The more I consider it, I think the Deep Voice longwave transmitter under development here on this website by part15.us members would lend itself perfectly to modification into a serious SCA generator.
Right now the schematic for the oscillator/PLL portion has not been posted, but here's what it offers....
The operating frequency is selected by the PLL keypad, and the SCA frequencies could therefore be selected very conveniently.
The PLL kit and oscillator being used can be FM modulated, and the accompanying literature that comes with the PLL kit tells how.
The second stage of the transmitter is a buffer stage, and I guess would have plenty of power to achieve the injection level for SCA use, but also the final stage has a power control and could be brought way down as low as needed, and has the added protection of an output filter to subdue the harmonics.
Take a look at what we have so far, and very soon I will add the latest schematic.
http://kdxradio.com/deepvoice.html
Despite having worked for many years at FM stations I was not involved with engineering until I built a low power FM tube transmitter from an electronics magazine, followed by a friend building for me a 3-transistor 1-Watt transmitter.
One fascinating aspect was the transmitter pre-emphasis which places a rising curve toward the top of the audio frequency spectrum so that a de-emphasis circuit in the radio smooths the frequency response back to normal while also reducing the presence of FM noise, which can be quite loud otherwise.
Earlier in this thread it was mentioned that right on top of the rise in audio frequencies is a filter which yanks anything above about 15kHz back down so as not to collide with the 19kHz pilot tone which turns on the stereo light in the radios.
A subcarrier generator would need to be injected into the transmitter all the way on the other side of the preemphasis and the pilot filter, which means it could not simply be plugged into the audio input jacks.
Then there's the stereo generator, which in essence is a sub-carrier generator already built into stereo transmitters.....
I'm thinking that building a subcarrier generator will be the easy part of the project. Getting it properly injected to the transmitter will take some advanced engineering know how.
Analog FM broadcast exciters/transmitters normally provide several input ports used to modulate the transmitter. One is used for the FM stereo signal (L+R mono, 19 kHz pilot, and L-R stereo subchannel). The stereo generator itself provides the filters needed to protect the 19 kHz pilot, and the baseband spectrum above 53 kHz -- which is the upper limit of the L-R subcarrier spectrum.
Several other ports allow external SCA generators to be injected into the baseband signal that is applied to the modulated oscillator of the FM exciter. The ports provide mutual isolation so that the various generators do not interact with each other.
The SCA generator outputs are frequency modulated subcarriers that might be on 57 kHz, 67 kHz, or 92 kHz, (typically). Sometimes several of those are used at once. The maximum deviation of the main FM carrier by each SCA subcarier is below 6%, typically.
But just to note that the usefulness of SCA on an unlicensed FM meeting §15.239 will be just about negligible due to the low power and low modulation. Even a pure mono FM signal with +/-75 kHz deviation that meets §15.239 barely reaches several hundred feet, unless some elaborate receive hardware is used.
Experimentation can be instructional, but OTOH it may be useful to anticipate some of the predictable obstacles.
....not a problem with watts. In the nano-watt world, a big nano problem as far as usability...well maybe I need to control something on the other side of the house...(many other devices available on the market to do that already)....or maybe just to dabble and piddle with (best reason).
OP is looking for something to experiment with, thus I don't think commercial exciters need be mentioned in this topic or are even applicable in the discussion as those are not only intended for licensed radio service, but are way..way...WAY out of a tinkerer's budget..unless said tinkerer has the bucks and just wants to find something to dump the bucks.
OP might want to check the DYI forums, some of the folks there are modifying Veronica FM transmitters to inject RDS sub carriers for their Christmas light displays.
I wonder if any of them are in compliance?
RFB
I don't think commercial exciters need be mentioned
Perhaps others have a different point of view.
If you do the math, 600 to 800 feet range is possible to a sensitive car radio, line of sight, even with the stringent U.S. rules (hardly elaborate receiving equipment). The several hundred feet that the FCC uses applies to an ordinary, consumer type radio.
And in Canada, we're allowed 4 times the field strength, so count on 4 times the range, with no obstructions, of course.
But even in Canada, it's nanowatts we're dealing with, so the increased field strength allowed probably doesn't make all that much difference with SCA.
