The noise phenomena arranged in the order of their dominance when plotted as a function of increasing frequency are:
Transit time noise, Flicker noise, White noise
Flicker noise, White noise, Transit time noise
White noise, Flicker noise, Transit time noise
Transit time noise, White noise, Flicker noise
ANSWER
The correct option is (b): Flicker noise, White noise, Transit time noise.
Official answer: BSource: ISRO's revised and final answer key of 16 February 2024
Flicker () noise has a power spectral density that falls roughly as , so it dominates at the lowest frequencies.
White noise (thermal and shot noise) has a flat spectrum. Once the term has dropped below this flat level, white noise dominates over the middle band of frequencies.
Transit-time noise appears at very high frequencies. There the carrier transit time becomes comparable to the signal period, and the noise rises with frequency, so it dominates at the top end.
Ordering by increasing frequency gives: Flicker noise, then White noise, then Transit-time noise.
Option (a) and option (d) both start with transit-time noise, which dominates only at the highest frequencies, so they are wrong. Option (c) starts with white noise, but at low frequency the noise is larger than the flat white level, so it is wrong. Only option (b) matches the order.
KEY IDEA
A noise spectrum has three regions: (flicker) noise at low frequency, a flat white-noise floor in the middle, and a rise from transit-time effects at very high frequency.
WATCH OUT FOR
Putting white noise first because it is the most familiar kind of noise. At low frequency the term sits above the flat white level, so flicker noise dominates there.
From the ISRO EC 2023 paper. Every question of this paper · The full paper (PDF, ISRO) · All 2 ISRO Noise & shielding (IN) questions, with notes
