RF Tuner: Adjacent Channel Interference Rejection Filters

[brainbaby]

Hi guys

The IF section of both monochrome and colour receivers must include rejection filters to suppress adjacent channel interference...

Why exactly so, since the work of rf tuner in television receiver is to select a particular channel and as the required channel is selected it then passes to the mixer where IF frequencies are generated, then why the author says that IF section should consist of rejection filters to suppress ACI. I mean that already this work is accomplished by the tv tuner where only required channel is selected from the band of channels. So does the provision of rejection filters in the IF section really make sense.

thanks

 

[Averagesupernova]

Why exactly so, since the work of rf tuner in television receiver is to select a particular channel and as the required channel is selected it then passes to the mixer where IF frequencies are generated, then why the author says that IF section should consist of rejection filters to suppress ACI. I mean that already this work is accomplished by the tv tuner where only required channel is selected from the band of channels. So does the provision of rejection filters in the IF section really make sense.

thanks

View attachment 220964

By your logic of the work already being done in the tuner we should not even need the bandpass section of the IF stage. Your assumptions are wrong.
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The modern TV tuner for NTSC, which I assume you are referring to simply shifts a band of frequencies to the intermediate frequency. This frequency is 45.75 MHz. While tuners do have a tracking filter on the input side to aid in suppressing adjacent channel signals the main filtering is done in the IF stage. I assume PAL is similar if not identical.

 

[brainbaby]

By your logic of the work already being done in the tuner we should not even need the bandpass section of the IF stage

I said...

rf tuner in television receiver is to select a particular channel and as the required channel is selected it then passes to the mixer

text which supported this is below..

you said..

the main filtering is done in the IF stage

text which supported this is..

Isn't both explanations seems contradictory to each other...

 

[Averagesupernova]

I don't believe I can explain any better than I already did.

While tuners do have a tracking filter on the input side to aid in suppressing adjacent channel signals the main filtering is done in the IF stage.

The one statement in the above quote agrees with your textbook. We cannot rely on the tuner from blocking 100% of the adjacent channel signals.

 

[brainbaby]

ok i agree to you ...
so can i believe that if we select a particular channel by setting the required local oscillator frequency then some portion of the adjacent channel manage to pass further to the IF section due to non ideal behaviour of filters in the tuner stage?

 

[Averagesupernova]

Yes that would be a safe thing to say. The typical tuner has what is referred to as preselection built in. This is an attempt to filter all of the channels out except the one we want. This is accomplished by what I referred to as the tracking filter. It is not perfect. It has to change its characteristics when tuning from one channel to another.

 

[tech99]

ok i agree to you ...
so can i believe that if we select a particular channel by setting the required local oscillator frequency then some portion of the adjacent channel manage to pass further to the IF section due to non ideal behaviour of filters in the tuner stage?

You will notice that at UHF, the adjacent channel is only separated by 8 MHz (PAL system) and if the carrier is at, say, 800 MHz this represents only 1% difference. It is not possible to obtain sufficient selectivity at 800 MHz using ordinary tuned circuits, and so the filtration is mainly achieved at IF. However, the tuner must provide all the image rejection.

 

[brainbaby]

however I agree that image rejection is one of the most important function of the tv tuner and to improve image rejection a higher IF is recommended according to your point

It is not possible to obtain sufficient selectivity at 800 MHz using ordinary tuned circuits, and so the filtration is mainly achieved at IF. However, the tuner must provide all the image rejection.

as the ideal responses are as follows
RF amplifier of tuner ideal response..

Mixer ideal response

Can you provide me NON IDEAL output response of all rf amplifier , mixer and IF section

 

[sophiecentaur]

Mixer ideal response

Your picture of the mixer ideal response would be a bit unrealistic. That response would be achieved within the IF stage, not within the mixer. All the mixer filter needs to do is to isolate the wanted mixing product from any others. I would suggest that the labels on your two diagrams are not referring to the appropriate stages in the receiver. The overall response up to the mixer output would be more like the first diagram and the overall response of the receiver would be like the second. The difference between the monochrome and colour responses will probably be to do with the requirement for compatibility and to ensure that not too much of the lower sideband of the colour subcarrier. Remember, it is a Vestigial Sideband (VSB) system so the slope across the carrier frequency needs to be symmetrical to control VSB distortion.
How much are you familiar with UHF TV systems? Are we discussing all this at the appropriate level?

 

[brainbaby]

first i need to define is what i think is mixing products , the mixing product is first the output of the rf amplifier which is a selected channel from the group of channels that along with am modulation and SSB with vestigial band is incident on the antenna..from there the content goes to the rf amplifier whose bandwidth is quite high to pass the required channel..the required channel frequency which passes through the rf amplifier beats up with the local oscillator frequency i.e heterodyned to yield an IF frequency which then passes to the IF section. In IF section vestigial side band correction takes place which modifies the response of the IF section and then further the output goes to the video detector where the modulation is removed and signal is demodulated...

