Understanding RF Mixer Specifications: How to specify them

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The performance of an RF mixer can be a pivotal element in the overall operation of an RF circuit design or system: selection of the correct mixer is key to the design.

Although many designs use small active mixers within the overall circuit made from discrete electronic components, for many other designs high performance mixer modules or integrated circuits are the answer.

Whether a mixer designed from discrete components, or a module or integrated circuit that is bought in, the specification and performance of the mixer is key.

Under specify the mixer and the performance of the whole RF circuit design may be compromised. Over-specify and costs are increased. Select the wrong type and even though it is high performance electronic component it may not work in the way that is intended, or some element of its performance may be wrong.

Selecting the right RF mixer is a key stage in the overall RF circuit design. With many hundreds or thousands to choose from, and from a variety of manufacturers an ordered selection process is essential.

RF Mixer basics

When looking at RF mixers for any RF circuit design there are a number of definitions that will be of interest.

Aspects like the ports, form of mixer and the like all play an important part of the specification.

There are three ports to any RF mixer / frequency mixer:

• RF:   This is the input used for the signal whose frequency is to be changed. It is typically a low level signal.
• LO:   This is the local oscillator signal and is at the specified level, higher than that of the RF input.
• IF:   This is the output port for the mixer.

There are also various forms of RF mixer which need to be understood. One of the first relates to the type of electronic components or devices within the mixer:

• Passive mixers:   Passive mixers typically use passive electronic components in the form of diodes as the switching element. As a result they cannot exhibit any gain, but many forms can provide excellent levels of performance.

  Passive mixers mainly use Schottky diodes because of their low turn-on voltage, but they need balanced transformers for most high performance designs and this can limited the frequency band over which they can operate.

  One key aspect of passive mixers is that they introduce what is called a conversion loss, explained later, and this can have an impact on the RF circuit design.

• Active mixers:   As the name of the active RF mixer implies, it contains active electronic components like a bipolar transistor, FET or even a vacuum tube / thermionic valve. These types of RF mixer are able to provide gain as well as proving the multiplication or RF mixer capability.

  Unlike passive mixers, active mixers can actually have a conversion gain, and this will have an affect on the RF design for the item.

RF mixers or frequency mixers can also be categorised according to whether they are balanced or unbalanced. This is an important decision to make.

• Unbalanced:   An unbalanced RF mixer is a basic form of RF mixer and one in which it simply mixes the two signals together and the output consists of the sum and difference signals as well as significant levels of the original RF signal and that of the local oscillator. As there is little isolation between the ports this can lead to increased levels of intermodulation distortion as well as the local oscillator and RF signals being present on the output.

• Balanced:   A balanced mixer is one in which the ports have a balanced or differential structure. Dependent upon the actual type there can be isolation between the different ports, and the LO and RF can be suppressed at the IF port. There are different types of balanced mixer: single balanced; double balanced and triple balanced (more correctly termed a doubly double balanced mixer).

Selecting the right type of RF mixer to meet the requirements of the circuit design is one of the key choices to be made.

Mixer package type specification

This decision is one of the first that can be made. The connection technology and requirements will be known early in the design. There are generally three types of package type:

• Surface mount technology:   RF mixers using surface mount technology are probably the smallest types in terms of area and can be mounted directly onto a printed circuit board. These are ideal where the whole circuit or system is printed circuit board based. However it is necessary to be aware of any special soldering restrictions, especially in terms of the solder reflow temperature, etc..

• Leaded component:   Some RF mixers will be available in the traditional leaded styles. These are normally used for low volume, through hole mounting on printed circuit boards.

• Connector:   In some instances a connector-ised RF mixer will be required, Often these come with either BNC or SMA connections, but other connectors may be requested including N or TNC types, but these tend to be less common or they may need to be requested as special items.

  These mixers tend to be used in larger rack based systems. Consideration of the size and connector type is necessary when choosing these options. Consider also the way these mixers will be mechanically mounted because many mixer manufacturers offer various options for this.

• Plug-in:   These mixers are through-hole mounted units. They have at least four pins and this enables them to be securely connected both electrically and mechanically. These may be used on through hole printed circuit boards. Typically these mixers have at least four pins, one each for the three signal lines and one for earth, although many may provide an earth or ground connection with each signal port.

Mixer local oscillator level

The local oscillator or LO input level is another key parameter to be considered. It may be a key factor in determining which set of mixers, or the mixer itself.

The higher the local oscillator input level, the higher the RF level that can be accommodated without running into issues with distortion, etc. Typically the local oscillator input should be 10dB above the highest anticipated RF signal. This keeps the mixer running within its linear operating range.

Mixer modules tend to be specified at various common levels, e.g. 7dBm, 10 dBM, 17 dBm, etc. These are sometimes referred to as level 7, level 10 or level 17 mixers. Other values are available for these mixers dependent upon the application, but these levels possibly form the most widely used values.

Unfortunately the higher power mixers tend to be more expensive, and amplifying the LO to the higher level so there is often a trade-off between performance and cost. Keeping the lowest LO level will not only keep the cost down, but also result in lower LO leakage within the system as well.

It is best to drive these mixers at levels approximately equal to the required drive input. Higher than this will particularly result in greater levels of LO leakage and other performance parameters may fall off.

