This page covers RF mixer basics which includes function, technical specifications,working of rf mixer and thumb rules in rf circuit design.It also provide link to rf mixer manufacturers.
RF mixer is often referred to as a frequency-translating device because of its purpose to perform either upconversion or downconversion of input signal. Because RF Mixer converts RF power at one frequency into power at another frequency to make signal processing easier and less expensive. Another reason of going for frequency conversion is to allow practical transmission of audio and other low frequency information through free space.
Acting as an upconverter,it will convert IF to RF and hence can be found in the transmit chain of a wireless device. Acting as a downconverter, it will convert RF to IF and is used in wireless receiver. RF mixer usually consists of single diode or multiple diodes or some other electronic device e.g. FET (Field Effect Transistor).
How RF mixer works
RF mixer is a three-port device. The three ports are usually denoted as radio frequency (RF), intermediate frequency (IF), and local oscillator
(LO). The RF and IF ports are bidirectional ports. Since a mixer has three ports, this means that it has nine S-parameters. Typically only five of these are tested
in practice as shown in the figure above.
Rf mixer is defined by following equation:
Fout= |nf1 +/- mf2|
where fout represents all output signals, f1 and f2 are the two input signals, n and m are the order of the harmonics, from zero (fundamental) to infinity.
Usually in all the mixers, second order products f1+f2 and f1-f2 will have the highest amplitudes
f1- f2 (actually: |f1- f2|)
2f1 and 2f2 are also second-order outputs. There are different types of mixers single ended,balanced(90 degree),balanced(180 degree), double balanced and image reject mixers.Practically mixers are of balanced type, in which even order harmonics will not be produced at the output due to the design. Hence one can find sum and difference of input frequences f1 and f2. 2f1 and 2f2 will not be produced. If there are no other outputs, if components are ideal (lossless) then the mixer circuit behaves as per equation below.
cos(w1)cos(w2) = [cos(w1 + w2)]/2 + [cos(w1 - w2)]/2
where cos(w1) and cos(w2) are the time domain representations of f1 and f2 input frequencies. The 1 by 2 factors indicates input amplitude is divided between two outputs. 1 by 2 factor represents 6 dB conversion loss of the mixer.
Following specifications need to be considered while purchasing rf mixer
Frequency ranges : the RF, LO, and IF ranges for which the mixer is designed.
Dynamic range: Input power range over which mixer is useful.
LO power level : the design or maximum LO power.
Conversion loss: the ratio of IF to RF power.
1-dB input compression level : the RF power at which the conversion loss increases by 1 dB.
IM intercept points: usually the IIP3.
Isolation: between the various ports, LO, RF, and IF; for example, how much is the LO power attenuated in getting to the IF/RF ports.
VSWR: at all the three ports RF,IF and LO.
Thumb Rules while using RF Mixer in the microwave circuit design
1.LO power should be at least 20dB higher than RF Power.
2.1dB compression point is 5 to 10 dB lower than LO Power.
3. Usually pads have been used at all the three ports of RF mixer. But When matching and intermodulation products are a problem use isolators in the place of pads. Refer our page on design of RF frequency converter to know the use of RF mixers in the microstrip based design.
RF mixers with high third order intercept point and higher LO/RF and LO/IF isolation are usually considered for the RF circuit design. These mixers provide best performance in the mixer based RF systems.
READ our page in the vendors section for rf mixer manufacturers or suppliers.
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