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. 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 9, 20212021-06-09
Reply by ●June 9, 20212021-06-09
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.Impossible? Why? He probably needs about 10 mW of RF for a job like that, and 0.1% THD ought to be quite doable with a decent VCO design. Whether the OP can make it work remains to be seen. 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 9, 20212021-06-09
On 7.6.2021 07:56 AM, Edward Rawde wrote:> "Harry Dudley-Bestow" <harry.dudleybestow@gmail.com> wrote in message > news:669bc2aa-92e8-4290-a91b-6b3f34bff7bbn@googlegroups.com... >> 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? > > "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," > "The project needs to be RF because I've arbitrarily decided to do an RF > project :)" > "the idea of this project was a fun and educational thing, not necessarily a > practical one." > "would like to have a high quality audio signal." > "and is it possible to get audio that is more or less indistinguishable to > the kind you get over a copper cable?" > "I don't know what the efficiency of a FM radio transmitter is" > "Discrete PLL's can apparently be a bit tricky to do in discrete components" > "but I am well familiar with digital techniques" > "I just want to build one system" > "I'll drill a hole in my headphones and tap off the battery" > "Do you think I could avoid standing wave problems if I just occupied a huge > bandwidth" > "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." > "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." > > Now that we have a full requirement specification I think one of these would > be most suitable for use in the receiver: > > http://www.r-type.org/exhib/aaa1272.htm > > I've always wanted to use one but never have. It looks way more fun that a > dual gate mosfet. > > There are some practical circuits here: > > https://www.radiomuseum.org/forum/fm_demodulation_weniger_bekannte_verfahren_1.html > > It shouldn't be hard to make a low power transmitter. for example: > > https://www.electroschematics.com/low-power-fm-transmitter/ > > But if I wanted high quality audio over a distance of 10s of meters I'd > consider longitudinal pressure waves sent through the air, assuming it > didn't disturb anyone else. > Suitable transmitters are readily available and you've already got a > receiver or two.The specifications and the apparent lack of RF clue of the OP are in strong disagreement. IMHO, if he's honestly willing to do something, I'd suggest first to build a receiver for the 100 MHz FM band, with the required quality. I suspect that it will be pretty difficult to get the IF and detector string work at a HiFi level without proper test instuments. The transmitter ends be simple as soon as we want HiFi -level stability and distortion specs. -- -TV
Reply by ●June 9, 20212021-06-09
On 6/9/21 6:32 AM, Tauno Voipio wrote:> On 7.6.2021 07:56 AM, Edward Rawde wrote: >> "Harry Dudley-Bestow" <harry.dudleybestow@gmail.com> wrote in message >> news:669bc2aa-92e8-4290-a91b-6b3f34bff7bbn@googlegroups.com... >>> 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? >> >> "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," >> "The project needs to be RF because I've arbitrarily decided to do an RF >> project :)" >> "the idea of this project was a fun and educational thing, not >> necessarily a >> practical one." >> "would like to have a high quality audio signal." >> "and is it possible to get audio that is more or less >> indistinguishable to >> the kind you get over a copper cable?" >> "I don't know what the efficiency of a FM radio transmitter is" >> "Discrete PLL's can apparently be a bit tricky to do in discrete >> components" >> "but I am well familiar with digital techniques" >> "I just want to build one system" >> "I'll drill a hole in my headphones and tap off the battery" >> "Do you think I could avoid standing wave problems if I just occupied >> a huge >> bandwidth" >> "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." >> "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." >> >> Now that we have a full requirement specification I think one of these >> would >> be most suitable for use in the receiver: >> >> http://www.r-type.org/exhib/aaa1272.htm >> >> I've always wanted to use one but never have. It looks way more fun >> that a >> dual gate mosfet. >> >> There are some practical circuits here: >> >> https://www.radiomuseum.org/forum/fm_demodulation_weniger_bekannte_verfahren_1.html >> >> >> It shouldn't