The standing wave is not actually a wave

That was my point, in post #54

 

Summarizing my original question, before I move on to FM:
It is now my understanding that broadcasting AM radio works by modulating (changing) the power applied to the transmitting antenna, all the while maintaining a constant frequency. Is this correct?

I'll worry about the receiving antenna (and corresponding demodulator) later on.

 

Right. Amplitude Modulation is accomplished by modulating the amplitude of the voltage at the transmitting antenna (with a single frequency).
Frequency Modulation has a constant power carrier wave with a changing frequency (which is usually changing by a rather small percentage of the carrier wave frequency). Figure a 100MHz commercial broadcast transmitter trying to deliver +/- audio range variations of the 100MHz carrier frequency.

 

Summarizing my original question, before I move on to FM:
It is now my understanding that broadcasting AM radio works by modulating (changing) the power applied to the transmitting antenna, all the while maintaining a constant frequency. Is this correct?

I'll worry about the receiving antenna (and corresponding demodulator) later on.

That's probably a reasonable way to look at it, but I don't know if it is the best. The question that is open is whether you are modulating the power or the voltage, even though one dictates the other. What it comes down to is if you want to double the signal level, do you double the power or do you double the voltage? I suppose, at least in theory, would could do either -- the transmitter and receiver designs just have to be in agreement as to which. As to which is actually used in real radio circuits, I image that was determined by which approach resulted in the simpler circuits way, way back in the day. My impression has always been that it was the voltage that was modulated, hence the amplitude was directly proportional to the input signal, but I could very well be wrong.

 

Right

Excellent... a simple answer to a simple question...

Now... let's talk about F.M.... I feel it's about to get a little more complicated. But not too far away from my grasp, hopefully.

So an F.M. broadcasting antenna transmits at a constant power (I'm guessing as high as it's legally possible) and shifts frequencies to encode the signal.

Let's consider that signal to be audio, for now:
- This tells me that each station has to have a frequency "window" through which it can transmit. Because if humans can hear at frequencies between 20 to 20,000 Hz, then that window has to be at least about 30 or 40 KHz, to leave a small gap between stations. And if the signal is digital, maybe that window can be smaller, depending on the desired data transmission rate... am I off the mark here?

Thanks for answer too, Bahn. I too, was guessing that voltage was easier to control in the circuits of the old days... maybe modern transmitters have been developed that control both voltage and current, for efficiency's sake.

 

Right. Amplitude Modulation is accomplished by modulating the amplitude of the voltage at the transmitting antenna (with a single frequency).
Frequency Modulation has a constant power carrier wave with a changing frequency (which is usually changing by a rather small percentage of the carrier wave frequency). Figure a 100MHz commercial broadcast transmitter trying to deliver +/- audio range variations of the 100MHz carrier frequency.

Why is that hard to figure? It is done all the time. The civil aircraft band is in the 120 MHz range (108 MHz to 136 MHz) and it is all AM. In addition, military comms in the 242 MHz to 385 MHz are also AM.

 

Excellent... a simple answer to a simple question...

Now... let's talk about F.M.... I feel it's about to get a little more complicated. But not too far away from my grasp, hopefully.

So an F.M. broadcasting antenna transmits at a constant power (I'm guessing as high as it's legally possible) and shifts frequencies to encode the signal.

Let's consider that signal to be audio, for now:
- This tells me that each station has to have a frequency "window" through which it can transmit. Because if humans can hear at frequencies between 20 to 20,000 Hz, then that window has to be at least about 30 or 40 KHz, to leave a small gap between stations. And if the signal is digital, maybe that window can be smaller, depending on the desired data transmission rate... am I off the mark here?

Thanks for answer too, Bahn. I too, was guessing that voltage was easier to control in the circuits of the old days... maybe modern transmitters have been developed that control both voltage and current, for efficiency's sake.

The frequency deviation in an FM signal isn't based on the frequency of the modulating signal -- it is based on the instantaneous amplitude of that signal. So let's say that your input signal was a sine wave with an amplitude of 1 V at a frequency of 1 kHz. That might mean that your 100 MHz carrier signal is being shifted between 99.950 MHz and 100.050 MHz 1000 times a second. If you then changed the signal to 2 V at that same frequency, the transmitted signal would be varying between 99.900 MHz and 100.100 MHz 1000 times a second, so the frequency deviation doubled because the amplitude of the signal doubled. Now, if you changed the signal to a 1 V amplitude signal at 10 kHz, the frequency would still vary between 99.950 MHz and 100.050 MHz, it would simply shift back and forth between those extremes 10,000 times a second.