Lower than the required level, then the performance again falls, typically providing an increase in conversion loss. Running a mixer with the local oscillator at around -3dB of the required level may increase the conversion loss by 0.5dB or so. Also the third order intermodulation performance may be degraded slightly - which is hardly surprising since the diodes will not be switching as hard.

Mixer 1dB compression point specification

The 1dB compression point of a mixer is very important specification where spurious signals are concerned.

An ideal mixer would operate linearly, i.e. for every 1 dB increase in the RF input level, the output from the IF port would also increase. However a point is reached where the output cannot handle the signal, and it starts to level out. The 1 dB compression point, is the point at which the output deviates from the linear curve by 1 dB, i.e. it is 1 dB less than the plotted linear line. The specification normally refers to the RF input power level at which this compression occurs.

The 1 dB compression point is easy to measure and it provides a useful comparison between mixer to see what their high level performance is like. Obviously for high level signals the higher the 1 dB compression point the better.

The 1 dB compression point is linked in to other mixer parameters as well.

Maximum RF port power specification

In any RF circuit design, a power budget may be prepared showing the power levels at different stages of the circuit. Knowing how the power level varies, it is often possible to accurately determine the maximum power level entering the RF port of the mixer.

With a knowledge of this figure, selecting the required mixer is simply a case of choosing the mixer whose 1 dB compression point exceeds this value.

In terms of inputs where the signal levels varies over a very wide range, it is very important to ensure that the level does not exceed a safe value. This can be exemplified in that one of the major problem areas on some older spectrum analysers with no automatic input protection was the destruction of the input mixer when high level signals were applied when the engineer forgot to put an attenuator in circuit.

Conversion gain or loss

The conversion gain for a mixer is very important when undertaking the RF circuit design for a project as it will determine the signal levels after the RF mixer.

The conversion gain or loss is defined as the ratio of the desired output level to the RF input signal level.

It can be seen from this that the local oscillator level does not feature in this figure - the conversion gain is only interested in the levels of the wanted input and output signals.

As might be expected, passive mixers feature a conversion loss. Dependent upon the mixer in question this might be around 7dB or so, but it is very dependent upon the actual mixer itself.

Active RF mixers normally feature a conversion gain, and again the level is very dependent upon the actual mixer itself.

The main issue is that the level of the conversion gain, or loss is known so that the appropriate action can be taken in the earliest stages of the RF circuit design.

Frequency range

Although RF mixers tend to support wide-band operation, the actual frequency range to be used must obviously be covered by the mixer. Again if the mixer is over-specified in terms of either / both the bandwidth and top frequency, then costs may be more than they need to be.

Typically it is good practice for any RF circuit design is to select a mixer where the mid-band frequency range covers the intended operating range.

That said, the performance of many mixers extends outside their specified ranges, although with some increasing degree of degradation the further outside the operating range the frequency is.

Isolation

The isolation level between the ports is often important and it states the level of what may be termed the leakage between the different ports. The RF and local oscillator is not normally needed at the IF, and if, for example the local oscillator leaks through to the RF port, it could give rise to intermodulation distortion.

As might be expected the isolation is measured in terms of dB, comparing the signal entering one port, to the same signal level at the other port where it is not required.

It is found that mixer isolation tends to deteriorate with increasing frequency as the reactance of stray capacitance falls, and also the circuit imbalances become more apparent.

Third order intercept point, IP3 & third order intermodulation

One major issue with any RF mixer is the level of unwanted signals that are generated within the mixing process. Non-linearities within the mixer give rise to additional signals and these can cause issues in many ways dependent upon the circuit design or system in which they are used.

The third order intercept point of a mixer (or amplifier) is a hypothetical point where the power of the third order products will have the same power level as the fundamental.

The third order intercept point of a mixer of any other device is theoretical because it lies well beyond the saturation level of the device, and it many cases it would be well beyond the point at which damage occurred, especially in the case of a mixer.

The reason that the IP3 figure is useful is that it provides a very good guide or figure of merit for the distortion generated by the device as the power levels rise.

The IP3 point can be defined for either the input or output ports.

There are two main ways of defining the intercept points:

• Based upon the intermodulation products:   The most commonly used approach for determining the IP3 of the RF mixer. For this, the mixer is given two sine wave signals that have a small frequency difference.

  
  

  The intermodulation products then appear at spacing equal to the input tones, and the levels can be measured. The third order products appear at three times the frequency spacing of the two signals either side of them.

• Based upon harmonics:   An alternative method is to use a single signal, and then the products appear at multiples of the input tone. The third order product is at tree times the fundamental.

The input third order intercept point is often designated as IIP3 and the one of the output is designated OIP3. These intercept points differ in level by an amount equal to the small signal gain (or loss) of the mixer.

The selection of an RF mixer for any RF circuit design can have a major impact on the performance. Accordingly it is important to ensure that its performance meets the needs for the particular RF design in terms of its electrical performance, environmental specification, mechanical and connector parameters, as well as aspects such as whether it in a surface mount technology format for printed circuit board mounting and large scale production, etc.

Normally selecting the right mixer is a balance between aspects like electrical performance, mechanical aspects, and cost. This is not always easy, but by understanding the specifications and the impact they have on the overall performance of the RF circuit design, then the best compromise can be selected.

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