be hard to make a low power transmitter. for example: >> >> https://www.electroschematics.com/low-power-fm-transmitter/ >> >> But if I wanted high quality audio over a distance of 10s of meters I'd >> consider longitudinal pressure waves sent through the air, assuming it >> didn't disturb anyone else. >> Suitable transmitters are readily available and you've already got a >> receiver or two. > > > The specifications and the apparent lack of RF clue of the OP are in > strong disagreement. > > IMHO, if he's honestly willing to do something, I'd suggest first to > build a receiver for the 100 MHz FM band, with the required quality. > I suspect that it will be pretty difficult to get the IF and detector > string work at a HiFi level without proper test instuments. > > The transmitter ends be simple as soon as we want HiFi -level stability > and distortion specs. >Two reasonably well-matched hyperabrupt-varactor VCOs, the TX one free-running and the RX one phase locked, should get down to the 0.1% level with a bit of care. Of course the OP doesn't have an HP 339A to measure it on. 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 https://hobbs-eo.com
Reply by ●June 9, 20212021-06-09
On Wednesday, June 9, 2021 at 8:01:39 AM UTC-7, Phil Hobbs wrote:> On 6/9/21 6:32 AM, Tauno Voipio wrote: > > On 7.6.2021 07:56 AM, Edward Rawde wrote: > >> "Harry Dudley-Bestow" <harry.dud...@gmail.com> wrote in message > >> news:669bc2aa-92e8-4290...@googlegroups.com... > >>> 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? > >> > >> "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," > >> "The project needs to be RF because I've arbitrarily decided to do an RF > >> project :)" > >> "the idea of this project was a fun and educational thing, not > >> necessarily a > >> practical one." > >> "would like to have a high quality audio signal." > >> "and is it possible to get audio that is more or less > >> indistinguishable to > >> the kind you get over a copper cable?" > >> "I don't know what the efficiency of a FM radio transmitter is" > >> "Discrete PLL's can apparently be a bit tricky to do in discrete > >> components" > >> "but I am well familiar with digital techniques" > >> "I just want to build one system" > >> "I'll drill a hole in my headphones and tap off the battery" > >> "Do you think I could avoid standing wave problems if I just occupied > >> a huge > >> bandwidth" > >> "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." > >> "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." > >> > >> Now that we have a full requirement specification I think one of these > >> would > >> be most suitable for use in the receiver: > >> > >> http://www.r-type.org/exhib/aaa1272.htm > >> > >> I've always wanted to use one but never have. It looks way more fun > >> that a > >> dual gate mosfet. > >> > >> There are some practical circuits here: > >> > >> https://www.radiomuseum.org/forum/fm_demodulation_weniger_bekannte_verfahren_1.html > >> > >> > >> It shouldn't be hard to make a low power transmitter. for example: > >> > >> https://www.electroschematics.com/low-power-fm-transmitter/ > >> > >> But if I wanted high quality audio over a distance of 10s of meters I'd > >> consider longitudinal pressure waves sent through the air, assuming it > >> didn't disturb anyone else. > >> Suitable transmitters are readily available and you've already got a > >> receiver or two. > > > > > > The specifications and the apparent lack of RF clue of the OP are in > > strong disagreement. > > > > IMHO, if he's honestly willing to do something, I'd suggest first to > > build a receiver for the 100 MHz FM band, with the required quality. > > I suspect that it will be pretty difficult to get the IF and detector > > string work at a HiFi level without proper test instuments. > > > > The transmitter ends be simple as soon as we want HiFi -level stability > > and distortion specs. > > > Two reasonably well-matched hyperabrupt-varactor VCOs, the TX one > free-running and the RX one phase locked, should get down to the 0.1% > level with a bit of care. Of course the OP doesn't have an HP 339A to > measure it on. > 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 > https://hobbs-eo.comPhil I was planning on using the VCO's outlined by Jim Williams here: https://www.analog.com/media/en/technical-documentation/application-notes/an14f.pdf which seem a bit complicated, but approachable. The app note was written in 1986 though, are there hyperabrupt-varactor diodes available now that mean a much simpler design with the same performance is possible?
