How do I determine the required LO drive level for a mixer to achieve optimal performance?
LO Drive Selection
The mixer diodes switch between their on and off states under the LO drive. With insufficient LO power, the diodes do not fully switch, resulting in higher conversion loss, lower IP3, and degraded spurious suppression. With excessive LO power, the diodes may be damaged or the mixer's balance may be degraded by non-ideal diode behavior at extreme forward bias.
The standard Level naming convention (Level 7, 13, 17, 23) refers to the LO power in dBm required to achieve the specified mixer performance. Each +10 dB increase in LO power approximately doubles the IIP3. This makes high-level mixers the choice for receivers that must handle strong signals without intermodulation distortion.
The LO source must provide clean, stable power at the required level. Phase noise of the LO directly transfers to the downconverted signal. Any amplitude modulation or spurious content on the LO creates unwanted mixing products at the IF. The LO buffer amplifier should provide the required power with adequate margin (2-3 dB above the mixer's specified level).
Frequently Asked Questions
What happens if LO power is too low?
Conversion loss increases (2-5 dB degradation for 3 dB under-drive), IP3 drops proportionally, and even-order spurious suppression degrades. The mixer still functions but with reduced performance. Operating with LO power 1-2 dB below specification is usually acceptable.
What if LO power is too high?
Minimal improvement in conversion loss (already near optimum), slight increase in IP3, but risk of diode damage from excessive forward current and reverse voltage. Most mixers tolerate 2-3 dB over-drive without damage. Check the maximum LO power rating in the data sheet.
How do I generate the LO power?
For Level 7: a typical frequency synthesizer output (+0 to +5 dBm) plus a small gain block. For Level 13: requires a driver amplifier (+13 dBm output). For Level 17: requires a medium-power amplifier. The LO chain must maintain low phase noise while providing the required power.