Newbie surppressed carrier question

By reinserting a reference frequency in the receiver to heterodyne with the received sideband(s).

** You obviously have no idea how a radio receiver works.

Do some web searching.

...... Phil

That's like asking what makes an apple an orange, ie no sense at all.

The term "carrier" comes from the early days of radio, and I've never seen any articles about the topic from that period so I don't know how they named it.

But it's a misnomer, since the name implies it carries something, when in reality, it's not needed at all.

And the models that came from the early days of radio still persist in many explanations, so it's no wonder it can be confusing. What really sort of happened was that there was one explanation for "AM" and another when "SSB" came along, when in reality a common explanation fits all such signals.

So you have a standard "AM" broadcast transmitter. The carrier signal is amplified up to the final output stage, and that stage is plate modulated. That terminology suggests that the amplitude of the carrier is varied, but in reality it's not. It's a mixing process. That final stage that is plate modulated with audio is actually a mixer. It translates the audio signal up to radio frequencies, where all the original information is intact, it's merely at a radio frequency. That mixing process generates two sidebands. | | |

9.999MHz 10MHz 10.001MHz

The "carrier" is 10MHz. The audio signal is a 1KHz tone. The mixing process results in the difference (the 9.999MHz signal) and the sum (the 10.001MHz signal) at the output, ie the two sidebands.

It's more obvious with a fixed tone, but do the math on other audio frequencies and you'll see the sidebands dance around that "carrier" frequency.

Since it's a linear mixer, the audio amplitude of the modulating signal remains at radio frequency, so each of those sidebands will rise and fall as the audio modulating signal rises and falls.

Some of the mixup about how the carrier must actually vary in amplitude surely derives from the fact that if you put a simple RF voltmeter on the output of the transmitter, the signal would go up and down with the modulating signal. But since that voltmeter can't select between a sideband or the carrier, it sees the complete amplitude of all three signals, and thus it looks like the carrier level is varying.

It's only incidental that the carrier is also present at the antenna. It's there because the modulated stage is unbalanced, so the carrier signal feeds through to the output. But since you've also translated the modulating signal to RF, those sidebands, you no longer need the carrier to send it along.

"DSB", in this case double sideband with suppressed carrier, uses a balanced modulator stage to mix the audio signal with the "carrier". The result is the two sidebands, but the unneeded "carrier" is not present at the output. Like this:

| |

9.999MHz 10MHz 10.001MHz

"SSB", and more specifically single sideband with suppressed carrier, goes a step further. Since both sidebands are mirror images of each other, only one is needed to convey the modulating signal, so the transmitter removes one of the sidebands, and the mixing process is balanced so there's no "carrier" at the output. Like this:

| 9.999MHz 10MHz 10.001MHz

or this |

9.999MHz 10MHz 10.001MHz depending on whether the transmitter sends lower or upper sideband (ie the sum or the difference between the "carrier" and the modulating audio signal).

All three variants of amplitude modulation have their pros and cons. "AM", ie double sideband with a carrier, is really easy to demodulate. A simple diode will do the deed, and tuning is easy. (Note, that demodulator that seems to create a varying DC voltage to match the modulating signal is in reality a mixer, mixing the carrier from the incoming signal with its sidebands, which converts the radio frequency sidebands back down to audio.)

"DSB", ie double sideband with no carrier, gets rid of the carrier, which wastes power (since it's not actually needed), and since it's not there, it can't beat against a carrier from some other station on almost the same frequency, which results in odd tones coming out of the receiver.

"SSB", ie single sideband with no carrier, puts all the transmitted power in one sideband, which is all that's needed to convey what's in the modulating signal.

But, since the demodulation of all three is about mixing the RF sideband(s) back down to audio, at some point the receiver needs a signal on the "carrier" frequency to beat against the sideband(s) and mix it back down to audio. With "AM", the carrier is there, and as I pointed out, few even think in terms of a mixing process at the receiver.

But when the "carrier" is not sent out from the transmitter, one has to create a "carrier" at the receiver end. The problem then arises of where to put the locally generated "carrier". Because if it's not in the right place, the demodulated signal will not be an exact replicant of the original modulating signal.

