Receiver noise metrics: Which of the following is NOT a generally useful figure of merit for comparing the noise performance of different radio receivers?

Difficulty: Medium

Correct Answer: Input noise voltage (without specifying source impedance and bandwidth)

Explanation:


Introduction / Context:
Noisy front ends degrade weak-signal reception. Engineers therefore compare receivers using normalized noise metrics. This item assesses which metrics are standardized and portable across different systems and which are not.



Given Data / Assumptions:

  • We compare different receivers that may see different source impedances and bandwidths.
  • We seek a metric that remains meaningful across setups.


Concept / Approach:

Noise figure (NF) and noise temperature (Tn) are standardized: both relate output SNR to input SNR independent of absolute signal level, provided the source impedance and temperature are defined. Equivalent noise resistance also references noise back to the input with a known bandwidth and impedance. A bare 'input noise voltage' number, without source impedance and bandwidth, is not portable because thermal noise scales with both.



Step-by-Step Solution:

Thermal noise spectral density ~ 4kTR (V^2/Hz) depends on R and T.Total noise voltage ~ sqrt(4kTR*B) depends on bandwidth B and source R.Thus quoting a single 'input noise voltage' without R and B is ambiguous and not comparable.


Verification / Alternative check:

Receiver datasheets always pair noise voltages with the test source impedance and bandwidth. Cross-vendor comparisons typically use NF or Tn.



Why Other Options Are Wrong:

  • NF and Tn are standard, comparable figures of merit.
  • Equivalent noise resistance is meaningful when referred to a defined input and bandwidth.
  • C/N at a known point is also meaningful when the reference bandwidth is specified.


Common Pitfalls:

  • Comparing unreferenced noise voltages from different receivers and drawing incorrect conclusions.


Final Answer:

Input noise voltage (without specifying source impedance and bandwidth)

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