Understanding the Single Balanced Mixer

The single balanced mixer is often used to provide an improved level of performance over an unbalanced mixer where some suppression of the input signals is required.

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The single balanced mixer is the simplest of the balanced mixer and it is used where some suppression of an input signal is required.

The essence of a single balanced mixer is that one of its inputs is applied to a balanced or differential circuit. This means that either the local oscillator, LO or RF signal input, RF is suppressed at the output, but not both.

This suppression is not achieved by filters, but rather by cancelling out the relevant input signal using the balanced circuit.

Another advantage of a single balanced mixer over a single diode unbalanced mixer is that they create 50% fewer intermodulation products.

Most modern single balanced mixers use a 180° hybrid coupler, typically a centre tapped RF transformer and then two diodes, although other active devices like FETs could be used as the switching elements.

Balanced operation can also be achieved using balanced transistor or FET configurations. These are typically contained within integrated circuits where high levels of performance can be achieved.

Single balanced mixer using basic transformer

It is perfectly possible to design and build a basic single balanced mixer using a basic transformer, two diodes and a couple of other components.

The basic circuit is given below and it can be seen that it is quite straightforward.

This type of balanced mixer was widely used within circuit like those of single sideband transmitters in the 70s and 80s to generate the double sideband signal before this was filtered to provide single sideband.

it can be seen that the basic circuit is based around two diodes which are driven using a transformer from the carrier input - this was typically a crystal oscillator.

Audio was provided through the capacitor C2 to provide DC isolation, although there is a DC path to ground provided by R1.

The output from the mixer is passed to the IF transformer T2 to be filtered and amplified to give the signal sideband signal for further amplification and conversion to the required final frequency.

To get the optimum balance the diodes can be matched as far as possible - this may not be particularly easy, but having the same type and if any matching of the curve can be done then this might benefit the operation.

VR1 along with VC1 can be adjusted to provide the optimum carrier suppression - if necessary C4 and VC1 can be connected to the other side of the 'bridge' to provide the best balance and carrier suppression.

Single balanced mixer with 180° hybrid

Single-balanced mixers using a 180° hybrid are now the preferred form of single balanced mixer. They consist of just a few components as seen in the basic diagram below.

Both 90° and 180° hybrids can be used to for single-balanced mixers, but the 180° hybrid’s input ports are mutually isolated, enabling the LO and RF ports to be isolated from each other.

This prevents the LO signal from getting into the RF section and giving rise to additional intermodulation, etc and if used at the input of a receiver for example, it will reduce any radiation of the local oscillator signal.

In this circuit, the local oscillator is balanced and drives the diodes. he diodes should ideally be Schottky diodes because of their low forward on voltage and this reduces the requirement for higher LO drive and hence higher LO breakthrough.

The RF signal is suppressed from the IF output by a capacitor to ground as well as the series inductor. A more sophisticated low pass filter could be included here if needed.

It is found that at higher local oscillator injection levels, the diodes can self-bias. This results in a high conversion loss and isolation. To avoid this, the RF chokes connected from the input to the diodes from the hybrid to ground are included.

Single balanced mixers are often used where the highest performance levels are not needed and cost may be an issue. They provide good suppression of the local oscillator and as such they are ideal for many circuits.