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#1
Posted to rec.audio.tubes
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FM Transmitter
I've now added a pre-emphasis preamp so it's basically an operational
mono FM Transmitter. http://flipperhome.dyndns.org/FM%20Stereo.htm I was intrigued by the reactance tube technique and, by golly, it not only works but I've now done one of my own. |
#2
Posted to rec.audio.tubes
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FM Transmitter
In article ,
flipper wrote: I've now added a pre-emphasis preamp so it's basically an operational mono FM Transmitter. http://flipperhome.dyndns.org/FM%20Stereo.htm I was intrigued by the reactance tube technique and, by golly, it not only works but I've now done one of my own. Hi Flipper, How stable and accurate is the center frequency of your reactance tube modulated oscillator? Is it good enough to work with a digital tuner that tunes in 200 kHz steps? I have on old Jerrold CATV FM band modulator. I once drew a schematic, but who knows what became of it. IIRC it used a 10.7 MHz oscillator modulated by a reactance tube. The oscillator center frequency was stabilized by feedback from a 10.7 MHz discriminator. The oscillator was then mixed with a crystal oscillator to produce the final carrier frequency in the FM band. I wonder how linear this scheme is, using a single reactance tube to modulate a relatively low frequency oscillator a full +/- 75 kHz? Many old tube FM broadcast transmitters used modulated oscillators around this same general frequency, but developed the carrier frequency by using a chain of frequency multipliers which increase the deviation along with the frequency, easing the demands on the reactance tube modulator. I would think a better scheme, than Jerrold's scheme, would be to run the modulated oscillator at the carrier frequency, as you do, and then mix with a crystal oscillator to get down to the 10.7 MHz discriminator frequency. Some kind of buffer between the oscillator and antenna would also probably be desirable to improve stability. Perhaps even running the modulated oscillator at half the final carrier frequency, and using a doubler between the oscillator and antenna as a buffer. I will have to dig out the old Jerrold, draw another schematic, recap it, and try to get it running. One of your stereo coders might work nicely with the old Jerrold. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#3
Posted to rec.audio.tubes
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FM Transmitter
On Mon, 09 Jan 2012 22:43:42 -0600, John Byrns
wrote: In article , flipper wrote: I've now added a pre-emphasis preamp so it's basically an operational mono FM Transmitter. http://flipperhome.dyndns.org/FM%20Stereo.htm I was intrigued by the reactance tube technique and, by golly, it not only works but I've now done one of my own. Hi Flipper, How stable and accurate is the center frequency of your reactance tube modulated oscillator? Really now, just how accurate do you think my fingers are when squeeze-pull tuning that air coil? LOL 'Accuracy' isn't really an issue for me because it's intended use is for old analog tuners anyway, although, my cell phone digital tuner gets it okay. I'm not really sure 'how good' the cell phone gets it because there's distortion somewhere. I mean in any receiver, not just the digital. I think it's in the modulator and am working on a cathode NFB scheme, similar to what I did with the Twin Triode AM Transmitter, to see if I can improve that. Speaking of which, the 'Gort' AM Transmitters were a hit at Christmas. Did you see the 'sales ad' I did for them? LOL http://flipperhome.dyndns.org/Gort%20Docs.htm An On/Off switch sounds so much sexier as a "Command Knob," don't you think? Is it good enough to work with a digital tuner that tunes in 200 kHz steps? Dunno. My cell phone has 100KHz tuning. This is all 'brave new world' stuff for me and, frankly, at this stage I'm just tickled to death it oscillates where it should and that the modulator actually modulates. I have on old Jerrold CATV FM band modulator. I once drew a schematic, but who knows what became of it. IIRC it used a 10.7 MHz oscillator modulated by a reactance tube. The oscillator center frequency was stabilized by feedback from a 10.7 MHz discriminator. The oscillator was then mixed with a crystal oscillator to produce the final carrier frequency in the FM band. I wonder how linear this scheme is, using a single reactance tube to modulate a relatively low frequency oscillator a full +/- 75 kHz? Many old tube FM broadcast transmitters used modulated oscillators around this same general frequency, but developed the carrier frequency by using a chain of frequency multipliers which increase the deviation along with the frequency, easing the demands on the reactance tube modulator. They also usually used double, push pull, modulators. Jerrold modulating at 10.7 MHz gives me hope, though. I would think a better scheme, than Jerrold's scheme, would be to run the modulated oscillator at the carrier frequency, as you do, and then mix with a crystal oscillator to get down to the 10.7 MHz discriminator frequency. Some kind of buffer between the oscillator and antenna would also probably be desirable to improve stability. Perhaps even running the modulated oscillator at half the final carrier frequency, and using a doubler between the oscillator and antenna as a buffer. Yeah, I haven't gotten to the 'antenna' part yet. It spews quite a bit as is. The idea is simple, simple, simple, though, and I figured on little more than a secondary off the coil. Well, and probably going to something slug tunable rather than the boing boing air coil. If it drifts it doesn't seem enough to bother analog tuners because I've listened to it for hours without touching the dial. I will have to dig out the old Jerrold, draw another schematic, recap it, and try to get it running. One of your stereo coders might work nicely with the old Jerrold. I am flattered by the unfounded confidence |
