RF Mixer

RF mixers are important components for RF design. The correct operation of an RF mixer is essential to any RF design, and part of this process is to ensure that the correct RF mixer specification is generated to choose the correct component for the particular RF design or circuit. In view of the high levels of performance required in some RF circuits and RF designs, it is often appropriate to buy RF mixers as components from specialist suppliers. These RF mixers are able to provide very high levels of performance and normally better than those that could be obtained by using discrete components on a circuit board.

When determining the correct RF mixer for a particular RF design or circuit, there are a few key RF mixer specifications that need to be known. Some are easy to define, but others may need a little more knowledge of the particular circuit of RF design being undertaken.

RF mixer connections or ports

When specifying RF mixers, reference is often made to the three ports of the mixer. There are two input ports and one output port. These ports are all identified separately as each one has different characteristics. The signal input is often designated "RF" or "RF input". The other input is typically connected to the local oscillator and is normally termed "LO". The output is normally designated "IF" for intermediate frequency.

Circuit symbol for an RF mixer with the different ports identified

Key RF mixer specifications

Although many elements of the performance of an RF mixer, there are a number that are of key importance, and they are required for the basic specification of the mixer.

• Mixer type
• Frequency range
• Impedance
• Input levels
• Conversion loss / gain
• Isolation
• Noise figure
• Spurious outputs

RF mixer type

There is a wide variety of different RF design or circuit configurations for mixers. Typically the types that can be bought as circuit modules or items are of the double balanced diode ring variety. However it is also possible to design single diode mixers, balanced diode mixers, bipolar mixers, FET mixers, etc. As diode mixers do not have any gain, this must be acceptable for the circuit in question.

Frequency range

No single RF mixer will be able to operate at all frequencies. The circuit construction of components will determine the range over which the RF mixer can operate. Typically the double balanced mixers bought as circuit components contain transformers and these are the main frequency limiting element. Despite these frequency limitations, RF mixers are normally able to operate over considerable frequency ranges. However the frequency range must be considered when ordering an RF mixer.

RF mixer impedance

The standard impedance for RF mixers is 50 ohms. It is important to ensure that the source impedance for the inputs, and the load for the output is accurately matched to the required impedance. Often small attenuators may be added into the line to ensure that this is the case. If the ports are not accurately matched to the required impedance, mixer specifications such as the spurious signal and isolation will be impaired.

Input levels

It is important to ensure that the input levels for the RF mixer are met. The RF input will have a maximum input level it can tolerate. Beyond this the mixer may become overloaded and the levels of spurious signals will rise above their limits, along with the isolation falling outside its specification.

The LO input is designed to have a certain input level and this should be maintained reasonably accurately. If it rises too high then higher levels of LO signal will appear at the output. Additionally higher levels of spurious may be experienced. If the signal level is too low, then the conversion loss may increase. Typically a tolerance of + / - 3dB is normally acceptable.

There are a few standard LO input levels. 7 dB is standard for many RF mixing applications, whereas higher LO levels may be required where higher elvels of RF input level may be needed.

Mixer conversion loss

The conversion loss of an RF mixer is a measure of its efficiency. The RF mixer conversion loss is defined as the ratio of the level of one of the output sidebands to the level of the RF input. This ratio is expressed in dB.

As only half of the input power can ever exist in one of the sidebands, this means that the best conversion loss that can ever exist in a diode mixer is 3dB. However other losses are also present in the mixer. These occur as a result of diode insertion loss, spurious signal generation, transformer core loss, etc. As would be expected, the conversion loss of the RF mixer will deteriorate towards the edges of the specified frequency band, mainly as a result of the transformer losses increasing.

The conversion loss is also found to be a function of the carrier drive level, increasing particularly if the LO drive level is not correct. However it is also found that the conversion loss can be improved by providing a short circuit impedance at the output port for the undesired sideband. It is also necessary to provide external decoupling for the IF and RF ports of a single balanced mixer in order to achieve the specified conversion loss.

Isolation

The port to port isolation is one of the parameters of an RF mixer that is of particular importance in most RF mixer applications. RF mixer isolation is defined as the ratio of the signal power available into one port of the mixer to the measured power level of that signal at one of the other mixer ports in a 50 ohm system. This is measured in dB.

Most RF mixer designs aim for the maximum isolation from the LO to the RF ports as the LO signal is normally the highest level. The LO to RF isolation is generally slightly less at the higher frequencies, and this is often important to ensure that the LO signal does not enter the RF drive circuitry and cause problems such as intermodulation. Lastly the RF to IF isolation is the poorest.

The in a diode balanced or double balanced RF mixer, the isolation is a determined by the equality of the diode dynamic characteristics and the accuracy of the transformer's balance. It is also found that a high level of second harmonic in the LO signal will also degrade the isolation performance because the isolation deteriorates at high frequencies as a result of parasitic capacitances.

Noise figure

The noise figure for an RF mixer is important in radio receiver front end circuits. Any noise introduced by the RF mixer at an early stage in the receiver such as the first mixer will degrade the performance of the whole receiver. In this way the noise figure for the RF mixer is important. Above very low frequencies (typically around 10 kHz) the Schottky Barrier diodes used in balanced and double balanced diode mixers contribute negligible levels of noise. This means that the noise figure for the RF mixer is essentially its conversion loss.

Spurious signals

The basic mathematics used to illustrate the way in which an RF mixer works assumes that it is a perfect multiplier. Unfortunately this is not the case, and signals in addition to the sum and difference frequencies are generated. The transfer characteristics for the diodes mean that harmonics for the input signals are generated. If the RF mixer was perfectly balanced, then these components would be cancelled out, but again this is not the case, and as a result, the harmonics of the two input signals also mix together. The lower odd ordered mixer products will be the highest level at the IF port or output of the RF mixer.

The levels of the spurious signals are dependent upon a number of factors. These include the LO and RF input levels as well as parameters such as the load impedance, temperature, frequency, etc. Many of these unwanted products will fall outside the required passband of the mixer, but most manufactured RF mixers will have data for these unwanted products which can be obtained from the manufacturer.

Summary

RF mixers are an important element in most RF designs. Accordingly it is necessary to understand their limitations and know how to specify them to obtain the required performance.

Luiggi Escalante

CI 18.878.611

CRF

Fuente: http://www.radio-electronics.com/info/rf-technology-design/mixers/rf-mixers-specifications.php

---