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Can I modulate two different signals simultaneously?

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the_risk_master

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Can I modulate two different signals (eg digital, other analog) at the same time using FM and AM technique?

[The idea is to modulate the digital signal using AM technique and the analog info modulated in FM at tha same time]

If you guys have hgotten my idea and has also the idea how to make this possible
I am greatful to discuss it here, becasue this would be my final year project in my university..

MAny thanks to all,
the_risk_master
 

biff44

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Sure.

Traditionally, there are things called subcarriers where for broadcast TV they transmit the video at the carrier using AM, and modulate the audio on a subcarrier with FM.

More modern examples include OFDM, where there are perhaps 50 orthogonal modulations going on at the same time.
 

Borber

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Your question is unclear.
Two different signals (carriers) can be modulated using any modulation you want and I do not see why not in the same time.
If you are talking about using something like double modulation (AM and FM) on the same signal (carrier) with the same modulations signal there are some restrictions, but it is possible.

Explain yourself better.
 

the_risk_master

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Hi Sir Borber and Sir Biff and others,

Sir Borber, youre right Im talking on double modulation, sorry for confusing you
or may be Im just plain unclear in my statements. What my idea is, to transmit two different information (one analog, other digital pulse) I will transmit analog info using FM technique and for the digital info AM technique, with just single carrier, Im aware that it is possible, but I am not sure how to implement it on electronics (may be you can give me a block diagram if you have an idea) and lets
also discuss the restrictions that you have stated previously

many thanks
the_risk_master

please reply again, (others can also join discussing here)
 

Borber

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For realization of your idea you must cascade FM modulator and AM modulator after it. To get undistorted FM signal after FM demodulation the modulation depth of AM must be lower than 90%. Simple ASK can not be used.
 

BigBoss

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This technique is used since many years..
For instance PAL or SECAM..

The only thing is to do is to combine two modulated signals in a proper way and to modulate this " composed signal" on a single carrier with another type of modulation like FM, PM or AM or digital modulations..

But be very careful while doing that bacause of signals can disturb each other if you don't supply sufficient isolation while combining...
 

Borber

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TV vision carrier modulation is very different than the idea we are talking about. The combination of video signal componnents (luminance and crominance) is done in basis band before AM modulation of carrier. Kind of FDM is used. And the modulation of carrier is only AM.
This idea of using two types of modulation of carrier to transmit two different signals is strange and not usually used but is possible.
 

the_risk_master

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Thanks again Sir Borber and Sir BiBoss and others, for the replies

But what factors or limitations I may encouter while implementing this?
Sir Boreber did mention in his previous post about some restrictions regarding in this topic. what are these restrictions?
 

artem

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I think you wont be able to implement 100 percent amplitude modulation as your freq modulation info will be lost . But that is actually DSB not AM .
 

Borber

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As I said max. AM modulation is 90%. You should allways left min. 10% of amplitude of the carrier for undistorted FM transmission. 100% AM means that the carrier will be interrupted (amplitude equals 0).
AM modulation is pure DSB AM (carrier and two sidebands).
 

BigBoss

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Borber said:
TV vision carrier modulation is very different than the idea we are talking about. The combination of video signal componnents (luminance and crominance) is done in basis band before AM modulation of carrier. Kind of FDM is used. And the modulation of carrier is only AM.
This idea of using two types of modulation of carrier to transmit two different signals is strange and not usually used but is possible.

I don't think so..

Chroma carrier that 4.43MHz is modulated by Y-B and Y-G components which are orthogonaly separated and modulated by PAL (or SECAM any way..) modulation technique.

Sound is also FM modulated with a carrier 5.5MHz..

Then Baseband video signal is added into these composite signals..

So there are two modulated and one baseband signal to be upconverted at main carrier signal.

Briefly, many modulated signals can be composed into one signal and then this composite signal can be upconverted onto another main carrier.

So, few independently modulated signals can be carrier out by one..

Seems feasible..
 

