Mixer
Understanding RF Mixers
Frequency conversion is essential in virtually every RF system. Receivers convert high RF frequencies to lower intermediate frequencies (IF) where filtering and digitization are easier. Transmitters convert baseband or IF signals up to the desired RF frequency. Mixers perform this conversion using the mathematical property that multiplying two sinusoids produces sum and difference frequencies.
Mixer Types
- Passive diode mixers: Use Schottky diodes in single-ended, single-balanced, or double-balanced topologies. No DC power required. Conversion loss of 6-10 dB. Excellent linearity.
- Active mixers: Use transistors (Gilbert cell topology). Provide conversion gain instead of loss. Higher noise figure. Common in integrated circuits.
- Sub-harmonic mixers: Use the second or higher harmonic of the LO, reducing the required LO frequency by half. Used at mmWave frequencies where high-frequency LOs are expensive or unavailable.
- Image-reject mixers: Use two mixers and a 90-degree hybrid to suppress the image frequency. Eliminate the need for a separate image-reject filter.
Key Specifications
- Conversion loss/gain: Power difference between desired output and RF input, typically 6-10 dB loss for passive mixers.
- LO drive level: Required LO power for proper operation, typically +7 to +17 dBm for passive mixers.
- Port isolation: Leakage between LO, RF, and IF ports. Higher isolation reduces spurious signals.
- Spurious response: Unwanted mixing products (m x f_RF +/- n x f_LO). Documented in a spur chart.
f_IF = |f_RF - f_LO| (downconversion)
f_RF = f_IF + f_LO (upconversion)
Image frequency:
f_image = f_LO + f_IF (if f_RF = f_LO - f_IF)
f_image = f_LO - f_IF (if f_RF = f_LO + f_IF)
Conversion loss (dB):
CL = P_RF (dBm) - P_IF (dBm)
Example: f_RF=10 GHz, f_LO=9 GHz, f_IF=1 GHz
Image frequency = 8 GHz
Mixer Topology Comparison
| Type | Conversion | LO Drive | LO-RF Isolation | Spurious |
|---|---|---|---|---|
| Single-ended | -8 to -10 dB | +7 dBm | ~0 dB | Poor |
| Single-balanced | -7 to -9 dB | +7 dBm | 15 - 25 dB | Moderate |
| Double-balanced | -6 to -8 dB | +10 to +17 dBm | 25 - 40 dB | Good |
| Triple-balanced | -7 to -10 dB | +17 to +23 dBm | 30 - 45 dB | Excellent |
| Active (Gilbert cell) | +5 to +15 dB | -5 to +3 dBm | 20 - 30 dB | Moderate |
Frequently Asked Questions
What does a mixer do in RF?
A mixer combines an input RF signal with a local oscillator (LO) signal to produce output signals at the sum and difference frequencies. The desired output (usually the difference frequency) is selected by filtering. This frequency translation is the basis of superheterodyne receivers and transmitters.
What is the difference between a passive and active mixer?
A passive mixer uses diodes and has conversion loss (typically 6-10 dB), but excellent linearity and dynamic range. An active mixer uses transistors, provides conversion gain (typically 5-15 dB), but has higher noise figure and is more susceptible to overload.
What is the image frequency?
The image frequency is a second input frequency that produces the same IF output as the desired RF frequency. It is located at f_image = f_LO +/- f_IF (on the opposite side of the LO from the desired RF). Image signals must be suppressed by filtering or by using an image-reject mixer architecture.