Every superheterodyne receiver, every upconverting transmitter, every modern radio front end leans on one deceptively simple component: the mixer. It multiplies two signals to move one of them to a new frequency — and in doing so, it creates every sum-and-difference product in the book. Here is how to choose one and keep the unwanted products out of your data.
What a Mixer Actually Does
An ideal mixer is a multiplier: RF × LO produces the sum (RF + LO) and difference (|RF − LO|) frequencies. The wanted product becomes your IF; everything else is a spurious response to manage. Real mixers are imperfect multipliers — which is where conversion loss, isolation, and spurious products come from.
Mixer Types: The Trade Is Linearity vs Gain vs Power
| Type | Conversion | Strengths | Costs |
|---|---|---|---|
| Passive diode ring (double-balanced) | −5 to −9 dB (loss) | Wide bandwidth, excellent isolation and linearity, no DC power | Needs +7 to +17 dBm LO drive; loss must be made up downstream |
| Passive FET (GaN/SiC high-end) | −6 to −8 dB | Extreme linearity (high IP3), rugged | High LO power, careful biasing |
| Active (Gilbert cell, IC) | +0 to +10 dB (gain) | Low LO drive (0 dBm), integrated, DC-powered | Lower linearity headroom, noise figure, supply current |
The classic system decision: a passive double-balanced mixer (like the Mini-Circuits/Sygnet-style families) gives the best dynamic range but demands a beefy LO chain — a PA, amplifiers, and filtering just to feed the LO port. Active IC mixers trade some IP3 for a dramatically simpler LO path and net gain. Both are valid; the LO drive budget usually decides.
Conversion Loss and Noise Figure: Read Them Together
- Conversion loss (passive) is not just attenuation — the mixer's noise figure is close to its conversion loss, so a −7 dB mixer contributes ~7 dB NF at that stage. Cascade analysis (Friis) means the LNA ahead of it still dominates, but the IF amplifier after a lossy mixer matters more than designers expect.
- Active mixers quote conversion gain and NF separately — a 6 dB-gain mixer with 8 dB NF is possible, and both numbers matter.
- LO-to-RF and LO-to-IF isolation determine how much LO leaks into your output and radiates back through the antenna. Double-balanced construction cancels LO at both ports by symmetry — respect the balance in your layout, or the spec evaporates.
The Image Problem: Every Mixer Hears Two Frequencies
For a desired RF with LO at f_LO and IF at f_IF, the mixer cannot tell the desired signal from the image at f_LO ± f_IF on the other side — both convert to the same IF. Strategies, in rising order of complexity:
- High-side vs low-side injection planning: choose the LO side that places the image where a filter can kill it (e.g. out of the cellular band rather than inside it).
- Preselector/image filter: a bandpass filter ahead of the mixer attenuates the image — simplest, but filter bandwidth vs channel spacing fights back at high IFs... or rather low IFs, where the image sits close.
- High first IF: pushing the first IF far from the RF pushes the image far enough for a fixed filter to reject it — the architecture behind most classic superhet designs.
- Image-reject mixers: quadrature architectures (Hartley, Weaver, integrated IR mixers) cancel the image mathematically, buying 25–45 dB rejection with no filter — at the cost of two mixers, a quadrature LO, and amplitude/phase balance sensitivity.
Spur Planning: The m × n Chart
Real mixers produce m×RF ± n×LO products for small integers m, n. Before freezing a frequency plan, run the spur chart (most mixer vendors provide calculators): find where 2×2, 3×2, and half-IF spurs land, and confirm none falls in-band. In-band spurs cannot be filtered out — only architecture changes fix them.
Selection Workflow
- Fix the frequency plan first: RF range, IF, LO side, image location, spur scan.
- Decide passive vs active from the LO drive budget and linearity requirement (IP3 target).
- Budget conversion loss/gain and NF through the cascade; place gain where Friis says it helps.
- Verify port isolation and plan the layout to preserve balance (symmetric lines, solid ground).
JTDZ Tech supplies double-balanced passive mixers, active IC mixers, LO amplifier chains, and the filters around them from the major RF manufacturers. Send us your frequency plan and we will quote mixers that keep the spur chart quiet.
