# #326 Mixers

## 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: