RF cavities and related devices

[artis]

By minor ups and downs I meant the FM part of the analogue tv signal , I saw a bunch of captured scope images of the signal and between the AM modulated main signal one can see a much smaller in amplitude but higher in frequency signal "riding" the AM modulated signal at certain spots.
Yes i could identify the synchronizing pulses , having the highest peaks.

As for what I've learned from this topic, actually alot, now I have a basic visual and technical understanding of how a klystron and a TWT works, where they are mostly used etc I also learned AM radio and FM some time ago but with TV even though the technique is similar everything feels more complicated, also this helps me understand other fields that I'm pursuing at the moment better as EM physics is all sort of "interlaced" just like the analogue tv signal frames.

As to what Baluncore said ok I understand RF signals are just AM or FM modulated sinewaves both the carrier and the "carried" signal itself.
I have always had some problem with understanding bandwidth, I do understand the basic concept but ok here's a question, if we use a carrier frequency that is optimized for best Q in a particular klystron, (when you talked about power amplifiers I assume you referred to klystrons) would it then matter for the efficiency of the klystron whether it amplifies a 10Hz AM modulated signal onto a high frequency carrier or 20khz modulated one as in both cases the klystron beam is velocity modulated at some high frequency just the amplitude changes differ wrt to time?
When you said an "AM carrier will vary slowly in amplitude as it is modulated" then I have to ask what is the frequency spectrum of analogue tv both picture and sound if used without the high frequency carrier or in other words what is the actual transmitted frequency for a single channel for example?
One more thing, so digital TV over air transmitted through the same devices as analogue doesn't have the luxury of having square wave PWM signals passing down parts of a PCB so I assume they use the same sinewave RF signal with FM modulation? Is the digital info then transmitted with the help of FM where it is later (at the receiving end)decoded into a PWM signal (in a satellite decoder box or one attached to an antenna)? like higher frequency parts translates into longer square pulses while lower frequency parts into shorter pulses with more "dead" time and these pulses are then interpreted into 1 and 0 in order to make a useful signal at the end ?
I know this covers many topics but while we are at it I want to know, thanks:)

 

[sophiecentaur]

Endless rambling threads with multiple topics are not PF style. If you have a specific question then ask it in a new thread. You have a better chance of an answer if the thread title relates to your question.

 

[artis]

agreed too many questions in one thread make a mess but too many threads for simple matters also make a mess , hard to make it right.
can you at least please then go on and answer this last post of mine here and any other related questions I will then ask in new thread.

Thanks.

 

[sophiecentaur]

threads for simple matters

None of these matters are "simple". Each one is worthy of several threads in itself.
Your last post has a half a dozen questions that do not have one word answers.
Whatever gave you the idea that this is simple stuff?

 

[Baluncore]

@artis.
You need to thoroughly understand amplitude modulation and channel bandwidth before you try to reason further about RF transmission systems. To do that you need to understand what Fourier analysis and synthesis are, and you need to be able to think about the same signals in both the time domain and the frequency domain.

If you do not have the basic concepts necessary to discuss the situation, the exploratory path you are on will be frustrating and inefficient for all. You need to be interested in and study signal processing to acquire the concepts needed to reason about signals. You cannot ignore, deny or skip the subject. Nor can we deliver those concepts as footnotes in this one thread.

 

[Tom.G]

I have always had some problem with understanding bandwidth,

Bandwidth (needed): The highest frequency of the information-carrying signal.

carrier frequency that is optimized for best Q would it then matter for the efficiency of the klystron whether it amplifies a 10Hz AM modulated signal onto a high frequency carrier or 20khz

In a practical sense for this discussion, No. Using examples that were introduced above, a 500MHz carrier and a Q of 100, the available bandwidth would be 5MHz.

what is the frequency spectrum of analogue tv both picture and sound

The video signal is about 3MHz bandwidth. Tha audio channel is 15kHz, the same as FM radio. Since the audio is FM modulated on the TV signal, it is actually an additional carrier (sub-carrier) transmitted as part of the overall TV signal. This Audio Subcarrier is 4.5MHz and is frequency modulated to ±25kHz deviation from from 4.5MHz.

In an analog TV receiver, the 4.5MHz audio signal has it's own IF amplifier chain separate from the video. The audio chain feeds a conventional FM detector just as you find in an FM radio, and emits the audio signal.

Now, please get back to getting an understanding of the basics and limiting threads to one or a very few questions.

Hope this helps some.

Cheers,
Tom

p.s. I had the same problem you seem to be having trying to reach an understanding. I got into electronics as a teenager living in a rural town. There was not anyone around to answer my questions, so I turned to books and experimenting. When I ran across something new I would find a book or magazine article that gave a more basic level of the subject; this was my approach to tracking down the 'How' and the 'Why' of difficult subjects. If something was just too complicated, it would be put aside and revisited occasionally over several months... eventually becoming understandable as I learned more of the underlying stuff. Perhaps not the 'Ideal' approach but it worked with what was available.

