >> >> On that subject (forgive me if it's already been suggested) the >> Chinese sell cheap but powerful lasers and just a 50 cent 5mW one >> ought to be able to convey a modulated signal over 20 meters or so >> that the OP requires, surely? >> > >Presents a bit of a pointing problem, of course.Leaving that aside for a moment, what could be used to 'recieve' the modulated beam? Would a photodiode do it?
Reply by ●June 13, 20212021-06-13
On Tue, 8 Jun 2021 20:18:24 -0700 (PDT), Bill Sloman <bill.sloman@ieee.org> wrote:>On Wednesday, June 9, 2021 at 9:30:07 AM UTC+10, Brian Gregory wrote: >> On 05/06/2021 19:36, Harry Dudley-Bestow 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. >> > >> >> Daft question about an almost impossible to do and almost impossible to >> do legally project with more and more ludicrous restrictions being added >> rapidly with each message posted. >> >> This is clearly a very nasty troll of just about the worst kind.Eh???> >Not really. More a classical dumb newbie, of the more than usually incompetent sort. Trolls are malicious.You of all people would know. Couldn't resist a gratuitous put-down as usual. Never miss up on an opportunity to throw in a supercillious slur, eh, Bill?
Reply by ●June 13, 20212021-06-13
On Fri, 11 Jun 2021 12:18:24 +0300, Tauno Voipio <tauno.voipio@notused.fi.invalid> wrote:>On 10.6.2021 17:18 PM, Tom Del Rosso wrote: >> Phil Hobbs wrote: >>> Harry Dudley-Bestow wrote: >>>> Right, so this is the block diagram that I have come up with so far: >>>> https://imgur.com/a/ZIbZz4y >>>> >>>> Not sure about the transmitter side - didn't find much in the way of >>>> clear explanations on the internet but the art of electronics says >>>> on the topic of FM transmission "it is often best to modulate at low >>>> deviation, then use frequency multiplication to increase the >>>> modulation index." (page 899, 2nd ed) so the block diagram does >>>> that. I don't know how I would go about multiplying all the way up to >>>> 915MHz, since a VCO based on op amps obviously won't go that high. I >>>> suppose I'll have to find and buy an off the shelf part for that >>>> step. I'm also no sure about the sidebands. I suppose I want a single >>>> sideband (SSB), but I have not quite figured out yet how to go about >>>> putting that in. >>>> >>> >>> Ain't no such animal as SSB FM. >>> >>> For an educational project, it's worth going through the derivation of >>> the FM spectrum. You'll need Wolfram Alpha or a copy of Abramowitz & >>> Stegun to get the Bessel function expansion, but it's about a >>> third-year undergraduate problem. >>> >>> When you do it, you find that the FM spectrum is made up op of a >>> forest of sidebands. The nth-order sideband amplitude is >>> proportional to J_n(m), where m is the modulation index (see one of >>> my earlier posts). >>> The math is not difficult, and it's really quite pretty. >> >> A million different FM receivers have been made and sold. Were many of >> them designed by someone who derived the FM spectrum first? >> >> It doesn't make sense that something that's been done for 100 years >> should be less approachable than other hobby projects. There are several >> methods, so the answer should be how to do one of each. The transmitter >> can come later, being easier but more dangerous for interference. > > >What was done nearly 100 years ago did not have HiFi quality, as the >original post wants. For broadcast quality FM reception, a bandwidth >around 300 kHz is needed, with as linear phase as possible. I doubt >that this is achievable without either ready-made modules or good >measuring equipment. > >The suggested 900 MHz band poses a larger challenge to the hobbyist, >as the stray inductances and capacitances together with transmission- >line effects play a much larger role than on e.g. 100 MHz band.I agree. 900Mhz is not for entry-level experimenters/hobbyists for the reasons you state. I suspect if the OP doesn't come down by a factor of 10 on that he will be frustrated, disappointed and eventually give up.
