soopertaco's physics questions :D (exam revision,etc)

[soopertaco]

Okay guys I'm in dire need of some tutoring! haha
I have an electronics and photonics SAC on monday and i still only have a vague grasp on the light modulation aspect of photonics.
Would anyone be kind enough to exercise their knowledge and better themselves by giving me a run down on the whole process of this? <3


also, i know transistors aren't part of the core study design anymore but regardless there is so much irrelevant information in my textbook. What do we need to know and what should we/i ignore?? specifically information about phototransistors and their application in light intensity modulation...

Thanks in advance!! 

[Vincezor]

Okay guys I'm in dire need of some tutoring! haha
I have an electronics and photonics SAC on monday and i still only have a vague grasp on the light modulation aspect of photonics.
Would anyone be kind enough to exercise their knowledge and better themselves by giving me a run down on the whole process of this? <3


also, i know transistors aren't part of the core study design anymore but regardless there is so much irrelevant information in my textbook. What do we need to know and what should we/i ignore?? specifically information about phototransistors and their application in light intensity modulation...

Thanks in advance!! 

Okay I'll try my best with modulation - I admit I'm not very strong with it either, though I've been able to do most trial exam questions regarding modulation, so who knows

Modulation is the process of carrying an information signal with a carrier wave. So in our case, the light output from an LED or Laser Diode can be modulated by the output signal from any transducer (eg Microphone). This output signal alters the LED/Laser Diode's brightness (Light intensity?). As a result the light emitted from this LED carries the output signal. In order for the original output signal to be analysed it must be demodulated which essentially reproduces the original signal that it was carrying. The component that does this is usually a photodiode or phototransistor which converts the light energy from the LED back to an electric signal - this is the original information signal.

However, I don't think you need to go that in depth for the exam (I hope!) I think you just need to be able to outline what modulation and demodulation is, as well as what components are used at each "stage".

Maybe it's a good idea to understand the characteristics of photodiodes and phototransistors, such as their response times as well as their function. Also I'll mention that you probably need to understand how to interpret graphs of photodiodes (Light intensity is proportional to current in reverse bias).


Sorry if I made a mistake somewhere, I'm going to take a break now. Feel free to point out any mistakes I have made, I'll confirm and edit if necessary.

[Comma]

Just a note: light modulation can never be below zero.

If you think about it, it makes sense. As what is negative light intensity? The answer is, of course, that there isn't any. So therefore, when you're given the graphs of the modulated signal, the whole curve would be moved up so that none of it is below the axis.

Hope this helps

[soopertaco]

Wow thank you so much for taking the time to write that!

So let me get this straight:
stage one: an information signal is produced from an optoelectronic/electronic-opto or input/output transducer (I really get mixed up with these.. eg. micrphone) carries the information wave.

stage 2: the output signal from say an LD or LED (LD has a much narrower band, coherent light and efficient signal transmission with little energy loss) is modulated with the information wave an transmitted over some distance or destination.

stage 3: the emitted modulated light is carried to a receiver which is usually a Photodiode or phototransistor. (are these in reverse bias and if so why?) these component/s convert the modulated wave via demodulation back into the original information signal (data wave)

stage 4: i remember something that my teacher mentioned; it was the reason the signal was modulated via an input/output transducer... correct me if i'm wrong

Thanks for replying again, i know it was a big ask but it just seemed like such a confusing and clutter topic in my head.

just another question:
I know that photodiodes are more sensitive have reaction times in the nano where as phototransistors have reaction times in the mega (or is that milli?) is that all we need to know regarding these components?

and also.. regarding the amplitude of the carrier wave, information/data wave or signal, modulated wave and demodulated wave. I'm a little hazy with these but i'll tell you what i know and hopefully you can correct me
Carrier wave has a high frequency

data wave can either be analogue (taking any input value within a range) thus irregular amplitude or digital (only taking a limited number of values) thus having a amplitude (I don't know that much about analogue and digital systems, my teacher brushed over once in class)

modulated wave is also has an irregular amplitude as it is a combination of the carrier and data wave.

the demodulated wave is presented as the output of the original signal (data wave)??

[soopertaco]

@ comma so taking into account what you said:

the carrier wave has a high frequency but is never in the 4th quadrant (i didn't know what else to call it :p)

the data wave on the other hand can be both +ve and -ve??

the Modulated wave as you said has to be positive as light intensity can never be negative

and the demodulated/original data wave is...?

[Comma]

Yep, so the carrier wave is always positive... and as I understand it, the data wave can be positive or negative and the demodulated signal of this will reflect the original - i.e. if the data wave is both positive and negative (as in it goes in both the 1st and 4th quadrant) then the demodulated wave will also be positive and negative, the fact that it was transmitted by light doesn't affect the demodulated signal.

[soopertaco]

Just a question: often it is advantageous to use prestressed concrete for beams. explain how prestressed concrete is stronger than regular concrete? 2 marks

I've got a general idea but i just need help with the expression

[Shark 774]

Yep, so the carrier wave is always positive... and as I understand it, the data wave can be positive or negative and the demodulated signal of this will reflect the original - i.e. if the data wave is both positive and negative (as in it goes in both the 1st and 4th quadrant) then the demodulated wave will also be positive and negative, the fact that it was transmitted by light doesn't affect the demodulated signal.

The modulated wave can be negative, if the graph is AMPLITUDE rather than intensity.