You seem to be trying to approach PF more as a personal tutor than we here are comfortable with. Doing your own research to answer your questions will lead you to a much better understanding of the field. An alternative is taking some formal instructional courses either on-line or in person, somehow I don't see you as embracing this option though.

So start digging out answers/understanding yourself and when you have a particular rough spot or hard time about something, post your specific question on PF; after all, that's what we do here!

 

[sophiecentaur]

Bandwidth (needed): The highest frequency of the information-carrying signal.

It's actually the Range of frequencies [Edit: occupied] by the signal. A 700MHz TV signal will have a bandwidth of around 7MHz, the same as if the signal is mixed down to an IF frequency (say 45MHz) or up on a microwave carrier of many GHz. "Bandwidth" would often mean "Channel Bandwidth" in practice. The "Baseband Bandwidth' would refer to the highest frequency that the Video circuits would handle - it would be the trace on a 'scope of the signal that the Receiver extracts from the RF signal. The term is also used for the rate of data in a digital signal. Bit rate would perhaps be a better term but Bandwidth is the popular but misleading term that's used.

The video signal is about 3MHz bandwidth. Tha audio channel is 15kHz, the same as FM radio. Since the audio is FM modulated on the TV signal, it is actually an additional carrier (sub-carrier) transmitted as part of the overall TV signal. This Audio Subcarrier is 4.5MHz and is frequency modulated to ±25kHz deviation from from 4.5MHz.

For NTSC and PAL colour TV, the frequencies involved are a bit higher than this. A 6MHz sound carrier is used, with similar FM characteristics to Sound FM Radio and the colour (Sub-carrier) is at around 4.43 MHz on PAL.

@artis Read Read Read is the only way to get into this subject. It is vast!

 

[artis]

Well I actually understood AM modulation as done in AM radio for some years now. To put it simply, you take a high frequency CW carrier of sinusoidal shape (for efficiency transmitting over large distances) you take ordinary audible spectrum "voice" from a microphone and you make this low frequency audible waveform control the amplitude of the high frequency CW carrier , feed the result into a transmitting antenna then pick it up with a receiving one add a filter and you get back your low frequency modulated wave without the high frequency carrier.As for bandwidth the part I don't seem to get is this I think, say that you AM modulate your signal with some high frequency carrier , audible frequencies range up to about 16-20kHz but for this question assume they are up to 100 Khz , I still could modulate the whole spectrum from 0 to 100Khz on some high (Mhz region signal) right? Even though modulating it would require a broader bandwidth because the frequency spectrum is larger?
So what I am asking is probably this, if my carrier frequency is say 100Mhz (round numbers for simplicity) then why my bandwidth would only be a fraction of those 100Mhz instead of me being able to modulate everything from low audible few Khz up to signals that come close to the carrier frequency itself?

Wiki says that "Bandwidth is the difference between the upper and lower band of frequencies in a continuous band of frequencies"
So why can't the difference be all the way from zero to close to the carrier frequency itself, now I have a feeling it has something to do with harmonics and transmitter and receiver circuits having limited ability to "latch" on to certain frequencies without having to physically alter the circuit rather than the physical limitation of a signal over air itself ? Also I think that it might have to do with the fact that certain electronics are designed to be more efficient at their design frequencies (because one can't make a device "for everything") so in theory a klystron and a transmitter could AM modulate everything from zero to the carrier frequency but most of the band would be very inefficient and there would be "noise" problems at the receiver etc , am I getting this right?

Trust me I read its just that the way I formulate my questions I can't find answers easily to them without probably reading few dozen books but please don't judge.

 

[sophiecentaur]

So why can't the difference be all the way from zero to close to the carrier frequency itself,

Because that would take up the whole of the spectrum so you could send only one signal over a particular wire or radio connection. If you "understood" the modulation used for AM radio, wouldn't that be clear to you? You have heard about Radio Channels, no doubt. Each channel will be a few kHz wide and uses a different central carrier frequency.
Read all of the Wiki article about AM and try to avoid skimming between highlights. You can't just extrapolate at random from Sound Radio and assume that comments translate straight to the operation of a klystron. A Klystron cavity is tuned to a narrow band of, say 1% of the carrier frequency. How would you expect it to transfer RF power onto or off an electron beam? Have you ever heard of an MF Klystron? Why would that be?

am I getting this right?

More aimless Q and A here. It just doesn't work this way. Ask a specific question on a separate thread and you may get a useful answer.

 

[artis]

at this point your simply angry. Ofcourse I know it would take up the whole "channel" I wasn't asking the question with respect to how commercial radio stations do their business I was curious from the point of physics, but instead of a simple answer you are teaching me a "lesson" again
different carrier frequencies for each channel is I think so that the radio could tune and latch onto each different station with the help of heterodyne.

