What is the 1 dB compression point of a mixer and how does it differ from an amplifier P1dB?
Mixer Compression
In a passive diode mixer, compression occurs when the RF signal becomes large enough to affect the diode switching process. At low RF levels, the diodes are switched solely by the LO signal. As the RF level approaches the LO level, the RF signal starts to forward-bias the diodes during the nominally off portion of the LO cycle (or reverse-bias them during the on portion), changing the effective switching waveform and reducing the conversion efficiency.
| 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
Is mixer P1dB the same as amplifier P1dB?
The definition is the same (1 dB deviation from linear), but the mechanism is different. Amplifier P1dB is intrinsic to the transistor's transconductance saturation. Mixer P1dB is determined by the LO power relative to the RF signal level. Increasing LO power increases mixer P1dB; there is no equivalent knob for amplifier P1dB.
What about two-tone P1dB?
Two-tone compression uses two equal-power RF signals. The effective P1dB per tone is approximately 3 dB lower than single-tone P1dB because the total RF power is doubled. This is important for communications signals that contain multiple carriers.
How does IF loading affect P1dB?
Poor IF port matching can reduce the effective P1dB by 1-2 dB because reflected IF power re-enters the mixer and affects the diode operating point. Always terminate the IF port with its specified impedance (usually 50 Ω) for best linearity.