Calculating dB output from this example

Question

This is my extra credit assignment so don't tell me answers, but please guide me how I should do this. I need to learn.

Question states: Determine the power output of the receiver in watts and in the appropriate measure of dB

let: Pin = 2W

diagram: --> [ G1 = 1/4 ] --> [ G2 = 1/10 ] --> [ G3 = 20 ] -->

How should I start?

After some thinking..

I'm not sure but G might represent the General formula which is G = Pout/Pin

EDIT

Is this correct?

Pin(dB) = 10log(2W/1W) = 3.01dB --- as input

dB = 10log(1/4) + 10log(1/10) + 10log(20) = -3.01dB --- As output

So the output at the destination is 0?

What are G1, G2 and G3 and what are the numbers 1/4, 1/10 and 20 associated with them? — Chris Taylor, Nov 30 '11 at 08:27

score 1 · Accepted Answer · answered Nov 30 '11 at 20:55

1

You did it correctly! Great job!

answered Nov 30 '11 at 20:55

HelpNeeder

1

Thank you! In my class professor got the same answer. – HelpNeeder Nov 30 '11 at 20:55

score 0 · Answer 2 · answered Nov 30 '11 at 17:23

0

Hint: Search for transfer function and Bode Plot

answered Nov 30 '11 at 17:23

@HelpNeeder I have a feeling about the question but you provide so little info about the diagram that I don't even know, if we are talking about the same thing. Transfer Function is a function that relates an input to an output. Its bode plot shows how the power is distributed among frequency spectrum but again, this is guessing. – Nov 30 '11 at 17:45
@HelpNeeder Just tell me what Diagram > means. – Nov 30 '11 at 17:52
This is basically what looks like on my problem: -->[G1]-->[G2]-->[G3]--> Meaning that I have 3 boxes, first 2 are reducing a signal, and last one acts as amplifier. So I have 2W input and what the output should be after last box. – HelpNeeder Nov 30 '11 at 17:55
@HelpNeeder :D See, this is completely different from what you ask. OK, they are simple gain blocks in series hence you can multiply to obtain just one block and the $G$ blocks are processing the signal I suppose.Do you know how to calculate the power of the signal? – Nov 30 '11 at 18:05
I have an idea, but I'm not sure if I'm correct. What I know is in my last edit in OP. – HelpNeeder Nov 30 '11 at 18:09
@HelpNeeder Let say, we have a constant signal with magnitude 1, it has 1 unit of power per second and if it is amplified by 2 then the output will have 4. What you need is to apply the same principal to your problem. – Nov 30 '11 at 18:16
I don't understand how you got 4. So you saying 2W x 1/4 should be output of my first block? – HelpNeeder Nov 30 '11 at 18:33

2 Answers2