With "SSB", that means you will have audio that is low or high pitched, though you can tune the locally generated "carrier" so it sounds about right, the human brain will know. And the brain will tolerate a certain level of "off frequency".

"DSB" is trickier, because the locally generated "carrier" has to be in exactly the right place, ie right between the sidebands. Otherwise, that 9.999MHz signal will not translate back down to the original 1KHz, it will be at 1.1KHz, say. But, the upper sideband will not translate to 1KHz either, it will land at at zero Hertz, you won't hear it. But, since the two sidebands do not mix back down to the same frequency, they will not clash, and the receiver output will not be listenable.

The simplest way to fix that is to put a filter in the receiver to knock out one of the sidebands, so the rest of the receiver sees an "SSB" signal. There's no difference between stripping off the signal at the transmitter or at the receiver (well in terms of demodulating that signal).

But then you lose one advantage of sending two sidebands, which is the redundancy of the two sidebands. If you use a demodulator that makes use of both sidebands, then interference to one will not be a problem, and fading to one will not be a problem.

So if the receiver is not stripping off one sideband, the locally generated "carrier" has to use information from those sidebands to tune it to the exact frequency. Various methods exist to do that, going from simple to complicated.

But despite the complications, all three types of amplitude modulated signals are demodulated by a mixer and a "carrier". It's clear in an "SSB" receiver, where the mixer is usually called a product detector, and you see the locally generated "carrier" in the form of the BFO, beat frequency oscillator. The methods to demodulate "DSB" use that same basic notion of a product detector and BFO, with added circuitry to automatically tune the BFO to the exactly needed frequency.

No to try to figure out your question, a receiver can differentiate between signals because the various sidebands of a signal are on different frequencies.

Michael

Receivers select a band of RF frequency to pass, centered around where the carrier would be. The modulation produces side bands on one or both sides of that carrier frequency. The passes band includes both the carrier and the sideband/s. If the carrier is missing, the sideband/s still fit through that same pass band.

The AM process is really a form of multiplication The result of multiplying two sinusoids, if you used a plain old multiplier, would be signals at the sum and difference frequencies, and none at either of the original frequencies. The only difference with AM is there is effectively a constant added to the signal input so that it never goes through zero, which means the carrier is always present. If you looked at a spectrum of the broadcast signal when there is only a pure sine signal input to the modulator, you'd see 3 peaks: The carrier frequency, the carrier plus the signal, and the carrier minus the signal.

In a real broadcast, there are of course many input frequencies, but the above still holds. The side peaks become bands whose width is the bandwidth of the incoming signal. Let's say the signal covers a 5 kHz bandwidth (I think this is typical for broadcast AM). Then a station at 1 MHz would have sidebands extending

5 kHz to either side, down to 995 kHz and up to 1005 kHz. The next station on the dial will be at least 10 kHz away (farther, in practice). so the sidebands will not overlap. If the carrier is removed (say by using a multiplier instread of an AM modulator) the sidebands stay in the same positions and don't interfere.

You can see how AM sidebands and multipliers work with my free Daqarta signal generator and your Windows sound card. (You don't have to purchase anything: The signal generator continues to work after the trial period, along with most everything else except inputs... which you don't need for these experiments.)

You can see waveform, spectrum, and spectrogram of the signal, and play around with modulation depth and type. The only difference compared to a broadcast situation is that the carrier will be in the audio range as well. So, for example, you can set the carrier to

10 kHz and try modulating signals between 0 and 5 kHz, giving sidebands from 5 kHz to 15 kHz. This is actually an advantage over RF, since things are proportionally more spread out and easier to see on the spectrum display.

Note that Daqarta's AM modulator works a bit differently from that in a transmitter (see the Help for full details): When you set Depth to 200% you get pure multiplication, which is the supressed-carrier condition.

If you have any trouble with this, I'd be glad to answer questions.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

You must be talking about sidebands? If so, a signal is created at the receiver that zero beats to the original signal used at the transmitter generating the side band offsets. Solution of side band transmissions allows one to concentrate it's use of space all in a useable window unlike AM.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

That's what he said.

That is correct.

