What is the difference between a double balanced mixer and a triple balanced mixer?
Triple Balanced Mixer
The standard double-balanced mixer has a limitation: the IF port bandwidth is limited on the low end by the IF balun (transformer). The IF balun cannot pass DC or very low frequencies, typically limiting the IF to above 1-10 MHz depending on the transformer core. This prevents use of DBMs in direct-conversion or very-low-IF architectures.
| 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
When should I use a TBM?
When you need IF response to DC (direct conversion, spectrum analyzers), when all three ports must cover the same wideband frequency range (multi-purpose test equipment), or when the best possible spurious performance is needed. TBMs are standard in high-performance spectrum analyzers and EW receivers.
Is the LO power requirement a problem?
TBMs typically require 3-6 dB more LO power than an equivalent DBM. A Level 17 DBM needs +17 dBm; the equivalent TBM needs +20 to +23 dBm. This is the primary practical limitation, requiring a more powerful (and often more expensive) LO chain.
What about harmonic mixers?
Harmonic mixers (subharmonic mixers) use the LO harmonics generated by the diode nonlinearity for mixing. They allow the LO to operate at a fraction of the required fundamental frequency. This is different from balanced mixer topology and can be combined with balanced configurations.