Assertion (A) : Noise figure is defined as the ratio of the signal to noise power at the input terminal to the signal to noise power at the output terminal of the system. Reason (R) : The noise figure will be less than 1 for an ideal receiver, which introduces no noise of its own. Select your answer using the codes given below :
(A) is true, but (R) is false
The assertion states the definition correctly :
\(F=\dfrac{(S/N)_{input}}{(S/N)_{output}}\)
It measures how much the signal-to-noise ratio degrades in passing through the system.
The reason is false because it gets the ideal value wrong. Every real stage adds noise of its own, so the output signal-to-noise ratio can never be better than the input's. Therefore
\(F\ \ge\ 1\qquad\text{always}\)
and an ideal noiseless receiver achieves exactly \(F=1\), or 0 dB — never less. A value below 1 would mean the amplifier had improved the signal-to-noise ratio, which would violate the second law of thermodynamics as surely as any perpetual-motion machine. With (A) true and (R) false, the code is 3.
| Device | Typical F | In dB |
|---|---|---|
| Ideal (noiseless) | 1 | 0 |
| Cooled parametric amplifier | 1.1 | 0.4 |
| Low-noise GaAs FET | 1.6 | 2 |
| Ordinary receiver | 6 – 10 | 8 – 10 |
The equivalent noise temperature restates the same information in kelvin and is preferred in satellite and radio-astronomy work, where noise figures cluster uncomfortably close to 0 dB:
\(T_{e}=(F-1)\,T_{0}\qquad T_{0}=290\ \text{K}\)
An ideal receiver has \(T_{e}=0\ \text{K}\), which again is the floor, not something to be beaten.
Friis's formula shows why the first stage dominates :
\(F_{total}=F_{1}+\dfrac{F_{2}-1}{G_{1}}+\dfrac{F_{3}-1}{G_{1}G_{2}}+\cdots\)
Each later stage's contribution is divided by the gain preceding it, so a low-noise amplifier at the very front of a receiver essentially sets the whole system's noise figure. This is precisely why satellite dishes mount the LNA at the feed rather than at the far end of the cable — putting a lossy cable first would add its loss directly to \(F_{1}\).
Hence, (A) is true but (R) is false — the ideal noise figure is exactly 1, never less.
Similar Questions
Which of the following are useful in comparing the noise performance of receivers ?
1. Input noise voltage
2. Equivalent noise resistance
3. Noise temperature
4. Noise figureSelect the correct answer.
The minimum receivable signal in a radar receiver which has an IF bandwidth of 2.5 MHz and a 9-dB noise figure is : (Take T as 290° K)
A two stage receiving system operates at 30 MHz bandwidth. Calculate noise figure (F) of the system for following values of G1 (dB) and arrange them in ascending order. Assuming F1 (dB) = 2 dB, F2 (dB) = 9 dB
A. G1 = 10 dB
B. G1 = 20 dB
C. G1 = 50 dB
D. G1 = 30 dB
E. G1 = 40 dB
Choose the correct answer from the options given below :
Read the following statements.
(a) Industrial noise is usually of impulse type.
(b) HF mixers are generally noisier than HF amplifiers.
(c) Thermal noise is independent of frequency at which it is measured.
(d) Impulse noise voltage is independent of the bandwidth.
Which of the above statements are correct ?
A mixer stage has a noise figure of 20 dB and is preceded by an amplifier that has a noise figure of 9 dB and an available power gain of 15 dB. The overall noise figure is :
List – I List – II a. Shot Noise i. Resistance b. Johnson Noise ii. Diode c. Current Noise iii. P-N junction d. Partition Noise iv. Triode Codes :
Important Questions from Noise Temperature and Noise Figure
Which of the following is not a primary source of external noise?
If each stage had a gain of 10 dB, and Noise Figure of 10 dB, then the overall Noise figure of a two-stage cascade amplifier will be
An antenna pointing in a certain direction has a noise temperature of 50 K. The ambient temperature is 290 K. The antenna is connected to a pre-amplifier that has a noise figure of 2 dB and an available gain of 40 dB over an effective bandwidth of 12 MHz. The effective input noise temperature Te for the amplifier and the noise power Pao at the output of the preamplifier, respectively, are
The noise figure of an amplifier is 3 dB. Its noise temperature will be about
Which of the following are useful in comparing the noise performance of receivers ?
1. Input noise voltage
2. Equivalent noise resistance
3. Noise temperature
4. Noise figureSelect the correct answer.
