Double balanced mixer tutorial

- overview, tutorial and information about balanced mixers, and how the balanced mixer and double balanced mixer can be used.

Double balanced mixers are able to provide very high levels of performance for many RF design and radio communications applications. They can either be built during the RF design stage of a product, or they can be bought as modules to include in an RF circuit. Although costly some of these double balanced mixers can provide high levels of performance without the need for expending considerable amounts of development time in a specialised area of RF design.

Like all other RF mixers, double balanced mixers have the same three inputs. These are the RF input which is connected to the incoming signal that is to have its frequency converted. It is normally at a lower level than the second input. The second input is known as the local oscillator or LO input. This takes in the internal local oscillator signal that is used to convert the RF signal to the new frequency. The level of the LO input is normally relatively high when compared to the RF signal. The third port of the double balanced mixer is naturally the output. The signal on the output of an ideal RF mixer should contain only the mixer products, i.e. the sum and difference frequencies of the two input signals.

Circuit symbol for an RF mixer with the different ports identified

Need for balanced mixers

Many forms of mixer are not balanced and as a result they allow through considerable levels of the local oscillator and RF signals. These are normally not wanted and normally they would have to be removed by filtering which is often inconvenient and expensive. The solution is to balance the mixer to remove the input signals.

There are two types of RF mixer that are balanced:

• Single balanced mixer:   Often called just a balanced mixer, this type of mixer will suppress either the LO or RF signal but not both



• Double balanced mixer   Unlike the single balanced mixer, the double balanced mixer suppresses both of the input signals.

In most applications it is necessary to suppress both the RF and LO input signals and therefore double balanced mixers are more widely used of the two types.

Reversing switch mixers

Double balanced mixers are a form of what is termed a "reversing switch mixer." Reversing switch mixers operate by using electronic switches in a bridge formation to reverse the input RF signal under the action of the local oscillator used as a square wave switching signal. They normally offer significant advantages over analogue mixers for radio communications and general RF design applications as they are able to offer better levels of dynamic range and noise. In view of this fact, they are normally used in high performance applications where noise and dynamic range are of importance - e.g. in the front end of a radio receiver or spectrum analyzer.

Double balanced mixer basics

The most common form of double balanced mixer is the diode double balanced mixer. In its simplest form it consists of two unbalanced to balanced transformers and a diode ring consisting of four diodes as shown.

Basic diode double balanced mixer

Although the design of the RF mixer looks straightforward, high performance mixers are designed and built to exacting standards to achieve the high levels of performance needed.

One of the key specifications for a double balanced mixer is whether any of the LO or RF signals appear at the IF port. This depends upon the diode and transformer uniformity. In addition to this the circuit offers high isolation between the RF and IF ports because the balanced diode switching precludes direct connection between T1 and T2.

Double balanced mixer components

Although there are comparatively few components in a double balanced mixer, their individual performance is crucial to the performance of the RF mixer as a whole.

Normally Schottky barrier diodes are used for the diode ring. They offer a low on resistance and they also have a good high frequency response. Ordinary signal diodes may be used for low performance applications, although the cost difference is small. It is found that the diode forward voltage drop for the diodes determines the optimum local oscillator drive level. RF mixers requiring to handle a high RF input level will need a correspondingly high LO input level. As a rule of thumb the LO signal level should be a minimum of 20dB higher than either the RF or IF signals. This ensures that the LO signal rather than the RF or IF signals switch the RF mixer, and this is a key element in reducing intermodulation distortion, IMD, and also maximising the dynamic range.

To increase the required drive level, it is possible to place multiple diodes in each leg. The most common LO drive level for a double balanced mixer is probably +7dBm. However they can be obtained with a variety of drive levels. Values of 0, +3, +7, +10, +13, +17, +23, and +27 dBm are normally available.

The transformers are also critical to the performance of the RF mixer. The matching of the transformers and the individual legs are important in determining the balance of the RF mixer. The transformer also plays an important role in determining the conversion loss and drive level of the RF mixer. As the transformers are wound on a ferrite core, the core loss, copper loss and impedance mismatch all contribute to the transformer losses.

Practical aspects

Using double balanced mixer units is comparatively simple, and if a few precautions are observed they will provide excellent performance and reliable service. However a few precautions should be observed to ensure the optimum performance.

• Use the right drive level   In order to ensure the correct operation of the mixer it is necessary to ensure that the correct specified drive level is used. In this way the diodes in the RF mixer will switch correctly.



• Choose the right level mixer for the RF design   In a similar vein to using the specified drive level, the particular RF mixer should be chosen so that the drive level is sufficiently high for the particular RF design. Normally the LO drive should be at least 20 dB higher than the highest expected RF or IF signal anticipated. This will ensure the optimum IMD and dynamic range.



• Ensure the ports are accurately matched   Diode double balanced mixers are termination impedance sensitive. They must be terminated with the correct resistive load or source impedance (normally 50 ohms). A wideband resistive output is particularly important if it is to achieve the highest dynamic range. This can be achieved by using an attenuator pad in the line. Although this can be used for the LO port, this approach is not normally suitable for the RF and IF ports as it would impair the noise figure. Instead accurate matching of the amplifier stages preceding and following the mixer is one solution.

Summary

Double balanced mixers are particularly useful RF components that can be used for many RF design applications. Although the manufactured items may appear expensive, their use saves considerable sums be avoiding specialised development of a high performance circuit element.