What is a resistive FET mixer and what are its advantages at millimeter wave frequencies?
Resistive FET Mixer
The resistive FET mixer exploits the FET channel's ability to switch between a low resistance (Rds_on = 2-10 Ω for GaAs or GaN HEMTs) and a very high resistance (Rds_off > 10 kΩ) under gate voltage control. By applying the LO to the gate, the FET becomes a variable resistor that modulates the RF signal passing between drain and source. No DC drain voltage is applied; the FET operates entirely as a passive switch.
| 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
How does it compare to a diode mixer?
Similar conversion loss (6-10 dB). Better linearity (IIP3 5-10 dB higher at the same LO power). Comparable noise figure. Easier MMIC integration. The main disadvantage is limited bandwidth compared to multi-octave Schottky diode mixers.
Can I use it below 10 GHz?
Yes, but at lower frequencies, diode-based DBMs are cheaper and simpler. Resistive FET mixers are most advantageous above 20 GHz where their integration and linearity benefits outweigh the simplicity of discrete diode mixers.
What about the LO drive requirement?
The LO must swing the FET gate over its full on-off range. Typical LO power: +3 to +13 dBm depending on the FET's pinch-off voltage and gate capacitance. Lower pinch-off voltage devices require less LO power.