Reply by ●June 9, 20212021-06-09
On 6/9/21 11:12 AM, Harry Dudley-Bestow wrote:> On Wednesday, June 9, 2021 at 8:01:39 AM UTC-7, Phil Hobbs wrote: >> On 6/9/21 6:32 AM, Tauno Voipio wrote: >>> On 7.6.2021 07:56 AM, Edward Rawde wrote: >>>> "Harry Dudley-Bestow" <harry.dud...@gmail.com> wrote in message >>>> news:669bc2aa-92e8-4290...@googlegroups.com... >>>>> 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? >>>> >>>> "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," "The project needs to be RF >>>> because I've arbitrarily decided to do an RF project :)" "the >>>> idea of this project was a fun and educational thing, not >>>> necessarily a practical one." "would like to have a high >>>> quality audio signal." "and is it possible to get audio that is >>>> more or less indistinguishable to the kind you get over a >>>> copper cable?" "I don't know what the efficiency of a FM radio >>>> transmitter is" "Discrete PLL's can apparently be a bit tricky >>>> to do in discrete components" "but I am well familiar with >>>> digital techniques" "I just want to build one system" "I'll >>>> drill a hole in my headphones and tap off the battery" "Do you >>>> think I could avoid standing wave problems if I just occupied >>>> a huge bandwidth" "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." "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." >>>> >>>> Now that we have a full requirement specification I think one >>>> of these would be most suitable for use in the receiver: >>>> >>>> http://www.r-type.org/exhib/aaa1272.htm >>>> >>>> I've always wanted to use one but never have. It looks way more >>>> fun that a dual gate mosfet. >>>> >>>> There are some practical circuits here: >>>> >>>> https://www.radiomuseum.org/forum/fm_demodulation_weniger_bekannte_verfahren_1.html >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> It shouldn't be hard to make a low power transmitter. for >>>> example: >>>> >>>> https://www.electroschematics.com/low-power-fm-transmitter/ >>>> >>>> But if I wanted high quality audio over a distance of 10s of >>>> meters I'd consider longitudinal pressure waves sent through >>>> the air, assuming it didn't disturb anyone else. Suitable >>>> transmitters are readily available and you've already got a >>>> receiver or two. >>> >>> >>> The specifications and the apparent lack of RF clue of the OP are >>> in strong disagreement. >>> >>> IMHO, if he's honestly willing to do something, I'd suggest first >>> to build a receiver for the 100 MHz FM band, with the required >>> quality. I suspect that it will be pretty difficult to get the IF >>> and detector string work at a HiFi level without proper test >>> instuments. >>> >>> The transmitter ends be simple as soon as we want HiFi -level >>> stability and distortion specs. >>> >> Two reasonably well-matched hyperabrupt-varactor VCOs, the TX one >> free-running and the RX one phase locked, should get down to the >> 0.1% level with a bit of care. Of course the OP doesn't have an HP >> 339A to measure it on. Cheers> Phil I was planning on using the VCO's outlined by Jim Williams > here: > > https://www.analog.com/media/en/technical-documentation/application-notes/an14f.pdf > > which seem a bit complicated,You can say that again. JW's stuff was always interesting and fun, but he had this tendency to use _way_ too many parts.> but approachable. The app note was written in 1986 though, are there > hyperabrupt-varactor diodes available now that mean a much simpler > design with the same performance is possible?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. Control the varactor using a pot connected between the power supply and ground, with a 100k series resistor from there to the varactors. Measure the voltage at the wiper, not on the oscillator side. (Note that varactors work in reverse bias.) A 470-nH to 1-uH inductor might be about right--it'll make the frequency low enough to look at on an inexpensive scope. You'll want to get a cheap Chinese frequency counter to test it with, and a DVM if you don't have one. If you can beg or borrow a decent oscilloscope, that will be a great help. Couple the counter to the source of the JFET via a 1k resistor to avoid loading it down. If you make two of them using the same parts and the same layout, you can measure their frequency vs. voltage curves. The THD will be determined by how closely they coincide. 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 https://hobbs-eo.com
Reply by ●June 9, 20212021-06-09
