How do I calculate the conversion loss and noise figure of a passive diode mixer?
Mixer Loss and Noise
Conversion loss in a passive diode mixer comes from several mechanisms: (1) the intrinsic loss of the mixing process (the diode switching waveform is not a perfect square wave), (2) the diode series resistance (which dissipates power), (3) the balun and hybrid losses in balanced designs, and (4) mismatch loss between the mixer ports and the external impedances.
| 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
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Why is SSB NF 3 dB worse?
Because the image frequency band contributes noise but not signal. The mixer converts noise from both RF and image frequencies to the IF, but only one sideband contains the desired signal. The noise power at the IF is doubled relative to the signal, adding 3 dB to the noise figure.
How does LO power affect conversion loss?
Conversion loss decreases as LO power increases (harder diode switching) until the optimum LO level is reached. Typical optimum: +7 dBm for Level 7 mixers, +13 dBm for Level 13, +17 dBm for Level 17. Beyond the optimum, CL increases slightly due to diode heating and nonlinear capacitance effects.
Can an active mixer have conversion gain?
Yes. Active mixers (Gilbert cell, FET ring) provide conversion gain of 5-15 dB because the mixing transistors also amplify the signal. Active mixers have NF of 8-15 dB (higher than passive mixers), but the system NF contribution is lower because subsequent stage noise is suppressed by the conversion gain.