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The performance of an RF mixer can be a pivotal element in the overall operation of an RF circuit or system and selection of the correct mixer is key to the design.
Although many designs use small active mixers within the overall circuit, for many other designs high performance mixer modules are the answer. However for these the selection of the right one is particularly important. Under specify and the performance of the whole unit may be marginal. Over-specify and costs are increased. Select the wrong type and even though it is high performance type it will not work correctly.
Selecting the right RF mixer module is a key stage in the overall design. With many hundreds or thousands to choose from, and from a variety of manufacturers an ordered selection process is essential.
RF Mixer basicsMost of the mixers used within RF mixer modules are of the form of a double balanced passive mixer. These use a diode ring and matching transformers in a form of circuit shown below.
The three terminals are the RF, LO and IF.
- 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 a number of key decisions to be made. Considering these will help ensure that the optimum type of mixer is selected or chosen.
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: This type of mixer is probably the smallest type 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..
- 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 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 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 1dM 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 design, a power budget may be prepared showing the power levels t different stages. 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.
Although mixers tend to support wideband 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 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.