How do I select a mixer for minimum conversion loss and maximum IP3?
High-Performance Mixer Selection
Mixer selection for maximum dynamic range focuses on two specifications: conversion loss (which sets the noise figure floor) and IIP3 (which sets the linearity ceiling). The dynamic range of the mixer stage is proportional to IIP3 - NF, so both parameters must be optimized simultaneously.
| 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 |
Frequently Asked Questions
Is there a tradeoff between CL and IP3?
Not directly. Both improve with higher LO power. The tradeoff is between LO power (cost, complexity) and RF performance (CL, IP3). At a given LO level, custom mixer designs can optimize either CL or IP3, but the improvements are typically < 1 dB.
What about image-enhanced mixers?
Image-enhanced mixers terminate the image port with a reactive load that reflects the image back into the mixer. This constructive interference at the image frequency improves conversion loss by 1-2 dB at the expense of narrower bandwidth. Used in narrowband receivers where the image frequency is well-defined.
How do I test a mixer's IP3?
Two-tone test: apply two closely-spaced RF tones at equal power and measure the third-order intermodulation products at the IF output. IIP3 = Pin + (Pdesired - PIM3)/2, where Pin is the input power of each tone. Use the same LO power as specified in the data sheet.