Reply by ●June 13, 20212021-06-13
On Wed, 9 Jun 2021 11:35:52 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>I'd start with a CPH3910 JFET Colpitts oscillator using a dual >hyperabrupt varactor such as a BB201 as the tank cap / voltage divider. > That'll have too much gain, but the tuning will be pretty linear over a >good part of the range, and since it's a one-off, you can keep the >amplitude reasonable by reducing the drain current.Hyperabrupt and linear and two words that don't belong in the same sentence. ;-)
Reply by ●June 13, 20212021-06-13
On Sunday, June 13, 2021 at 8:37:03 PM UTC+10, Cursitor Doom wrote:> On Tue, 8 Jun 2021 20:18:24 -0700 (PDT), Bill Sloman > <bill....@ieee.org> wrote: > > >On Wednesday, June 9, 2021 at 9:30:07 AM UTC+10, Brian Gregory wrote: > >> On 05/06/2021 19:36, Harry Dudley-Bestow 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. > >> > > >> > >> Daft question about an almost impossible to do and almost impossible to > >> do legally project with more and more ludicrous restrictions being added > >> rapidly with each message posted. > >> > >> This is clearly a very nasty troll of just about the worst kind. > Eh??? > > > >Not really. More a classical dumb newbie, of the more than usually incompetent sort. Trolls are malicious. > > You of all people would know.You, Flyguy and John Larkin remind me of it more or less nonstop. Happily all of you are remarkably pathetic in your malice. Jim Thompson once claimed to have denounced me to the FBI which struck me as excessive.> Couldn't resist a gratuitous put-down as usual.That put-down was well-earned, rather than gratuitous.> Never miss up on an opportunity to throw in a supercillious slur, eh, Bill?It's hard to avoid being supercilious when confronted by people who offer a lot to be supercillious about. You'd be a prime example of somebody who must get that reaction frequently. -- Bill Sloman, Sydney
Reply by ●June 13, 20212021-06-13
On Sun, 13 Jun 2021 06:37:43 -0700 (PDT), Bill Sloman <bill.sloman@ieee.org> wrote:>On Sunday, June 13, 2021 at 8:37:03 PM UTC+10, Cursitor Doom wrote: >> On Tue, 8 Jun 2021 20:18:24 -0700 (PDT), Bill Sloman >> <bill....@ieee.org> wrote: >> >> >On Wednesday, June 9, 2021 at 9:30:07 AM UTC+10, Brian Gregory wrote: >> >> On 05/06/2021 19:36, Harry Dudley-Bestow 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. >> >> > >> >> >> >> Daft question about an almost impossible to do and almost impossible to >> >> do legally project with more and more ludicrous restrictions being added >> >> rapidly with each message posted. >> >> >> >> This is clearly a very nasty troll of just about the worst kind. >> Eh??? >> > >> >Not really. More a classical dumb newbie, of the more than usually incompetent sort. Trolls are malicious. >> >> You of all people would know. > >You, Flyguy and John Larkin remind me of it more or less nonstop. Happily all of you are remarkably pathetic in your malice. Jim Thompson once claimed to have denounced me to the FBI which struck me as excessive.Oh really? THAT struck you as excessive, but your wish to see John and I hanged for disagreeing with you is perfectly reasonable??>> Couldn't resist a gratuitous put-down as usual. > >That put-down was well-earned, rather than gratuitous.No it wasn't. The OP's question - from a technical standpoint at any rate - was 100% on topic for this group.> >> Never miss up on an opportunity to throw in a supercillious slur, eh, Bill? > >It's hard to avoid being supercilious when confronted by people who offer a lot to be supercillious about. You'd be a prime example of somebody who must get that reaction frequently.As ever, you demonstrate a grandiose sense of self-worth. I believe I've already informed you of what that trait is symptomatic of.
Reply by ●June 13, 20212021-06-13
Cursitor Doom wrote:> On Wed, 9 Jun 2021 11:35:52 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> I'd start with a CPH3910 JFET Colpitts oscillator using a dual >> hyperabrupt varactor such as a BB201 as the tank cap / voltage divider. >> That'll have too much gain, but the tuning will be pretty linear over a >> good part of the range, and since it's a one-off, you can keep the >> amplitude reasonable by reducing the drain current. > > Hyperabrupt and linear and two words that don't belong in the same > sentence. ;-) >Not so. To get linear tuning, you need a varactor whose C(V) curve goes as 1/(V-V0)**2 in the range of interest. There are fancy hyperabrupts whose doping profile is minutely adjusted to make them follow that curve accurately, but generally a dual hyperabrupt will exhibit a range of a few volts someplace where that's a pretty good approximation. Using the dual varactor with no pad caps won't make for the most stable tuning, but it'll have a wide range, for sure. That way you can find the sweet spot. Not what you'd do in production, at least not without external AFC, but just the ticket for a one-off, I think. 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 13, 20212021-06-13
Cursitor Doom wrote:> On Wed, 9 Jun 2021 03:50:06 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> Cursitor Doom wrote: >>> On Mon, 7 Jun 2021 09:29:30 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>> Depends for what. The OP is probably not going to be using direct >>>> digitization at 900 MHz for a battery-powerd hobby project, and I'm not >>>> going to be using $20k worth of lithium niobate modulators to put FM >>>> sidebands on my diode laser, even if they worked at my wavelength.S >>> >>> On that subject (forgive me if it's already been suggested) the >>> Chinese sell cheap but powerful lasers and just a 50 cent 5mW one >>> ought to be able to convey a modulated signal over 20 meters or so >>> that the OP requires, surely? >>> >> >> Presents a bit of a pointing problem, of course. > > Leaving that aside for a moment, what could be used to 'recieve' the > modulated beam? Would a photodiode do it? >Sure. Lowish-performance audio is simple. Making it work reliably in real life would involve avoiding mode hopping, for one thing, and that doesn't happen by accident. Current-modulating a diode laser produces both AM and FM, but a photodiode detects only AM. You can detect the FM, but it needs an interferometer, which makes the pointing problem much, much worse. 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