There are a lot of benefits to FM or AM (and, of course, some disadvantages otherwise AM would have died decades ago). On of the advantages is that you can directly transmit analog telemetry data even if it has DC content. With AM, if you transmit telemetry data you can't recover it very easily because the amplitude drops with distance, so the best you can do it recover relative amplitude information, which is just fine for audio, but to recover absolute amplitude you have to include some kind of reference. Even then, you have the problem that low level signals get lost in the noise. With FM the received signal can recover the absolute amplitude directly from the frequency that is received and this isn't affected by distance (though it is affected by relative movement that results in Doppler shifting), so as long as you can receive the signal at all, you can recover the data. Also, a signal that is at a constant zero value is just as easy to receive as a signal that is at a constant high value, they are simple two different frequencies at the same amplitude.

 

With FM the received signal can recover the absolute amplitude directly from the frequency that is received and this isn't affected by distance (though it is affected by relative movement that results in Doppler shifting), so as long as you can receive the signal at all, you can recover the data.

Very interesting... I'll need to re-read your answer more carefully to make sure I understood what you said... But from what I'm gathering here, it looks like an F.M. receiver is a bit more complex than a transmitter. But A.M. doesn't seem to be the case. Is that right?

 

Why is that hard to figure?

Who said anything was hard to figure and why quote me to say that?

 

Very interesting... I'll need to re-read your answer more carefully to make sure I understood what you said... But from what I'm gathering here, it looks like an F.M. receiver is a bit more complex than a transmitter. But A.M. doesn't seem to be the case. Is that right?

The typical double side-band with carrier A.M is just one type of many types of carrier amplitude modulation so the complexity of the 'A.M' receiver can range from a crystal set, to a very complex digitally synthesized receivers with many carrier modes (AM,CW,USB,LSB,ISB) we once used for long range HF communication, to thumb-drive sized software defined radio systems on a chip today.

I find that FM is easier understood today using IQ signals as compared to the old analog methods of using frequency domain concepts but you need to understand both as the modulated signal has ‘theoretically’ an infinite bandwidth and an infinite number of side-bands. The old rule of thumb for actual needed bandwidth was the Carson bandwidth rule.

http://www.rfcafe.com/references/electrical/frequency-modulation.htm

 

let's say that your input signal was a sine wave with an amplitude of 1 V at a frequency of 1 kHz. That might mean that your 100 MHz carrier signal is being shifted between 99.950 MHz and 100.050 MHz 1000 times a second. If you then changed the signal to 2 V at that same frequency, the transmitted signal would be varying between 99.900 MHz and 100.100 MHz 1000 times a second,

That's a lot better take on it than I had. I was thinking the carrier frequency deviation was the frequency of the audio signal and completely forgot the amplitude information had to be wedged in there somehow. It's been a lot of years since I even thought about how an FM transmit signal was made.

F.M. receiver is a bit more complex than a transmitter. But A.M. doesn't seem to be the case.

An AM receiver is so simple! Look at a crystal radio and be amazed at how easy it is.

 

Who said anything was hard to figure and why quote me to say that?

Fine. Then what, exactly, did you mean by, "Figure a 100MHz commercial broadcast transmitter trying to deliver +/- audio range variations of the 100MHz carrier frequency."? What is the point you were trying to make?

 

I'm thinking this thing about recovering absolute amplitude of a DC level would require a crystal locked center frequency at the receiver. A lot of old FM receivers had a bit of circuitry to pull the tuning frequency toward the carrier wave frequency. It was called, "AFC" (automatic frequency control) and it was considered to be a convenience. I think if you were trying to decode a DC level, that circuit would queer the process by re-tuning the receiver to the frequency which was meant to be representing the DC level.

Fine. Then what, exactly, did you mean by, "Figure a 100MHz commercial broadcast transmitter trying to deliver +/- audio range variations of the 100MHz carrier frequency."? What is the point you were trying to make?

