15219 pwm transmitter

Don't you just hate it when ppl. abbrev. things?

Robert,

Maybe I err that pwm means Pulse Width Modulation and if so you can enlighten me. Maybe you are thinking of a traditional PWM system where the signal is demodulated at the receiver?

In any case, a while back I looked into a PWM scheme and I still have an interest in this. My thinking approaches this a bit differently and I don't know if it is what you have in mind. Perhaps it could be used to perform the function of a modulator/final in one transmitter stage to produce an AM signal. Being an ON-OFF technique it could yield high efficiency.

At this point it has been a thought experiment to apply filtering to the output of a PWM circuit before the antenna to remove the harmonic energy from the pulse signal and leave the AM waveform to be radiated. Basically, the entire AM signal (carrier with modulation) would be generated in the PWM stage. It would require some fairly fast digital circuitry but not beyond what is available but unfortunately fast digital usually means power lost as heat.

I haven't given it much thought beyond this but it may be work a closer look.

Neil

The filtering will be the big consideration, which may end up robbing any gain obtained by filter losses.

Would such a design begin to take the price range a bit beyond most budgets..possibly.

Would be neat to see a design under test, that is testing into the outdoor antenna system seeing if it holds that efficiency and how long before re-tune is needed of the antenna system.

Sure would make things easier if we had a little more wiggle room with power and antenna size.

Something the organization should begin to develop is a plan for action to introduce some changes to the rules and allow a bit more flexibility.

RFB

here is a youtube video of a 500mw tx on 800khz

I too have looked at PWM for modulation, but it doesn't provide any advantage to Part 15 transmitters.

PWM is great for increasing the efficiency of the modulator stage. However, the modulator efficiency isn't a factor in the Part 15.219 rules, so a less efficient modulator is just as good as long as it drives to 100% modulation.

How about development of a true hi-level balanced modulator that could work in a PWM final circuit?

RFB

OK. I'll bite. How about it?

i would basically like to see a true professional grade transmitter design. i believe the BE boxes use some sort of digitally created AM running into a class e amplifier.

i would like to see a system capable of producing the full quality audio of a professional modern am transmitter and have a high efficiency final RF section and ATU.

i think the amt5k has come very close to this goal but there is room for a little improvement beyond the amt5k's design as decent as it is.

here is a website covering class e and pwm design.
http://www.classeradio.com/

Robert,

You posted "i think the amt5k has come very close to this goal but there is room for a little improvement beyond the amt5k's design as decent as it is."

I am not trying to challenge you but I am interested in what operational improvement you think could be made. Does PWM offer an advantage that you see over conventional high efficiency AM designs?

Neil

according to the guy in that video pwm has a much wider and cleaner frequency response than analog modulation methods.

here is a circuit description...

"PWM test circuit one, a little test circuit put together by Boomer. It shows the circuit on the bench putting out a signal on 800 khz, the AM mediumwave band.

The modulator chip is a TL494, set for single ended output, 50 to 60 khz, emitter follower, through a low pass filter to the transmitter's final stage. keep in mind that the 494 is driving the modulated stage directly, and the power output is in the area of 500 milliwatts (.5 watt).

The modulation can reach down to DC for bottomless bass, and indeed the bass sounds excellent! The distortion you hear is the camera choking on all of the bass. The low frequency response here is only limited by the laptop's output section and a 2.2 uF and 100k input resistor.

On the high end, it's mostly flat with a few db peak above 5 khz, then a roll off above 8 khz, based on the sound, though I haven't tested the HF response yet.

This circuit is more complex than the simplest AM transmitter you could build, but it has the potential for higher audio quality and better frequency response. The efficiency is higher than most other circuits you could think of."

If I understand the write up at the site you linked correctly the circuit described is used to modulate a final RF amplifier and is essentially an audio amplifier. What I had in mind was to use the PWM circuit to generate the complete modulated RF signal.

There seems to be no advantage to having a DC audio amplifier response other than to provide bias for a final amplifier as is done in the Ramsey AM-25 circuit. The audio response in the Ramsey circuit is not DC due to capacitive coupling though the amplifier is capable of this operation.