On 6/9/21 11:35 AM, Phil Hobbs wrote:> On 6/9/21 11:12 AM, Harry Dudley-Bestow wrote: >> On Wednesday, June 9, 2021 at 8:01:39 AM UTC-7, Phil Hobbs wrote: >>> On 6/9/21 6:32 AM, Tauno Voipio wrote: >>>> On 7.6.2021 07:56 AM, Edward Rawde wrote: >>>>> "Harry Dudley-Bestow" <harry.dud...@gmail.com> wrote in message >>>>> news:669bc2aa-92e8-4290...@googlegroups.com... >>>>>> 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? >>>>> >>>>> "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," "The project needs to be RF >>>>> because I've arbitrarily decided to do an RF project :)" "the >>>>> idea of this project was a fun and educational thing, not >>>>> necessarily a practical one." "would like to have a high >>>>> quality audio signal." "and is it possible to get audio that is >>>>> more or less indistinguishable to the kind you get over a >>>>> copper cable?" "I don't know what the efficiency of a FM radio >>>>> transmitter is" "Discrete PLL's can apparently be a bit tricky >>>>> to do in discrete components" "but I am well familiar with >>>>> digital techniques" "I just want to build one system" "I'll >>>>> drill a hole in my headphones and tap off the battery" "Do you >>>>> think I could avoid standing wave problems if I just occupied >>>>> a huge bandwidth" "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." "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." >>>>> >>>>> Now that we have a full requirement specification I think one >>>>> of these would be most suitable for use in the receiver: >>>>> >>>>> http://www.r-type.org/exhib/aaa1272.htm >>>>> >>>>> I've always wanted to use one but never have. It looks way more >>>>> fun that a dual gate mosfet. >>>>> >>>>> There are some practical circuits here: >>>>> >>>>> https://www.radiomuseum.org/forum/fm_demodulation_weniger_bekannte_verfahren_1.html >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> It shouldn't be hard to make a low power transmitter. for >>>>> example: >>>>> >>>>> https://www.electroschematics.com/low-power-fm-transmitter/ >>>>> >>>>> But if I wanted high quality audio over a distance of 10s of >>>>> meters I'd consider longitudinal pressure waves sent through >>>>> the air, assuming it didn't disturb anyone else. Suitable >>>>> transmitters are readily available and you've already got a >>>>> receiver or two. >>>> >>>> >>>> The specifications and the apparent lack of RF clue of the OP are >>>> in strong disagreement. >>>> >>>> IMHO, if he's honestly willing to do something, I'd suggest first >>>> to build a receiver for the 100 MHz FM band, with the required >>>> quality. I suspect that it will be pretty difficult to get the IF >>>> and detector string work at a HiFi level without proper test >>>> instuments. >>>> >>>> The transmitter ends be simple as soon as we want HiFi -level >>>> stability and distortion specs. >>>> >>> Two reasonably well-matched hyperabrupt-varactor VCOs, the TX one >>> free-running and the RX one phase locked, should get down to the >>> 0.1% level with a bit of care. Of course the OP doesn't have an HP >>> 339A to measure it on. Cheers > >> Phil I was planning on using the VCO's outlined by Jim Williams >> here: >> >> https://www.analog.com/media/en/technical-documentation/application-notes/an14f.pdf >> >> which seem a bit complicated, > > You can say that again. JW's stuff was always interesting and fun, but > he had this tendency to use _way_ too many parts. > >> but approachable. The app note was written in 1986 though, are there >> hyperabrupt-varactor diodes available now that mean a much simpler >> design with the same performance is possible? > > 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. > > Control the varactor using a pot connected between the power supply and > ground, with a 100k series resistor from there to the varactors. > Measure the voltage at the wiper, not on the oscillator side. (Note > that varactors work in reverse bias.) > > A 470-nH to 1-uH inductor might be about right--it'll make the frequency > low enough to look at on an inexpensive scope. > You'll want to get a cheap Chinese frequency counter to test it with, > and a DVM if you don't have one. If you can beg or borrow a decent > oscilloscope, that will be a great help. > > Couple the counter to the source of the JFET via a 1k resistor to avoid > loading it down. > > If you make two of them using the same parts and the same layout, you > can measure their frequency vs. voltage curves. The THD will be > determined by how closely they coincide. > > Cheers > > Phil Hobbs > > > > >Oh, and the varactors are wired in series opposing (common cathode), with the (positive) bias applied at the midpoint. -- 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 https://hobbs-eo.com
