Mixer Conversion Loss & Noise Figure
A mixer's conversion loss Lc is the ratio of available RF input power to available IF output power. Because a passive mixer also adds noise, its noise figure is set by Lc and the diode noise-temperature ratio t. A double-sideband (DSB) measurement gives a figure ~3 dB lower than the single-sideband (SSB) figure that matters for a real receiver. This tool computes the SSB and DSB mixer noise figure and cascades the mixer with the IF amplifier to give the front-end noise figure.
Equations & Parameters ▸
\(L_c=\dfrac{P_{RF,\,avail}}{P_{IF,\,avail}}\), conversion gain \(=-L_c\) (dB).
\(F_{DSB}=L_c\,t,\qquad F_{SSB}=2L_c\,t\ \Rightarrow\ F_{SSB}=F_{DSB}+3\text{ dB}\)
Cascade with IF amp: \(F_{sys}=F_{mix}+(F_{IF}-1)L_c\) (linear).
\(F_{DSB}=L_c\,t,\qquad F_{SSB}=2L_c\,t\ \Rightarrow\ F_{SSB}=F_{DSB}+3\text{ dB}\)
Cascade with IF amp: \(F_{sys}=F_{mix}+(F_{IF}-1)L_c\) (linear).
| Lc | Conversion loss (dB, positive). |
| t | Diode noise-temperature ratio (≈1 for a good Schottky diode). |
| FIF | IF amplifier noise figure (dB). |
| Sideband | Which mixer NF definition to cascade with the IF amp. |
Reference: D. M. Pozar, Microwave Engineering, 4th ed., §13.3.
Inputs
dB
Positive≈ 1
dB
dBFor cascade
Results
Mixer alone
Conversion gain—
NF (DSB)—
NF (SSB)—
Mixer + IF amplifier
Sideband used—
Front-end NF—
Front-end noise temp—
Down-conversion