How do I calculate the required LO to RF isolation in a mixer for a given application?
LO-RF Isolation Budget
The LO-RF isolation budget accounts for all mechanisms by which LO power can reach the RF port: internal mixer leakage, PCB board coupling, cable coupling, and conducted coupling through shared power supplies. The total isolation is the parallel combination of all leakage paths (the worst path dominates). Typical contributions: mixer internal isolation: 30-40 dB. PCB layout isolation: 40-60 dB (with proper shielding). Cable isolation: 60-80 dB (with good shielding).
| 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
What if the LO and RF are close in frequency?
When fLO is close to fRF (low-IF or direct-conversion), the bandpass filter cannot distinguish between the LO and RF frequencies, so the filter provides minimal additional isolation. In this case, the mixer's internal isolation (30-40 dB) is the only protection. This is one reason why direct-conversion receivers suffer from LO leakage problems.
Does LO-IF isolation matter?
Yes. LO leakage to the IF port creates a large signal that can saturate the IF amplifier or ADC. This is especially problematic when the LO frequency falls within the IF bandwidth, which occurs in some frequency plans. IF filtering or a DC block (for direct conversion) mitigates this.
Can I achieve more than 40 dB from the mixer alone?
Standard DBMs: 30-40 dB. Premium hand-matched DBMs: 40-50 dB. Triple-balanced mixers: 40-50 dB. Isolation beyond 50 dB from the mixer alone requires special construction and careful testing. The practical total isolation is usually achieved through mixer + filter + layout shielding.