What is the noise figure of a passive mixer and how does conversion loss factor in?
Passive Mixer Noise Performance
A passive mixer uses diodes (Schottky barrier diodes in most microwave designs) to perform frequency conversion without any active gain element. Since the mixer is fundamentally a passive, lossy device, its noise figure follows the same thermodynamic principle as any other passive component: the noise figure equals the loss.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
Frequently Asked Questions
Does the LO contribute noise?
The LO signal itself carries AM noise that can convert to IF noise through the mixer. Good LO sources with low AM noise are important for minimizing this contribution, especially for narrowband receivers where close-in LO noise affects the noise floor near the IF center frequency.
How does an active mixer compare?
Active mixers (using FETs or Gilbert cells) can provide conversion gain, meaning their noise figure is lower than a passive mixer. However, active mixers generally have lower linearity (IP3 and P1dB) than passive diode mixers. The choice depends on whether noise or linearity is the priority.
Why specify SSB noise figure for mixers?
Most receivers process only one sideband. The mixer accepts noise from both sidebands but signal from only one, so SSB noise figure correctly accounts for this and should be used in the cascade analysis. DSB NF is 3 dB lower and is only appropriate when both sidebands carry desired signal.