There are no sideband offsets. They still bracket, precisely, where the suppressed carrier would have been.

Not at all. A signal of two sidebands with the carrier suppressed (The topic), occupies the same bandwidth as a double sideband with carrier signal.

Bandwidth is reduced by half, by suppressing one of the two sidebands.

>

I think you're trying to pull this to one side here. My comparison to AM was for power efficiency of use when it comes to side band signals for the transmitter.

Think about it../

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

lobotomy"

Hide quoted text -

Double sideband signal with or without a carrier occupies same amount (band-width) of frequency spectrum. Single sideband, without carrier (SSB) occupies half bandwidth. Correct?

I thought about it. I think your reply to the OP contained bad errors, and now you are trying lie out of it.

Part of your post: >>> Solution of side band transmissions allows one to concentrate it's >>> use of space all in a useable window unlike AM.

...says nothing about power efficiency. "Space" in your statement means bandwidth.

If anyone is trying to "pull this to one side," it's you.

I see you pulling this crap on other posters, but you won't get by with it here. The best you can is STFU and go study the topic. And while I'm at it, the OP didn't ask about power efficiency anyhow. What you did was typical of someone who doesn't know crap about what they are posting about.

Think about it real good.

Yes

No, not quite.

Single sideband, with or without carrier (SSB) occupies half bandwidth.

A carrier has no bandwidth.

^^^ No, You think about it real Good Ass ^^^

I see you're just like some one else here. (no names mentioned)

It's obvious if you can't read between the lines, you're not any

better than those you speak of, Who ever they maybe..

Go play some where else, your petty bull shit is not welcome here.

And I do not withdraw anything I said since there is nothing wrong with

what I said. If you really think other wise, you do have a problem with

reading or, your just looking to be an ass, if that is that case, do it

elsewhere.

I can't speak for the rest here but I can say this, your little kiddy acts do not impress me at all.

Grow the f*ck Up!

-- "I'd rather have a bottle in front of me than a frontal lobotomy"

Where do I go to read Good Ass? Or did you mean something else?

Sure, no names mentioned because you're a chicken shit coward. He was correct and so am I: You are too ignorant to accept when you make a mistake. You hope to bury it rather than have the OP get a correct, meaningful answer,

I read fine, and I understand the language just fine, and I know BS when I see it.

Smoke screen. You really think this will fool someone?

Again...Smoke screen. You really think this will fool someone?

I'm reasonably certain the ass has exposed himself.

If you were a little impressed with your betters, you might be better off.

By the way, your gutter language use means YOU LOSE.

You are too ignorant for words. You lurk and lurk and collect half-correct information and then spew it out to screw-up beginners who ask questions.

Ignorant, you took the works out of my mouth.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

Pervert.

It looks like you've had both. :(

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

A receiver is tuned to a particular frequency and is designed to demodulate a particular type of modulation.

In the case of a suppressed carrier transmission, be it single side band (SSB) or dual sideband (not used much) the receiver is tuned to the sideband which is transmitted. In the case of SSB, the sideband covers the same spectrum as half an unsuppressed transmission, minus the bandwidth used the carrier.

Normally, a phased lock loop is used to restore a replica of the carrier in the receiver and a balanced bridge is used for demodulation.

Distinguishing between "stations" is achieved by accurately tuning to the narrow transmitted frequency spectrum and filtering out other transmissions.

Sean

Dear God, what must this guy think of the lot of you! I've heard more maturity in a kindergarten playground. He asked a relatively simple technical question and all you can do is bicker and insult each other.

I dropped back into this group to see if things had improved at all, but they obviously haven't. It's still the same old backbiting and ego trips.

To hell with the lot of you.

By the way, the development that my company was working on, the circuit that I designed that doubles the range and speed of DSL; the company is fully funded, the invention is patented and it is selling to telephone companies as I write this. So to those of you who said that I was not intelligent enough to invent such a circuit and in the vernacular that you seem so addicted to on this newsgroup; - - - -

Nah, you're not worth it.

Sean

Back at you, too,

B.S.

Whoever said it, was likely correct.

My, my!

Don't let the door whop you in the backside on your way out.

If it's patented, then post the patent numbers.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida