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A doubly Balanced Frequency Mixer

THE IDEAL MIXER#

a perfect mixer

MIXERS MAY BE CLASSIFIED BY THEIR TOPOLOGY#

  • An unbalanced mixer: in addition to producing a product signal, allows both input signals to pass through and appear as components in the output. This can be constructed using a single diode.
  • A single balanced mixer: is arranged with one of its inputs applied to a balanced differential circuit so that either the local oscillator (LO) or signal input (RF) is suppressed at the output, but not both.
  • A double balanced mixer: has both its inputs applied to differential circuits, so that neither of the input signals and only the product signal appears at the output. Double balanced mixers are more complex and require higher drive levels than unbalanced and single balanced designs.
  • CMOS switching mixers: are closer to the concept of the ideal frequency mixer. These are used for both Double Sideband / Direct conversion (simple type), and for quadrature sampling detectors (complex type).

A DOUBLY BALANCED MIXER#

double balanced

As with all mixers presented here, this radio frequency mixer has a Local Oscillator running at 7 MHz.

This is a doubly balanced mixer, which uses two trifilar wound transformers. It also uses 1N4148 standard signal diodes, although you could use BAT45 Schottky diodes.

Because it is doubly balanced, there is a supposed to be a high rejection of the local oscillator appearing at the mixed output. So, the idea of this mixer is to produce only the sum and difference at the output. This circuit can be simulated using LTspice or another simulator. However, simulation has its limits. Eventually, it is crucial to build the device and test it under laboratory conditions using the necessary test equipment.

  1. Oscilloscope
  2. Signal generator with at least two outputs
  3. Spectrum analyser — preferably with IP3 intermodulation test modes

Note: It is a myth that a mixer has a natural Z or impedance of 50 ohms on all ports. The actual effect is that the impedance is determined by what is presented to these ports. In terms of power reflections and transfer efficiency, it is evidently advantageous to ensure that all ports possess the same impedance.