#4
Posted to rec.audio.tubes
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FM Transmitter
In article ,
flipper wrote: On Mon, 09 Jan 2012 22:43:42 -0600, John Byrns wrote: In article , flipper wrote: I've now added a pre-emphasis preamp so it's basically an operational mono FM Transmitter. http://flipperhome.dyndns.org/FM%20Stereo.htm I was intrigued by the reactance tube technique and, by golly, it not only works but I've now done one of my own. Hi Flipper, How stable and accurate is the center frequency of your reactance tube modulated oscillator? Really now, just how accurate do you think my fingers are when squeeze-pull tuning that air coil? LOL 'Accuracy' isn't really an issue for me because it's intended use is for old analog tuners anyway, although, my cell phone digital tuner gets it okay. I have a small FM "transmitter" that I built many years ago using a single transistor oscillator modulated by a varactor diode and it is reasonably stable. It occurred to me that the reactance tube might not be as stable as a varactor? I'm not really sure 'how good' the cell phone gets it because there's distortion somewhere. I mean in any receiver, not just the digital. I think it's in the modulator and am working on a cathode NFB scheme, similar to what I did with the Twin Triode AM Transmitter, to see if I can improve that. Speaking of which, the 'Gort' AM Transmitters were a hit at Christmas. Did you see the 'sales ad' I did for them? LOL http://flipperhome.dyndns.org/Gort%20Docs.htm No, but I have now! An On/Off switch sounds so much sexier as a "Command Knob," don't you think? The "command knob" would be even sexier if it controlled a step start circuit like in a real transmitter. Is it good enough to work with a digital tuner that tunes in 200 kHz steps? Dunno. My cell phone has 100KHz tuning. This is all 'brave new world' stuff for me and, frankly, at this stage I'm just tickled to death it oscillates where it should and that the modulator actually modulates. I have on old Jerrold CATV FM band modulator. I once drew a schematic, but who knows what became of it. IIRC it used a 10.7 MHz oscillator modulated by a reactance tube. The oscillator center frequency was stabilized by feedback from a 10.7 MHz discriminator. The oscillator was then mixed with a crystal oscillator to produce the final carrier frequency in the FM band. I wonder how linear this scheme is, using a single reactance tube to modulate a relatively low frequency oscillator a full +/- 75 kHz? Many old tube FM broadcast transmitters used modulated oscillators around this same general frequency, but developed the carrier frequency by using a chain of frequency multipliers which increase the deviation along with the frequency, easing the demands on the reactance tube modulator. They also usually used double, push pull, modulators. True, although a 10 kW Westinghouse FM transmitter I knew many years ago used a variable resistance modulator. A 6H6 dual diode was connected in series with either the capacitor or inductor of the oscillator tank circuit, I forget which, current from an audio controlled a variable current source passed through the diode varying its resistance. The change in resistance caused the resonant frequency of the oscillator tank circuit to vary, producing FM. Audio distortion of the basic scheme was probably high because a discriminator was used to provide audio feedback around the modulator. The carrier's center frequency was controlled by a separate, and much more complex phase detector circuit. Jerrold modulating at 10.7 MHz gives me hope, though. I would think a better scheme, than Jerrold's scheme, would be to run the modulated oscillator at the carrier frequency, as you do, and then mix with a crystal oscillator to get down to the 10.7 MHz discriminator frequency. Some kind of buffer between the oscillator and antenna would also probably be desirable to improve stability. Perhaps even running the modulated oscillator at half the final carrier frequency, and using a doubler between the oscillator and antenna as a buffer. Yeah, I haven't gotten to the 'antenna' part yet. It spews quite a bit as is. The idea is simple, simple, simple, though, and I figured on little more than a secondary off the coil. Well, and probably going to something slug tunable rather than the boing boing air coil. My one transistor transmitter used relatively few turns, probably about half a dozen, of relatively heavy wire that I wound around a #2 lead pencil as a winding form. I can't be sure exactly how many turns I used because the coil is soldered inside a tin shield. At any rate I never noticed any great problen with microphonics, I will have to fire it up and see if it is at all microphonic. The air core coil is reminiscent of the coils found in many old tube FM tuners. If it drifts it doesn't seem enough to bother analog tuners because I've listened to it for hours without touching the dial. I will have to dig out the old Jerrold, draw another schematic, recap it, and try to get it running. One of your stereo coders might work nicely with the old Jerrold. I am flattered by the unfounded confidence -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#5
Posted to rec.audio.tubes
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FM Transmitter
On Tue, 10 Jan 2012 22:42:05 -0600, John Byrns