"Perhaps others have a different point of view."
Perhaps. But I doubt a Part 15 forum would be the proper place to be mentioning about commercial exciters in relation to a 15.239 setup. Perhaps VE would be the proper place?
RFB
"And in Canada, we're allowed 4 times the field strength, so count on 4 times the range, with no obstructions, of course."
That is one example of how the stronger field strengths allowed in other countries do not cause the heavens to tremble and flowers morph into radioactive Venus fly traps.
If someone could provide the explanation as to why in other nations a stronger field strength is allowed and not in the US..I do mean a REAL answer and not a political answer, you got my attention.
RFB
The U.S. organizations that lobby on behalf of the broadcast industry are steadfast in crushing even a gnat of competition, ignoring common sense in the pursuit of a scorched earth policy.
The listeners are not so stupid that they'd listen to junk in the absence of quality, they simply don't listen to radio.
The U.S. is into abandoned mall space, even on the radio dial.
Other countries tend to have more intellectualism in the ruling class.
"The U.S. is into abandoned mall space, even on the radio dial."
True. Even if the abandoned mall space is crumbling apart, there are those who do a good job of convincing a potential investor that it's up to code and the latest gig. And since research is something falling to the wayside, it's easy for these scam artists to sell fake ice to Eskimos.
The spectrum is no different. What was once valuable is now worthless and they still can auction off spectrum as if it was the hey day of happy radio.
"Other countries tend to have more intellectualism in the ruling class."
I think other countries also have more intellectualism in the population as well as in their leadership. In the US..its a passive public, thus allows the less than low regard to current ruling class to get away with what they do.
It isn't a matter of the left hand not knowing what the right hand is doing. It is now a matter of the mind being manipulated to thinking less is better...across the board. But so far Carl's explanation carries more wattage than the crap from NAB or the FCC.
RFB
At first I was just curious about the professional SCA generator linked by kc8gpd.
I was amazed how someone could think of so many circuits to include in a device that can also be built in a fairly simple form, at least for hobby work.
But as I reflect some more, I now consider it a great point of reference to see "how did they do this" and "how did they do that".
For example, it appears in the pro unit that the output impedance is high, around 10k. Hmmm. Since in essence it is a LW transmitter I was expecting the standard 50-ohms. What this means is that converting the Deep Voice LW transmitter for use as an SCA generator will require a different output circuit.
Just because a commercial SCA generator has a high impedance output does not mean that a home built SCA generator connecting to a home built transmitter has to follow that high impedance standard.
Why create more work for yourself when it's unnecessary?
"The more they overtake the plumbing, the easier it is to stop up the drain" Scotty - Star Trek III - The Search For Spock.
RFB
To obtain 4x field strength requires increasing the power 16x, which also increases the sq. ft (meters, yards, etc) of area covered approx. 16 times also, at least for FM line of sight.
After this tip for future SCA experimentation I am bowing out of the pursuit, but I will follow with interest any work done by others.
The schematic for the first Part 15 Long Wave Transmitter known as "Deep Voice" is now finalized with the posting of an oscillator/PLL diagram.
As we have discussed, that project lends itself to conversion into an SCA Generator. Here are the main points so far as I know:
For an SCA Generator
1.) The AM modulation transformer would be omitted;
2.) The input for FM modulation already exists on the DataKit PLL circuit, which is unused on the AM version of Deep Voice, but which is available for the SCA version;
3.) if needed, a pre-emphasis network could be added from standard design sources for audio sources;
4.) a data input does not require pre-emphasis;
4.) The audio card is not needed for the SCA version. A more tailored audio pre-amp may or may not be needed;;
5.) The RF output might be better handled with a volume control rather than a power control on the SCA version.
This is the only place I will post these notes, so if you have thoughts of pursuing this course of action clip this and save it.
Here is the Deep Voice Transmitter site:
http://www.kdxradio.com/deepvoice.html
SCA generated is needed for experimentation. I have read the comments here today about the 4046 IC and the Deep Voice Transmitter, but how do I put this altogther? How come I don't see a schematic or anything for a SCA version of a Deep Voice transmitter that uses frequencies from 20 kHz to 100 kHz? It would have been great to see one built for the SCA FM band.