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A Single 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.

THE SINGLE BALANCED MIXER CIRCUIT#

bal-mod

CIRCUIT DESCRIPTION#

The circuit shown is a single balanced frequency mixer; it takes the form of two diodes and a trifilar wound transformer. The value of each turn is approximately 53 µH. At 7 MHz this presents a reactive load, or impedance, of approximately 2.3 K ohms.

This type of mixer is often used as a product detector in a receiver.

The diodes act as an imperfect switch. We can improve these imperfections, by using BAT45 Schottky Diodes.

These do not have a P/N junction. Instead, they have a Metal ⇾ P junction. This is much more homogeneous than two semiconductor junctions. This results in advantageous characteristics:

  1. Lower voltage drop.
  2. Lower capacitance.
  3. Faster switching times.

The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.

Note: in the circuit shown, we see the mixer used for transmission. However, as mentioned, it can equally be used as a product detector, and often this is the case.

Also note: that 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.