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Old 22nd June 2005, 11:13 PM   #1
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Default Low Power FM stereo transmitter?

My primary goal is to be able to transmit either my MP3/WMA or Dish Network music stations to other stereo systems around my house. Secondary goal is to learn something about what I am doing in the process. So I'd like recomendations on a DIY kit or plans with easily sourceable parts to build a high quality FM stereo transmitter. I'd also like to stay legal but I'm not sure where that places me in the grand scheme of things. Anybody done this before? Got a sucsess or disaster story to share? Bonus karma points if you are able to share plans that use vacuum tube circuts!

TIA,
Bryan

PS. I figure I'll probalby end up with something solid state but I'd love to do this with tubes somehow so I posted this here. Tube lovers seem to be friendlier anyway. If you have a suggestion on a better place to ask I'd be happy knowing that too.
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Old 22nd June 2005, 11:22 PM   #2
EC8010 is offline EC8010  United Kingdom
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It's undoubtedly illegal. All things that are fun are illegal.

You need to have a look at some ham radio books. The ARRL manual or RSGB handbook might be a good start. Unfortunately, I doubt if either will have FM modulators. FM uses a lot of spectrum, you see, so I doubt if it was ever licenced to hams. One possible source of information would be video tape machines, but beware that they use narrow-band FM modulation that discards most of the sidebands. Experimenting with an aerial attached would be very naughty and couldn't be condoned.
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Old 22nd June 2005, 11:35 PM   #3
SY is offline SY  United States
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EC: Hams were happily using FM in their VHF bands at the time I got my ticket (let's see, you would have been about 7 or 8 then...). One of my early triumphs was converting a police radio to 2 meters. It was there that I was initiated into the cult of Reactance Modulators.

In any case, my old ARRL VHF book shows several transmitters, all of which would be totally illegal to use for this purpose. It's a good thing that I've read it so that I know what not to do....
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Old 22nd June 2005, 11:49 PM   #4
rdf is offline rdf  Canada
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Hmmmm, I thought very low power FMs were legal, in the 100 mW range depending on country if I recall. Small enough to be portable.
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Old 23rd June 2005, 12:04 AM   #5
gmarsh is offline gmarsh  Canada
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It's perfectly legal to make and operate your own 88-108MHz FM transmitter, provided:

- the radiated FM power is below a certain field strength at a certain distance from the radiation source; there's a table in the FCC books.
- your signal does not interfere with any FCC licensed broadcast station. If you broadcast on or near the broadcast frequency of an existing FM station in your area so that your next door neighbour can't pick up that station, it's illegal even if your field strength lies within limits.

See FCC Part 15 rules for all the juicy bits.

Making a mono tube FM station isn't that hard; there's plenty of transmitter tubes which can be turned into voltage controlled oscillators in the FM band. I've worked with a bit of VHF FM gear that did exactly this.

Generating the composite stereo signal won't be that fun however. The basic process is:

- low pass fillter L, R with a 15KHz cutoff.
- apply 75uS pre-emphasis to L,R
- convert L and R into L+R, L-R
- generate a 19KHz pilot tone, with as few harmonics as possible.
- generate a 38KHz tone, phase-locked to the 19KHz pilot.
- Mix L-R with the 38KHz tone.
- sum together 45.5% of L+R, 9% pilot, and 45.5% 38KHz*(L-R).

The "hard" part is doing the 38KHz/L-R mix - you shouldn't have any leftover 38KHz or L-R. Broadcasting a 38KHz tone can confuse pilot tone recovery PLLs in receivers and effect stereo separation, and broadcasting L-R on top of L+R isn't good either.

If you pull this off with tubes, i'd be impressed. In my professional career, I've taken the easy way out and did it with DSP.
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Old 23rd June 2005, 12:16 AM   #6
paulb is offline paulb  Canada
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I'm in the middle of building this kit:
http://www.northcountryradio.com/Kitpages/mpx96.htm
I believe they've posted the schematic so you can look at it. You'd have to be a masochist to do this with vacuum tubes. Maybe just the RF sections?
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Old 23rd June 2005, 12:50 AM   #7
rdf is offline rdf  Canada
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" In my professional career.." + Halifax = Nautel?

