What is a frequency multiplier and how does it differ from a mixer in terms of signal generation?
Multiplier vs Mixer
Frequency multipliers exploit the nonlinear I-V characteristic of a device to generate harmonics of the input signal. The Nth harmonic is selected by a bandpass filter at N×fin. The conversion efficiency from fundamental to Nth harmonic decreases rapidly with N because the harmonic content of the nonlinear waveform drops off. For practical Schottky diode multipliers: ×2 efficiency = -8 to -12 dB, ×3 = -12 to -18 dB, ×5 = -20 to -25 dB.
| 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
Which gives better phase noise?
Depends on available sources. A multiplier from a low-noise 10 GHz source to 40 GHz degrades phase noise by 12 dB. A mixer combining 35 GHz and 5 GHz sources gives phase noise equal to the noisier source. If the 35 GHz source has better than (10 GHz source - 12 dB) phase noise, the mixer approach wins.
Can I use a mixer as a multiplier?
Not directly. A mixer's output frequency is always the sum or difference of its two inputs. However, if both ports are driven by the same signal: fout = fin + fin = 2×fin. This is an inefficient frequency doubler (conversion loss > 10 dB). Dedicated multiplier circuits are more efficient.
What about active multipliers?
Active multipliers use transistors biased for harmonic generation and include amplification, achieving positive conversion gain (+5 to +10 dB for ×2). They are more complex than passive diode multipliers but produce higher output power and can include filtering on-chip. GaAs and GaN MMIC active multipliers are common.