Reply by ●June 9, 20212021-06-09
On Wednesday, June 9, 2021 at 8:36:10 AM UTC-7, Phil Hobbs wrote:> You can say that again. JW's stuff was always interesting and fun, but > he had this tendency to use _way_ too many parts. > > but approachable. The app note was written in 1986 though, are there > > hyperabrupt-varactor diodes available now that mean a much simpler > > design with the same performance is possible? > 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.Yep the art of electronics has a reasonable section on varactor tuned resonant circuits and they are *much* simpler, to the point where I can look at them and just understand what's going on. I assume that for 915MHz operation I will still need a frequency multiplier of some description but getting just the VCO working properly seems like a good first step here.> Control the varactor using a pot connected between the power supply and > ground, with a 100k series resistor from there to the varactors. > Measure the voltage at the wiper, not on the oscillator side. (Note > that varactors work in reverse bias.) > > A 470-nH to 1-uH inductor might be about right--it'll make the frequency > low enough to look at on an inexpensive scope. > You'll want to get a cheap Chinese frequency counter to test it with, > and a DVM if you don't have one. If you can beg or borrow a decent > oscilloscope, that will be a great help.I'm not worried about my ability to measure most of this stuff, about the only thing I can't borrow stuff to do is look at time series 915MHz signals.> Oh, and the varactors are wired in series opposing (common cathode), > with the (positive) bias applied at the midpoint.Yes art of electronics mentions this as a way to cancel the effects of the actual output oscillation from changing the varactor capacitance itself.
Reply by ●June 9, 20212021-06-09
Harry Dudley-Bestow wrote:> On Wednesday, June 9, 2021 at 8:36:10 AM UTC-7, Phil Hobbs wrote: >> You can say that again. JW's stuff was always interesting and fun, >> but he had this tendency to use _way_ too many parts. >>> but approachable. The app note was written in 1986 though, are >>> there hyperabrupt-varactor diodes available now that mean a much >>> simpler design with the same performance is possible? >> 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. > Yep the art of electronics has a reasonable section on varactor tuned > resonant circuits and they are *much* simpler, to the point where I > can look at them and just understand what's going on. I assume that > for 915MHz operation I will still need a frequency multiplier of some > description but getting just the VCO working properly seems like a > good first step here.Right. Building a well-behaved 900-MHz LC oscillator is too hard for a beginner. With the oscillator as described there will be no problem whatever getting enough deviation, so you'll probably want to frequency shift instead of multiplying. And I wouldn't worry about getting up to 900 MHz anyway--just stay out of the aircraft and public safety bands, keep the radiated power way down (1 mW maybe), and mobody will know or care.>> Control the varactor using a pot connected between the power supply >> and ground, with a 100k series resistor from there to the >> varactors. Measure the voltage at the wiper, not on the oscillator >> side. (Note that varactors work in reverse bias.) >> >> A 470-nH to 1-uH inductor might be about right--it'll make the >> frequency low enough to look at on an inexpensive scope. You'll >> want to get a cheap Chinese frequency counter to test it with, and >> a DVM if you don't have one. If you can beg or borrow a decent >> oscilloscope, that will be a great help. > I'm not worried about my ability to measure most of this stuff, about > the only thing I can't borrow stuff to do is look at time series > 915MHz signals. > >> Oh, and the varactors are wired in series opposing (common >> cathode), with the (positive) bias applied at the midpoint.> Yes art of electronics mentions this as a way to cancel the effects > of the actual output oscillation from changing the varactor > capacitance itself.Right. Otherwise you can get some very strange parametric effects, including subharmonic generation (parametric frequency division). 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 10, 20212021-06-10
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. -- Defund the Thought Police
