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.
| 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
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.