What is the difference between a fundamental mixer and a subharmonic mixer?
Mixer LO Frequency Selection
In a fundamental mixer, the diodes are driven directly by the LO signal, and the nonlinear I-V characteristic generates mixing products at fRF ± fLO. The LO must be at the full required frequency, which can be challenging above 60-100 GHz where signal sources become expensive, inefficient, and difficult to distribute.
| 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
When should I use subharmonic?
Above 60-100 GHz where fundamental LO sources become expensive or unavailable. At W-band (75-110 GHz): a subharmonic mixer with 40-55 GHz LO is more practical than a fundamental mixer requiring 75-110 GHz LO. Below 60 GHz: fundamental mixing is almost always preferred for its lower conversion loss.
Can I use a higher subharmonic?
Third-subharmonic (3× LO) and fourth-subharmonic (4× LO) mixers exist but have progressively higher conversion loss (10-15 dB and 15-20 dB respectively). They are used only at the highest frequencies (200+ GHz) where even a half-frequency LO is impractical.
What about LO phase noise?
Subharmonic mixers multiply the LO phase noise by 20·log10(n), where n is the harmonic number. A 2× subharmonic mixer degrades the effective LO phase noise by 6 dB. This must be accounted for in the system noise budget.