Neil

Few loudspeaker systems reliably reproduce bass down at the bottom of human hearing, and only certain audio content actually contains the lowest octave range, so talk of "audio response down to DC" has little meaning in a practical sense.

Ultra low frequency waves must be contained within the noise-to-ceiling headroom of the signal, limited at 100% modulation (or slightly more in some systems), and prolonged low frequency tones can cause vocal or mid-range sounds to "wubble", which is a way of saying that the mid-tones become amplitude modulated by the low tones, like thumping on your chest while singing a pure "Aaaaaaaaaaaaaaah!"

What I'd like to see is a well designed, CERTIFIED (hopefully both in Canada and the U.S.) transmitter that is as efficient as the AMT5K claims it is (and seems to be supported by anecdotal evidence). Kits are fine and dandy for what they are, and you're always going to have guys that want to tinker. But most people that I've seen and heard from want more of a turnkey type system - either because they don't have the technical skills to build that kit or because they don't have the time or inclination (i.e., are more interested in the other aspects of the hobby, and yes, that includes moi).

Plus, if you are eventually inspected by the FCC, you'll have far less trouble with a certified transmitter.

I found the big hullaballoo about the Talking House and ATU somewhat ironic, because they are supposed to be certified (details of the certification testing notwithstanding). But there are plenty of finished kits out there and you are essentially relying on the goodwill and integrity of the kit builder (not to mention the kit manufacturer) to come out at the end with a Part 15 compliant device.

What Artisan has expressed is a good description of the problem I've run into trying to sell people on the idea of getting started with part 15.

The problem begins with a normally luke warm feeling about radio, and any interest dies right away when we talk about kits, antennas, grounds and audio connections.

It's a fact there are people with no taste for the technical.

Yet those of us who understand the magic of using a radio station for program transmission, realize that those who "tune it out" are really missing something.

We tried to help.

People seem to pursue part 15 broadcasting for different reasons. My interest is, and always has been, in the technical aspects of transmitters and antennas. It began when a neighbor gave me a phonograph with a single tube inside but it had no speaker. A bit of fiddling with it revealed that it was a phono-oscillator and my part 15 hobby began.

To make a station work I had to learn how to perform the programming activities such as cuing record and tape cuts so the timing would come out right, microphone techniques so I didn't "pop" or fade while speaking, mixing, and so on. Point being that to make it work I had to learn things secondary to my prime interest.

Now reverse this and consider someone who wants to get into part 15 broadcasting but is deterred by the transmitter and antenna and other technical things. They are going to have to learn some things beyond their prime motivation just as did I.

For those interested in starting a part 15 broadcast station here's some advice. Start small. Proceed slowly and learn as you go. Get a C-Crane or another certified FM transmitter and an audio source such as CD or iPod player, plug them together and you are on the air. We hope you will be hooked on the idea of having your own radio station and if so start building a real functioning studio. A very simple way to do this is to use a PC with an audio card and a inexpensive microphone such as a USB microphone. Install ZaraRadio and learn to use the mixer on the PC. The basics are now all in place and you just have to learn to use them.

If your interest grows, so will your acquired knowledge and you will learn what you need to grow the station, including kit building for AM transmitters if needed and other technical things. It can be done step by step.

That being said, here's something for beginners to consider as they read the posts here. Many of us have quite a bit of experience in technology and programming and our conversations can be intimidating. We are literally pushing the state of the art here with Part 15 AM transmitters, antennas, and program development and that is why posts often get so complicated. Don't let this scare you! Jump right in and post questions, comments, or whatever is on your mind. We want to help anyone who is interested regardless of their prior knowledge. The regular posters here enjoy what we do and would very much like to see others share the fun.

Now a bit of advice for our regular posters. Keep what I posted in mind and don't overwhelm a newcomer. Try to recall when you were a newbie and what it was like. There is no need to impress anyone with what we know but there is a need to offer help when it is sought.

Neil