I am bit confused with some theories regarding the "Noise in communication channels". I tried to solve this question many times but still could not obtain an answer .. If there is an expert in this field .. please be kind enough to help me.. thank you..

this is the question

A television receiver consist of an amplifier having a gain of 20 followed by a mixer with the gain of 25. the bandwidth of the front end operating at temperature 270K is 10MHz. amplifier has 600 ohm input resister , a noise equelent resistance of 200 ohm and a 20kohm outout resistor. for the mxer, these values are 47K,2K and 680K . calculate the equlent input-noise resistance of this receiver front end?

 

equelent

Equivalent maybe?

 

There are three resistances of interest.

R3 - mixer output resistance
R2 - [(amplifier output resistance) || (mixer input resistance)] + mixer noise resistance
R1 - (amplifier input resistance) + (amplifier noise resistance)

The net noise resistance referred to the input would then be

Rnet=R1+R2/(A1)^2+R3/(A1*A2)^2

Where
A1 {amp gain} = 20
and
A2 {mixer gain} = 25

 

Thank you very much T_n_k for your valuable reply ..... According to the equation we need only the values of Resistors and Gains to calculate the answer. Does it mean that the answer is not depened on the operating bandwidth and the tremerature? Are they extra data(not usefull)?

can you please send me some materials which will help me to understand theoies behind this. thank you ...

 

There are dozens of references on the topic of noise on the web.

Maybe try Google Books - for your particular question use a search term such as ..

"equivalent noise resistance cascaded amplifiers"

 

Does it mean that the answer is not depened on the operating bandwidth and the tremerature? Are they extra data(not usefull)?

The other data are not relevant to the problem solution

noise voltage is related to [noise] resistance by the relationship

\(e_n=\sqrt{4KTR\(\Delta f\)}\)

But since you are only asking about equivalent noise resistance and not actual noise voltage then the additional information is not required.

 

thanks a lot T_n_k... can you plz help me with this matter..This is last one..

The block diagram of a wireless receiver frond-end circuit is shown in below. A noise source with a power of -70 dBm is applied to the receiver input. The physical temperature of the system is 270 K. The operating frequency band is 1850-1860 MHz.
(please take a look at the attached file...)

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a) What is the equivalent noise temperature of the source over the operating bandwidth?

b) What is the overall noise figure of the receiver?

c) What is the equivalent noise temperature of the overall system?

d) What is the total power output (in mW) of the receiver?