wrote: In article , flipper wrote: On Mon, 09 Jan 2012 22:43:42 -0600, John Byrns wrote: In article , flipper wrote: I've now added a pre-emphasis preamp so it's basically an operational mono FM Transmitter. http://flipperhome.dyndns.org/FM%20Stereo.htm I was intrigued by the reactance tube technique and, by golly, it not only works but I've now done one of my own. Hi Flipper, How stable and accurate is the center frequency of your reactance tube modulated oscillator? Really now, just how accurate do you think my fingers are when squeeze-pull tuning that air coil? LOL 'Accuracy' isn't really an issue for me because it's intended use is for old analog tuners anyway, although, my cell phone digital tuner gets it okay. I have a small FM "transmitter" that I built many years ago using a single transistor oscillator modulated by a varactor diode and it is reasonably stable. It occurred to me that the reactance tube might not be as stable as a varactor? I don't know. The reactance tube operates by modulating a reactance derived from the R-C lead-lag (depending on what reactance it's emulating) filter driving it's grid so drift on those would affect it. The amount of reactance fed into the tank is then proportional to gm, so how much does that drift? I can tell you one thing, though. The one I built isn't terribly linear, which isn't too much of a surprise since there's nothing but an 820 under the cathode (actually 470 with the 6BL8 on the breadboard) I've since revised the thing with a 6AU6 preamp and NFB from the modulator cathode. That made a huge difference and now it sounds, as Tony the Tiger would say, Grrrrreat! If anything I may have a hair too much pre-emphasis because I'm using rather loose tolerance junk bin parts on the breadboard. Doesn't matter for most things but it does for the filter. I'm not really sure 'how good' the cell phone gets it because there's distortion somewhere. I mean in any receiver, not just the digital. I think it's in the modulator and am working on a cathode NFB scheme, similar to what I did with the Twin Triode AM Transmitter, to see if I can improve that. Speaking of which, the 'Gort' AM Transmitters were a hit at Christmas. Did you see the 'sales ad' I did for them? LOL http://flipperhome.dyndns.org/Gort%20Docs.htm No, but I have now! An On/Off switch sounds so much sexier as a "Command Knob," don't you think? The "command knob" would be even sexier if it controlled a step start circuit like in a real transmitter. What's a "step start?" Is it good enough to work with a digital tuner that tunes in 200 kHz steps? Dunno. My cell phone has 100KHz tuning. This is all 'brave new world' stuff for me and, frankly, at this stage I'm just tickled to death it oscillates where it should and that the modulator actually modulates. I have on old Jerrold CATV FM band modulator. I once drew a schematic, but who knows what became of it. IIRC it used a 10.7 MHz oscillator modulated by a reactance tube. The oscillator center frequency was stabilized by feedback from a 10.7 MHz discriminator. The oscillator was then mixed with a crystal oscillator to produce the final carrier frequency in the FM band. I wonder how linear this scheme is, using a single reactance tube to modulate a relatively low frequency oscillator a full +/- 75 kHz? Many old tube FM broadcast transmitters used modulated oscillators around this same general frequency, but developed the carrier frequency by using a chain of frequency multipliers which increase the deviation along with the frequency, easing the demands on the reactance tube modulator. They also usually used double, push pull, modulators. True, although a 10 kW Westinghouse FM transmitter I knew many years ago used a variable resistance modulator. A 6H6 dual diode was connected in series with either the capacitor or inductor of the oscillator tank circuit, I forget which, current from an audio controlled a variable current source passed through the diode varying its resistance. The change in resistance caused the resonant frequency of the oscillator tank circuit to vary, producing FM. Audio distortion of the basic scheme was probably high because a discriminator was used to provide audio feedback around the modulator. The carrier's center frequency was controlled by a separate, and much more complex phase detector circuit. Yeah, I read about one of those. I also saw a compressor that worked on the same principle: rectified signal, buffered, drove current though a diode in series with a pass resistor forming a divider. Jerrold modulating at 10.7 MHz gives me hope, though. I would think a better scheme, than Jerrold's scheme, would be to run the modulated oscillator at the carrier frequency, as you do, and then mix with a crystal oscillator to get down to the 10.7 MHz discriminator frequency. Some kind of buffer between the oscillator and antenna would also probably be desirable to improve stability. Perhaps even running the modulated oscillator at half the final carrier frequency, and using a doubler between the oscillator and antenna as a buffer. Yeah, I haven't gotten to the 'antenna' part yet. It spews quite a bit as is. The idea is simple, simple, simple, though, and I figured on little more than a secondary off the coil. Well, and probably going to something slug tunable rather than the boing boing air coil. My one transistor transmitter used relatively few turns, probably about half a dozen, of relatively heavy wire that I wound around a #2 lead pencil as a winding form. I can't be sure exactly how many turns I used because the coil is soldered inside a tin shield. At any rate I never noticed any great problen with microphonics, I will have to fire it up and see if it is at all microphonic. The air core coil is reminiscent of the coils found in many old tube FM tuners. I don't notice any great problem when using it but you can tell for sure the transmitter's on by tapping the base. Thump, boing, thump, boing. My coil's considerably larger in diameter, though. If it drifts it doesn't seem enough to bother analog tuners because I've listened to it for hours without touching the dial. I will have to dig out the old Jerrold, draw another schematic, recap it, and try to get it running. One of your stereo coders might work nicely with the old Jerrold. I am flattered by the unfounded confidence |
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