modulation techniques for part 15

Most low power Tx's will take advantage of using the "low level" approach, that is using a balanced modulator after the buffer stage and inserting audio there.

Though there are a few examples of "high-level" modulation as you described by using a current amplifier on the final's B+ or through a modulation transformer, both of which also require an RF choke to prevent that final RF signal from looping back into the audio circuits. Note this method requires your audio power to be 1/2 that of the power level of the carrier at the finals output. So if your carrier is 1 watt, you need .5 watts audio drive to fully modulate that to 100 percent.

The neat advantage of the low level method is that you can operate with either the carrier plus upper and lower side bands (AM), or without the carrier for SSB mode, all by merely flipping a logic state command to the balanced modulator chip, at least those with that capability and there are a bunch of them, or using the carrier null output pin of the modulator chip.

What information is out there in the handbooks and such, mostly talk about high level modulation through the plate or a screen, all of which carry high voltage potentials. But the techniques are basically the same for flea power levels as that from a Part 15 transmitter. Basically you scale down the design parameters to fit the low power levels being worked with. Instead of a modulation transformer that needs to handle 600 plate volts B+ and 300 volts audio signal, its bumped down to the working voltage of the low power TX and thus the audio signal voltage required is also less.

RFB

thanks for the tips. all the 'low level' diode ring mixer ICs i have seen at operate at really low levels (i am thinking roughly around 1 to 8dBm). if i built a diode ring mixer myself using shottky diodes and ft37-43 transformers do you think it would be able to operate at 100mW?

Here's a link to a description of DAM. Scroll down to "DX-50 AM Transmitter Operation in a Nutshell".
http://hawkins.pair.com/wcbs_wfan.html

If you have never seen Jim Hawkin's site and you are interested in broadcast history and technology you are in for a treat. The link above is for his site and here's the home page:
http://hawkins.pair.com/

The WLW and VOA Bethany pages are of special personal interest to me since I have visited both transmitter sites and used to drive directly by the WLW and VOA towers to and from work each day. Sadly the VOA site is gone. The antenna arrays and towers were a familiar landmark for those traveling between Dayton and Cincinnati on I-75.

Neil

"as i understand it, the basic idea is that you use your audio source to modulate the amplifier's power supply via either a transistor or a transformer. there seems to be lots of ways to do this but i am having trouble finding detailed explanations."

Carl, one of our contributors, started and maintained a thread where he detailed his development of a 13.56 MHz transmitter. You might find some good information here: http://part15.us/node/2870

I advise using high level modulation instead of low level modulation for an AM only transmitter. Low level modulation requires a linear final amplifier and its attendant lower efficiency compared to class C, D, or E. It is easy to produce the required audio power for 100 mW AM transmitters. An IC such as the LM386 audio power amplifier can readily drive a modulation transformer at these power levels. This chip looks like it could be used to supply both the modulation and power to the final since it is DC coupled and has self centering output which could be changed using the + or - input or by using an adjustable LM317 regulator for the LM386 supply. A transformer wouldn't be needed if this is the case. As a starting point, a class C, D, or E final designed for a 50 ohm load would require about 2.5 to 3 volts at 40 mA from whatever power supply is used for 100 mW operation.

I have seen nice tutorials on AM schemes and if I run across them I will post. Otherwise, a net search may turn up what you seek.

Neil

http://www.classeradio.com/

http://www.maxmcarter.com/classexmtr/circuitdescription.php

http://www.frn.net/vines/Forum2/HTML/002047.html

Whichever approach you take, don't let any of them scare you off. Either method works good and both have certain advantages over the other.

"Don't fear the Linear"

RFB

Neil, I have been considering modulating the Deep Voice long wave transmitter that we are planning with a transistor modulator injected at the emitter of the final. But if I read you correctly in your note up above, I think you might be saying we should stick with transformer modulation the same as Big Talker the SW transmitter?

Jim Hawkin's description of how high power AM RF combiners work is very fascinating. Stating that it is "like a reversed loudspeaker crossover network" gives a really clear perspective about what is actually happening.

Consider this idea: two transmitter exciters, consisting of two oscillator and two buffer sections on two frequencies, let's say 1550 and 1680. Second, we build the combining circuit to properly match the two signals, set the one final RF stage so that it is receiving an input of 100mW consisting of combined RF, one modulation section so both frequencies will have the same audio.

Would it pass inspection?

it might, but you'll have a bandwidth problem from the electrically short antenna

Carl commented "...I think you might be saying we should stick with transformer modulation the same as Big Talker the SW transmitter?"

This is a known workable method but I would consider a transformerless approach. The LM386 data sheet says it can produce 365 mW of audio power and this is not enough for a 1 watt tx. I mentioned using this for a 100 mW tx but was not very definite about this because I haven't tried it.

I have seen and used DC power amps based on op amps and a power transistor and if you go for a 1 watt transmitter the pass transistor will run fairly hot.

I'll sketch a circuit for you to see what is involved and post here tonight if I can.

Neil

The 7W Velleman Mono Amplifier Kit used to drive the Big Talker Shortwave has proven to be a very reliable performer in this application, and is readily available from many suppliers at a small price.

I have a circuit around here that I tried with the old Pixie2 SW Transmitter and it did work to modulate from a transistor putting a signal into the emitter of the final. I will find it and post it for inspection.

There is no real need for me, personally, to replace transformer modulation since I have extra transformers, but for novelty and invention sake it might be interesting to try something different. One less heavy part.

I have sketched a transformerless modulator/supply circuit. This is for illustration only to give an idea of what is involved. I have not built or simulated this and it is for a starting point only.

Link here: http://postimage.org/image/vfjth4ifx/

Neil

Your circuit is similar to the Wild Planet Radio DJ broadcaster toy

Where Wild Planet uses the LM386 to directly modulate the final(Q1-2N3904), yours boosts an op-amp output with a follower to apply modulation to the final.

Plus, your circuit has the added benefit of the offset level adjustment to set the symmetry of the audio swing.

Nice!

Those amplitude modulation examples are very interesting. It would be rewarding to find out how they perform.

On the matter of SW antenna, another issue also discussed on this train of comments, I found a HAM's article about success with a circularly polarized version of a center-fed dipole. Instead of stretching straight out from side-to-side, the sides wrap in a circle or loop like a giant horseshoe. The author said it compensates for the constant phase changes coming from the ionosphere. Of course at our power levels the ionosphere isn't much involved. If you'd like a link I will provide one.