It doesn't require a 20MHz excursion to represent a 20KHz change in information.

 

completely forgot the amplitude information had to be wedged in there somehow.

intersting point... how is amplitude encoded in an F.M. radio signal?

 

While I'm on that subject, the FCC allocates bandwidth and the required spacing between carrier frequencies. Just one sentence tells a lot:
"While all countries use FM channel center frequencies ending in 0.1, 0.3, 0.5, 0.7, and 0.9 MHz, some countries also use center frequencies ending in 0.0, 0.2, 0.4, 0.6, and 0.8 MHz."
https://en.wikipedia.org/wiki/FM_broadcast_band

That tells you that a transmitter is not allowed to modulate its carrier into the next guys transmit allocation. Further, it suggests that if broadcast stations are allocated at 0.2 MHz apart, the modulation can't be allowed more than +/- 0.1 MHz. I think the reality of it is that the FM modulation your car radio receives is a lot less than +/- 100KHz but I'm awfully rusty on this. Point being that the carrier frequency is a lot higher than the deviation required to transmit the information. The modulation amount can easily be less than 0.1% of the carrier frequency.

That's probably a reasonable way to look at it, but I don't know if it is the best.

I'm not even trying for the best. I'm trying for the basics because I think that's where Martinez is able to function today. If he suddenly runs off and leaves me in the dust, I will bow out and leave it to those more knowledgeable about the minutia.

 

how is amplitude encoded in an F.M. radio signal?

WBahn just told you. Amount of deviation from center frequency represents the amplitude, how fast the frequency switches between more and less frequency carries the frequency information about the signal.

the frequency deviation doubled because the amplitude of the signal doubled. Now, if you changed the signal to a 1 V amplitude signal at 10 kHz, the frequency would still vary between 99.950 MHz and 100.050 MHz, it would simply shift back and forth between those extremes 10,000 times a second.

 

I'm thinking this thing about recovering absolute amplitude of a DC level would require a crystal locked center frequency at the receiver. A lot of old FM receivers had a bit of circuitry to pull the tuning frequency toward the carrier wave frequency. It was called, "AFC" (automatic frequency control) and it was considered to be a convenience. I think if you were trying to decode a DC level, that circuit would queer the process by re-tuning the receiver to the frequency which was meant to be representing the DC level.

Yeah, I think it wouldn't work with that circuitry in place and that you'd have to rely on the accuracy of the oscillators at both ends. For telemetry with sufficiently loose tolerances that might be sufficient. For more sensitive applications you would probably still need to embed a reference of some kind, though that would probably be considerably easier and more reliable than with an AM system (at least in systems where the telemetry data is directly modulating the signal).

It doesn't require a 20MHz excursion to represent a 20KHz change in information.

Okay. I think, perhaps, I'm getting an idea of how you meant that statement to be interpreted and how I misinterpreted it.

 

WBahn just told you.

I think I'm beginning to see the light... it's like encoding two-dimensional information (amplitude and frequency) in a single time-variant signal...

I'm trying for the basics because I think that's where Martinez is able to function today.

You're right. But it has nothing to do with the day itself, and everything with the time... yesterday I got less than 4 hrs sleep, and my bed is seducing me in a way that I find impossible to resist... so nighty night... and thanks for taking the time to answer.
Tomorrow I'll re-read today's posts... see if clarity finally arrives...

 

nsaspook and WBahn are both giving you true information, but there's a lot of it to learn. When I first read about QAM256 I realized I was so far behind that I would never catch up.
When you resume this Thread, we will need you to help us find how far you're gotten and where you want to go next.

ps, there was a lot wrong in your post #65 but we're chewing through that really quickly.

The bandwidth allocation limits the amplitude information but the quickness of the switching between frequencies represents the frequency of the audio signal. That allows a huge audio frequency bandwidth because you can wobble a 100MHz signal way faster than 20KHz before you run into limitations concerning interference with the carrier frequency.

 

intersting point... how is amplitude encoded in an F.M. radio signal?

Here is a SDR video of a music FM channel with FM channels on either side. I then switch the frequency waterfall display bandwidth down to see the audio signal deviation better with the music analog signal.

https://flic.kr/p/dYvmbH

CB radio AM sample: https://flic.kr/p/dYvm98