You can take gmarsh's answers as gospel. (In a previous job happy customer running FM-4 Ser#'s 104 and 105. )
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Old 23rd June 2005, 11:14 AM   #8
gmarsh is offline gmarsh  Canada
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Quote:
Originally posted by rdf
" In my professional career.." + Halifax = Nautel?

You can take gmarsh's answers as gospel. (In a previous job happy customer running FM-4 Ser#'s 104 and 105. )
Good guess - I'm a research guy at Nautel.

Thanks for the kind words. You can't beat the FMxx series - they're hideous looking, they have practically no features, they're annoying to use (you have to open a panel and turn a pot to adjust output power) and they're obsolete relative to virtually everything else on the market. But they're indestructible - they're a good simple design that just plain works. We don't market them and people still call us up wanting to buy them.
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Old 23rd June 2005, 06:10 PM   #9
PRR is offline PRR  United States
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> transmit ...music stations to other stereo systems around my house.

Then get one of the many dongles sold for iPod and similar home transmission, and be happy.

> learn something about what I am doing in the process.

Unless you plan a career in FM transmission (which has become DSP and spec-sheet work), an FM transmitter will teach you many thing you will never need to know.

If you must fling solder, get a Ramsey Kit.

> plans that use vacuum tube circuts!

Ah. I don't have plans handy, but I do remember what FM VT transmitters looked like. BIG!

Simple: get a VT FM radio. It will probably be hot-chassis: get an isolation transformer!!!

Follow the original signal path. RF amp, mixer, IF strip, ratio-detector, volume pot, audio amp. To the side of the mixer is an ocsillator (LO). Next to that (if the radio was any good) is a reactance tube (plain pentode plus reactor) for AFC. Its grid runs back to the ratio detector, through an R-C network and possibly an AFC switch.

Snip the AFC at the reactance tube grid. Feed audio voltage. The oscillator will make FM.

The oscillator runs 10.7MC below the signal, so it tunes ~80MC-~100MC. You can pick it up on an FM radio, BUT the LO output was carefully designed to NOT radiate outside the set. It would muck-up other people's radios. Oh, you can catch it if you are very close, but for whole-house use you'll need to boost it. Seems to me you could shuffle the RF stage parts and build a power output stage. Even drive 75/300-ohm antenna (if the input coil is air-core; ferrite would saturate at mighty-low transmit level). Output power is unknown, spectral purity may be very bad: A $10 transmitter is legal only if you have a $100,000 test facility to verify FCC Part 15 compliance.

And an L-C tuned VT VHF transmitter will wander all over the dial as tubes heat and supply voltage drifts. Even with stabilized voltage, temp-oven, etc, stability will challenge most analog radios, and some digital-tuned radios refuse to grab a signal that isn't right on a legal frequency.

The Way They Did It in broadcasting: start with a crystal, solid frequency. Too solid: you can't pull a crystal off-pitch enough for wide-band FM. But a Phase Shift network can cause short-term frequency change without any possibiility of long-term variance from crystal freq. A Phase Modulator is a guitarist's Wah-Wah pedal except voltage controlled and many-many more stages. Looks like a small VT computer. In fact it takes too many stages to be practical: you phase-modulate to the equivalent of say +/-5KC deviation, and then to a string of frequency multipliers. 3MC at +/-5KC, multiplied-up 30 times, gives 90MC and +/-150KC deviation. A Phase Modulator ran many-dozen tubes.

Stereo is an add-on. Fisher made DEmodulators to convert the last FM Mono receivers to FM Stereo. Just three tubes, but a lot of coils. Making stereo is a lot harder than decoding it: gmarsh skimmed the surface. It "is" ordinary vacuum tube design, and perhaps easier than getting a solid FM signal to transmit. It isn't a trivial hobby.

If you haunt eBay used test gear areas, you can find signal generators that can test FM radios, sometimes FM Stereo, sometimes with external audio inputs. Restoring one of these would be more-sane than total DIY.
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Old 23rd June 2005, 06:19 PM   #10
paulb is offline paulb  Canada
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Quote:
Originally posted by PRR
If you must fling solder, get a Ramsey Kit.
I did (the FM-10). I finally gave up on the piece of junk and bought the MPX96 instead.
Any of the cheap transmitters probably use one of the single-chip transmitters made by Rohm or others. Their specs are terrible. The MPX96 uses more classic components that should give better distortion and frequency characteristics.
I hope. It's not working yet.
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