How do I design a frequency multiplier chain for millimeter wave LO generation?
Multiplier Chain Design
Frequency multiplication is the standard technique for generating signals at millimeter wave frequencies where direct oscillators are not available or have poor performance. The approach leverages the excellent phase noise performance of microwave oscillators (VCOs, YIG, DROs at 5-20 GHz) and multiplies them to the desired mmWave frequency.
| 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
Why not just use one big multiplier instead of a chain?
Higher-order multipliers (×4, ×5, ×8) have very poor efficiency (<1%) and generate many spurious products that are difficult to filter. Cascading lower-order multipliers (×2, ×3) with inter-stage filtering produces cleaner outputs with better total efficiency.
How do I choose the multiplication factor?
Prefer ×2 and ×3 for best efficiency. Avoid ×4 or higher in a single stage. For ×8: use ×2 × ×2 × ×2 (three doublers) or ×2 × ×4 if a clean ×4 is available. For ×6: use ×2 × ×3 or ×3 × ×2. The order of stages matters: place the tripler last for minimum spurious content.
What power output is achievable?
At W-band (75-110 GHz) from a 10 GHz source: typical output is -5 to +10 dBm from a ×8 chain. At D-band (110-170 GHz): -10 to 0 dBm from a ×12 chain. Higher power requires a mmWave power amplifier after the multiplier chain, which adds cost and complexity.