What is the noise figure contribution of a passive component like a filter or attenuator?
Why Passive Loss Equals Noise Figure
Every passive component in an RF signal path, whether it is a cable, filter, attenuator, connector, or switch, degrades the signal-to-noise ratio by exactly the amount of its insertion loss. This is a fundamental result of thermodynamics and has direct consequences for receiver system design.
The physical mechanism is straightforward. A passive component at temperature T radiates thermal noise power equal to kTB, where k is Boltzmann's constant and B is the bandwidth. When a signal passes through a lossy component, the signal power is reduced by the loss factor L, but the thermal noise from the component itself partially replaces the lost power. At the reference temperature of 290 K, the net effect is that the output SNR is degraded by exactly the loss factor.
This has critical design implications. Any loss placed before the LNA in a receiver chain adds directly to the system noise figure. A 0.5 dB cable loss before a 0.5 dB NF LNA results in a system noise figure of approximately 1.0 dB, effectively doubling the noise contribution. This is why satellite ground stations mount LNAs directly at the antenna feed, and why filter placement in a receiver front end requires careful tradeoff analysis between selectivity and noise performance.
Fpassive = 1/G = L (linear)
Te = Tphysical × (L - 1)
At 290 K: Te = 290 × (L - 1)
Frequently Asked Questions
Does this apply if the component is not at 290 K?
No. At temperatures other than 290 K, the noise figure changes. The equivalent noise temperature formula Te = T_physical × (L-1) gives the correct result at any temperature. At cryogenic temperatures, a lossy component contributes less noise than its insertion loss would suggest at room temperature.
Does connector loss count?
Yes. Every connector interface has some insertion loss, typically 0.1 to 0.3 dB per mated pair. In noise-critical systems, minimizing the number of connectors between the antenna and LNA is essential.
What about a circulator or isolator?
Circulators and isolators are passive devices with typical insertion loss of 0.3 to 0.7 dB. Their noise figure equals their insertion loss, just like any other passive component.