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README.md

#326 Mixers

All about frequency mixers.

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Notes

Frequency mixers are nonlinear devices that produce a signal at the sum and difference of the original frequencies of two signals applied to it. Other frequency components may also be present in a practical frequency mixer.

Commonly used to shift signals from one frequency range to another (heterodyning).

Given their use in RF, the input signals are conventionally notated as:

  • RF - input radio spectrum
  • LO - local oscillator

And the output:

  • IF - intermediate frequency.

Section 5 of Experimental Methods in RF Design provides thorough and very understandable introduction to mixers of various types.

Some mixer implementations I have experimented with:

  • LEAP#327 - double-balanced diode ring mixer

Theoretical Basis

cos(ω1t)cos(ω2t) = 1/2cos(ω1t - ω2t) + 1/2cos(ω1t + ω2t)
f(if) = ± m.f(rf) ± m.f(lo)
Components of f(if) Products
1st order f(lo), f(rf)
2nd order f(lo) - f(rf) or f(rf) - f(lo), f(lo) + f(rf)
3rd order 2f(lo) - f(rf), 2f(lo) + f(rf), f(lo) - 2f(rf), f(lo) + 2f(rf)
... etc

Typically want the upper or lower 2nd order products from the IF.

  • the upper product (sum) is often called upper sideband, and is probably the one we want
  • the lower product (difference) is often called lower sideband, and may not be desired because it exhibits spectrum inversion

Balanced Designs

So-called "balanced mixers" eliminate feed-through of the input signals. A double-blanaced design means both inputs (1st order products) are eliminated from the output.

Filtering

1st order products are minimised with single or double balanced design.

Filters are often used in practical designs, potentially on all ports:

  • RF - pre-selection filter
  • LO - to eliminate oscillator harmonics
  • IF - to select the desired product.

Active and Passive

Mixers can be classified as active or passive:

  • passive - typically using diodes, no additional power required
  • active - typically JFET, BJT or MOSFETS with additional biasing power. Or packaged as ICs often with filtering and amplification.

Example active mixer integrated circuits:

  • LT5560 - double-balanced broadband active mixer with integrated input amplifier
  • MAX2681 - double-balanced Gilbert-cell mixer with single-ended RF, LO, and IF port connections
  • SA602A - low-power VHF monolithic double-balanced mixer with input amplifier, on-board oscillator, and voltage regulator

Mixers are also available as packaged/connectorised units, for example:

Credits and References