Borber

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OK BigBoss. We can say TV is using some kind of multiplexing to transmitt four modulating signals (Y, Y-B, Y-G and sound). And multiplexing is usual method. But the_risk_master wants to use two different types of modulations on the same carrier to transmitt two modulating signals. Is it allso multiplexing?
 

wahabmalik

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I think, yes its possible. but the real challenge will be the modelling of the process and then designing approperiate filters.
you can first amplitude modulate the carrier. well i think, modulation index should be kept small <50%. This amplitude modulated signal is fed as carier to to FM modulator. The challenge here would be to analyse the effects of carrier amplitude on the performance of FM modulator and come up with a circuit desgn capable of meeting these challenges.

on receiver side, you can simply use an FM and AM receiver in parallel. AM uses the envelope of the recieved signal so it doesn't look much complex. I don't think the frequency of signal will effect the envelope detector much. FM demodulator can work in parallel, just clipping the recieved signal at 50% of its peak value and then using the normal FM receiver.

well, can you please tell me what's the motivation behind the idea. I mean what do you think is advantage of doing this. any advantage on bandwidth utilization or energy utilization. If you have analysed these, can you let us know?
Thank you.
 

artem

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Borber said:
As I said max. AM modulation is 90%. You should allways left min. 10% of amplitude of the carrier for undistorted FM transmission. 100% AM means that the carrier will be interrupted (amplitude equals 0).
AM modulation is pure DSB AM (carrier and two sidebands).

From where you get 90 percent of AM modulation but not let say 95 or 85? imho modulation parameters should be connected to acceptable SNR ratio for both modulations then numerics will make sense .
 

Borber

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This 90% I took from analogy with intercarrier sound principle in TV without much analysis. There vision carrier must not be modulated more than 90% for a sound SNR of I think 55dB.
I would compose modulators chain first of FM oscillator and second AM modulator. Reversed order requires PM for frequency modulation and modulation index will be lower. This is only a quick view into the problem.
 

artem

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I ment that implementation could require to have channels (channels established over AM and FM) quality of service to degrade equally when SNR decreases.
the_risk_master did not specify any RS, but only :

"The idea is to modulate the digital signal using AM technique and the analog info modulated in FM at tha same time"
 

the_risk_master

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Hi Sir Boreber, Sir Artem, Sir BigBoss, and others,

Thanks for the discussions youve been posting, but before we get any further from here. Below is a picture or diagram Ive been proposing of, I think will have problems in handling the digital info here using the ASK carried by FM. the reception might also be prone to errors (in digital information) can you help me improving this diagram?
 

melc

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The schematic presented has limited chance to work.
The OA has a PSRR which will don't care by the little variations of the AM signal ( a digital signal can't produce AM modulation but probably ASK, you need a DA converter for AM). Huge variation of supply will decrease the output liniarity of the FM signal.
Let's go from theory to practice. Which will be the FM carrier and the maximum frequency signal of the AM modulator you expect to run with this schematic?
 

artem

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2 the_risk_master

i am not expert in radio and data communication but i guess that you should specify your constraints in form of requirement specification first .
The implementation is last thing in design phase you could start with.
 

mehta_jp

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It is not as simple as it seems. I have worked on PAL TV Transmitters. in that scheme it has three components.
1 monochrome video: up to 5 mhz
2 audio: at 5.5 mhz & 200 khz bandwidth
3 chroma ( colour) : at 4.43 mhz

here, note that audio & video never collide in terms of bandwidth (*audio at 5.5 that is 500 khz away from video) a simple filter is suffice.
color information is in band of video (monochrome) but due to it is being in retrace blanking time of horizontal sync it is filtered out in time domin. so here (1) fm & am are not really combined in frequency domain (2) monochrome and colour signal are time multiplexed.
now comming to your point, consider for example that you have clock of 100 khz for digital signal & you restrict series of 1 followed by series of 0 to maximum of 10 ( by some coding technique). then minimum frequency of your digital signal will be 10 khz. you can use <10 khz for audio (am) signal. mix them at baseband and either use am or fm, it dosen't matter. imho this maximizes bandwidth utilization. at receiver demodulate & use high roll-off filters

jay
 

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