All the mixer filter needs to do is to isolate the wanted mixing product

If mixer does the bandpass action when where will heterodyning takes place...??

Rf amplifier only have a large bandwidth so that it could pass the require signal frequencies easily for that a large IF is needed for image correction which brings the distance of actual signal and its image counterpart to be large so that bandpass action of rf amplifier could easily cut through or separates the two which would be difficult if the image signal would be located very near to each other...

This RF action is not ideal so some portion of the channels of different bands passes through it which further get selected at the IF section...this seems agreeable to me to an extent...but you have confused me a bit...

 

[sophiecentaur]

If mixer does the bandpass action when where will heterodyning takes place...??

I think you are wasting time in categorising the names and functions, rather than just appreciating what actually happens. A Mixer circuit is more than just the non-linearity; it will involve some input and output selective filtering. The IF 'strip' does the definitive spectrum shaping.
I ask again your level of knowledge and experience of these things. If you are quoting from one source then I suggest you read around and compare what it says with other sources. I am not sure your purpose in posting your question.
For instance:

Rf amplifier only have a large bandwidth so that it could pass the require signal frequencies easily

If it were convenient. all the filtering, tuning and detection would be done at RF. In fact there are a number of devices, like Pagers, in which the whole of the processing is done at RF and a final digital stream comes out. The fact is that you cannot produce a tunable UHF filter with the required spectral shape. The RF amplifier / filter is necessarily wider band than you might choose. A single frequency channel filter can be made with much tighter spec but that is no good for a TV receiver with around an Octave of frequency range.
Have you ever tried to put together a UHF TV receiving system?

 

[Baluncore]

The IF section of both monochrome and colour receivers must include rejection filters to suppress adjacent channel interference...

The problem with real RF front-end mixers is that any asymmetry in construction will cross-modulate the biggest RF signals present, then sum the unwanted cross-mods into the target IF signal. If you put a 3dB RF attenuator before the mixer you will get a 6db reduction in cross-mods, with only a 3dB reduction in the wanted signal.

Analogue TV has a huge carrier with big synchronisation sidebands, a few MHz away is a quadrature colour carrier. There are two FM carriers for sound, one FM modulated by the L+R audio sum, the other smaller, with the L–R difference. In between the main carrier and the audio carriers are a few MHz of analogue picture brightness information. Any cross-mods that find their way into the picture part of the channel spectrum will appear as diagonal stripes on the screen. Delayed multi-path signals will appear as sideways shifted ghosts.

The frequency separation of the adjacent channel VSB AM and FM carriers, and the separation of co-channel colour and audio sub-carriers were very well defined, so a trap could be implemented at a fixed frequency in the IF if there were any problems.

For analogue TV, the immediately adjacent channel was not co-located at the same transmit site. To allocate four TV services to three transmitter sites, every third channel could be used at each site. At site 'A' channels 30, 33, 36 and 39. At site 'B' channels 31, 34, 37 and 40, then at site 'C', 29, 32, 35 and 38. Those channel groups all fall in the bandwidth of one fixed antenna. The antenna directional beamwidth will then provide the first level of adjacent channel rejection from other sites. The separation of carriers is then three times more, so adjacent channel interference is not such a problem. VSB AM TV was all a big compromise, frozen by immature standards, in a Freudian suspended development.

Cable TV used one cable for all signals so it gave much greater control over the amplitude of adjacent channels. There was no big hostile signal immediately adjacent to a weak target signal, the channels could be packed closer in the cable spectrum, separate from the rest of the hostile universe.

Analogue VSB AM TV was a disaster, mainly because it used AM for the vision. If it had used FM, then digital TV would not have needed to be implemented as soon. Nowadays we can use an image rejection mixer to down-convert QPSK modulation to baseband in one step, then digitally demodulate the several kilobit wide data stream. QPSK is spread spectrum with a suppressed carrier, so there is no huge carrier to cause cross-mods.

@brainbaby, I have to wonder why you are studying the intricate detail of a past failure, namely AM TV, while the rest of the world is spreading the spectrum and moving digital information reliably at unprecedented rates.

 

[sophiecentaur]

@brainbaby, I have to wonder why you are studying the intricate detail of a past failure, namely AM TV, while the rest of the world is spreading the spectrum and moving digital information reliably at unprecedented rates.

I wondered about that, myself. Years ago, nearly everyone in the World made the switchover to Digital broadcast TV to allow compatibility with virtually all other information inputs to the home.

 

[brainbaby]

I ask again your level of knowledge and experience of these things. If you are quoting from one source then I suggest you read around and compare what it says with other sources.

I consider myself to a beginner in television engineering...