I understand that the physical size of a rf cavity determines the frequency range it is suitable for but in AM modulation of a single carrier wave for example the frequency the cavity operates is fixed isn't it? just the amplitude changes so as long as there could be this one mystical radio station why couldn't they use the whole spectrum from zero up to carrier frequency the klystron should be fine with it?

So from a klystron point of view (since this is a thread about them) the klystrons that amplified the lower frequency AM channels had to be physically bit different (size, diameters etc) than the ones amplifying the high end of the AM? how about Tv?
Also the lower frequency would require more power put into the antenna for the same signal strength at distance x away?I will make new thread but honestly I have no guarantee that in this new thread you will show more empathy towards my curiosity.

 

[sophiecentaur]

at this point your simply angry.

No - just exasperated that you have been insisting on looking at things your way, which is very often totally wrong.

If you just want to know how much a particular pair of Nike Shoes will cost, then a straight numerical answer suffices but your questions very seldom have a straight possible answer to them because they are so often the wrong questions and not answerable without a lesson.

For instance, how can you suggest using a Klystron to produce a wideband signal that stretches from zero to 700MHz? I thought we had established that even a TV klystron (with specially de-tuned extra cavities) can only manage about 1% bandwidth. I think you would feel the same way as I do when faced with that sort of craziness about something that you actually know about. You claim to know all about the idea of channels so how can you ask about a klystron for DC to Daylight bandwidth? How long would the drift tube of a 1MHz klystron need to be and how big would the cavity be? Don't you realize just how outrageous these ideas are? Ask yourself "Can you buy one?". Why do you think the answer is No.

I suggest you sort out some basics of signals and modulation systems (in detail - not just scanning and saying to yourself "I recognise that - I know it".

If you start another thread about some small part of that topic then you may find someone who is not a quivering wreck will answer you and you may get a reasonable answer. But you have to show some respect for the subject (PF members have fairly thick skins as long as people are polite - which you are) and do not dismiss things as trivial.

 

[artis]

I like your input overall it's just that once you get "exasperated" your input deteriorates like the sound of an AM radio over a receiver with dried up electrolytic caps

I will stop posting and will have to get around frequency modulation and processing for good until I do something, the basics of klystrons would be clear now I also realize that a particular klystron can accept only a small range of frequencies if one hopes to get any decent efficiency out of it, that is fine with me.

The reason I asked about the spectrum is because in AM the carrier frequency is fixed so the klystron being "dumb" only works with that exact carrier frequency to which it is built and has a high Q correct? So why would it care if I modulated a 10Hz sine on that carrier and the next moment decided to modulate a 100Khz Sine on top of the same carrier, in both cases the cavity "works" with the carrier ?

In FM I would understand why a certain klystron can only amplify a specific band because since the information is frequency modulated a wider band (wider range of frequencies) would require the klystron to amplify all those frequencies as well as the "main" one, but I fail to see why the same would be for AM?
Is my logic here so wrong ? Or maybe you misunderstood me and thought I would like to amplify all those frequencies without modulation through the klystron , which I DID NOT intended.

To be honest you never really answered this question just pointed out how I don't know anything (to which I can't entirely object)

 

[sophiecentaur]

The reason I asked about the spectrum is because in AM the carrier frequency is fixed so the klystron being "dumb" only works with that exact carrier frequency to which it is built and has a high Q correct? So why would it care if I modulated a 10Hz sine on that carrier and the next moment decided to modulate a 100Khz Sine on top of the same carrier, in both cases the cavity "works" with the carrier ?

Here's just one answer. If your amplifier has a very narrow (high Q) pass band, centred exactly on the carrier frequency, the modulation sidebands are 'wiped off' the signal and you are left with just the carrier. The 6MHz sound carrier on the multiplex Sound + Vision modulated UHF signal has to get through the klystron (plus the vision sidebands that are on the 'other side' of the carrier. Analogue TV is really not the place to start because the carrier uses Vestigial Sideband Filtering (OMG!) of the Vision signal so that the lower frequency vision components are double sideband modulated and the higher frequency components are single sideband modulated. This is all to get a 'quart into a pint pot' and gives good definition without using the full RF bandwidth that would normally be necessary. (See what I mean?)
Stick with sound Radio AM and FM for the basics. Don't go near old fashioned Analogue TV because it is dying rapidly in favour of Digital signalling when you are (really!) ok with analogue easy modulation methods.

 

[berkeman]

Thread is closed for a bit. The Mentors are discussing how to handle this. Thanks for all who have been trying to help the OP with basic math and the associated intuitive understanding for his/her question.

EDIT / ADD -- Thread will stay closed for now. I'm having a detailed PM discussion with the OP to try to fill in some of the confusing spots, and also to help them find some learning resources at the right level for their background.