How do I calculate the noise figure of a mixer from its conversion loss?
Mixer Noise Analysis
The passive mixer noise figure closely tracks its conversion loss because the dominant noise mechanism is the thermal noise of the diode junction resistance. When the diode is in its on-state, it has a resistance of 5-20 Ω that generates thermal noise. When the diode is off, no noise is generated. The time-averaged noise contribution gives NF very close to CL.
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
How do I calculate system NF with a passive mixer?
In the Friis cascade: the mixer acts as a lossy two-port with NF = CL and gain = -CL (negative gain = loss). NF_system = NF_LNA + (NF_mixer - 1)/G_LNA + (NF_IF - 1)/(G_LNA × G_mixer). The mixer's conversion loss reduces the gain for subsequent stage noise suppression.
Does temperature affect mixer NF?
Slightly. Diode thermal noise scales with temperature (T/290) in the same way as resistor noise. A 50°C ambient temperature increase adds about 0.7 dB to the mixer NF. For precision receivers, this temperature variation must be included in the noise budget.
What about 1/f noise in mixers?
Passive diode mixers have very low 1/f noise (essentially none) because no DC current flows. Active mixers have significant 1/f noise from the bias current, with corner frequencies of 10 kHz to 10 MHz depending on the technology. For direct-conversion receivers, this 1/f noise degrades sensitivity for narrowband signals near DC.