What is the difference between a passive diode mixer and an active FET mixer?
Passive vs Active Mixer
The fundamental difference is that passive mixers have no power supply and produce conversion loss, while active mixers require DC bias and can provide conversion gain. This affects their role in the receiver signal chain: a passive mixer must be preceded by enough gain to overcome its conversion loss, while an active mixer contributes gain that helps suppress the noise of subsequent stages.
| 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
Which has better noise figure?
Passive mixers: NF ≈ conversion loss (6-8 dB typically). Active mixers: NF = 8-15 dB but with conversion gain. For system NF contribution: a passive mixer with 7 dB loss followed by an IF amplifier with 3 dB NF gives NF_system ≈ 10 dB. An active mixer with 12 dB NF and 10 dB gain gives NF_system ≈ 12 dB at the mixer but suppresses subsequent stage noise.
Can I use FETs as passive mixers?
Yes. Resistive FET mixers use the FET channel as a variable resistor (switching between low and high resistance with the LO). They achieve conversion loss similar to diode mixers (5-8 dB) but with better port isolation and easier integration in CMOS/GaAs ICs.
Which is better for direct conversion?
Active mixers are preferred for direct conversion (zero-IF) because they provide gain at DC/baseband where subsequent stages have high 1/f noise. Passive mixers work for direct conversion but require very low-noise baseband amplifiers to compensate for the conversion loss.