The following reference which I have taken is from..
television servicing vol 1...by patchett
The text here clearly states what probably sophiecentaur is talking about ...

The text clearly states that the tuner section of the receiver never provides any selectivity as the selectivity here means to separate sound and picture rf carrier amongst each other for this purpose IF section is designed all about.
What I mistakenly thought earlier was that the tuner does the selectivity by picking out a particular channel frequency from the bunch of channels...however this is a tuning action and should not be confused with selectivity.

But its true that the channel selection is provided by the tuner section where rejection of adjacent channel interference can dealt and my quote...

The IF section of both monochrome and colour receivers must include rejection filters to suppress adjacent channel interference...

doesn't makes a valid argument...

The separation of carriers is then three times more, so adjacent channel interference is not such a problem

Adjacent channel interference is not a major issue to solve out as it can be mitigated by external transmission strategies as claimed in post #12..

So now I think so that function of tuner and IF section is sorted out...
Function of tuner

1. Amplify the incoming signal and change the signal frequency into IF frequency
2.reduce noise
3.increase S/N ratio
4.shield radiation from oscillator to get coupled to the antenna
5.Image rejection ( *Image rejection only boost up channel selection not selectivity)

Hope so I ve found the right conclusion..

 

[sophiecentaur]

UHF AM Colour TV systems are a real labyrinth with many levels of complexity. It's really not the best thing to start with, learning about TV. As I read this thread, stuff keeps bubbling to the surface of my memory from decades ago. The RF and demodulation bits are only the start. Books and books have been written about the features of PAL and NTSC systems which were only done the way they were in order that the colour signal would be compatible with monochrome sets. How deep do you want to go with this? It is all water under the bridge.
The comment in that extract about there being "no selection" reads as total nonsense if taken out of context. A mixer will produce many products at many frequencies. The output of the mixer is where frequencies in the wanted channel are selected. (the IF band) At this stage, both vision and sound signals are there. Demodulation of the sound carrier involves the inter-carrier beat to produce and isolate the 5.5MHZ sound carrier. The frequency stability of old mixer systems couldn't be relied on to keep the sound carrier in the right spot for FM demodulation.
I seriously suggest that you should read more around this subject because 'servicing' manuals are not always good for 'education' in a topic. Some of the statements you are making about what you have understood are muddled. More reading could clear this problem up.

2.reduce noise

Nothing can do that. Noise power in a given bandwidth is with you for ever! *A low noise head amplifier may improve things if it has a better noise figure than a standard front end TV amplifier or it can increase the signal level to make up for down-lead loss of signal. But the IF stage can only ensure that no extra noise is introduced.

the selectivity here means to separate sound and picture rf carrier amongst each other for this purpose IF section is designed all about.

This again is misguided. You could not have inter carrier demodulation if only the IF sound carrier were present. Both carriers go into the sound stage mixer. The Vision IF has a cut at the sound carrier frequency to reduce the effect of a high level sound carrier in the vision demodulator.
*In a digital system, a signal can be regenerated to reduce the effect of added noise - as long as the noise level is not too high but this is strictly Analogue.

 

[brainbaby]

More reading could clear this problem up.

suggest me some sources

 

[sophiecentaur]

Do some Googling.

 

[Averagesupernova]

https://goo.gl/images/1xiY9p

 

[brainbaby]

https://goo.gl/images/1xiY9p

link doesn't seems working

 

[sophiecentaur]

link doesn't seems working

iWork's for me at this point in time.
Unfortunately it refers to a book which you would have to buy. That can bring on the pains for me unless I really need the information.

 

[brainbaby]

iWork's for me at this point in time.

strange ...how can it work if the page doesn't exist..

 

[sophiecentaur]

strange ...how can it work if the page doesn't exist..

It exists for me (twenty seconds ago). I see a black background with the image of a book about Video Systems.
But you can't expect consistency all over the web. Imo, it's a wonder it works at all. Try copy and paste into your browser address window.

 

[brainbaby]

I did this multiple times but what I have found is this

and this

I might expect you that you may have remembered the author

 

[sophiecentaur]

I did this multiple times but what I have found is this
View attachment 221141

and this

View attachment 221142

I might expect you that you may have remembered the author

The page that my link gave me was this one which is not an (ordinary) eBay link. Very strange. Can you see what I saw now?
Which part of the World are you in? That may be making a difference.

 

[Averagesupernova]

https://www.google.com/search?client=ms-android-verizon&ei=BnKVWtvgKaLa5gLUyYTACQ&q=basic+television+and+video+systems+bernard+grob&oq=basic+television+and+video+systems+bernard+grob&gs_l=mobile-gws-serp.3..0i30j0i8i30l4.3165.4920..5420...0...212.952.0j5j1...1..mobile-gws-wiz-serp...0i7i30j0i8i7i30.65GJwgzV5sg=
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Try that. It should get you somewhere.