What is the relationship between noise floor, bandwidth, and noise figure in a receiver?
Understanding Receiver Noise Floor
The noise floor defines the weakest signal a receiver can detect. It is set by three factors: the thermal noise power density, the receiver bandwidth, and the receiver noise figure. Understanding this relationship is fundamental to every receiver design, link budget calculation, and sensitivity specification.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
Frequently Asked Questions
Why is the thermal noise floor -174 dBm/Hz?
This comes from kT at 290 K: (1.38×10⁻²³ J/K)(290 K) = 4.00×10⁻²¹ W/Hz. Converting to dBm: 10×log10(4.00×10⁻²¹/0.001) = -174. This is a fundamental physical limit at room temperature.
Does the noise floor change with temperature?
Yes. The -174 dBm/Hz value assumes 290 K. At cryogenic temperatures (77 K), the thermal floor drops to -180 dBm/Hz. In hot environments (350 K), it rises to -173 dBm/Hz. For most terrestrial systems, 290 K is a valid approximation.
How does processing gain affect the noise floor?
Signal processing techniques like coherent integration, spread spectrum despreading, or matched filtering can extract signals below the noise floor. Processing gain does not change the physical noise floor but allows detection of signals below it by effectively narrowing the bandwidth after reception.