RF and microwave mixer is a 3 port device used to mix two RF signals, which are applied to the input ports of the mixer (port 1 & 2), and generate new RF signals at the 3rd port at the frequency, which is the sum or difference of the provided RF signals to the mixer.
In RF mixers along with the required signals, other RF frequencies may also get generated, which will be the combination of harmonics of the signal feed to the mixer.
RF Mixers are widely used in RF circuits to shift signals from one frequency to another, this process is known as heterodyning, for the further processing of the received signal or the transmission of the processed signal. RF and microwave mixers are primarily used to modulate the original signal with the carrier signal in radio communication transmitters.
Intermediate frequency (IF) signals in the range of 70 MHz will Mix with a high-frequency signal like an S-Band signal to generate an output signal in the range of S-band, and this signal can be transmitted to a long distance without any loss.
In this RF mixer article, we will cover,
RF mixers or Microwave frequency mixers are used in all aspects of RF circuit design for Radio communication, Receivers, and Transmitters to radar systems. In fact, RF mixers are used anywhere that RF signals are used.
An RF is a passive component or active device, that converts an RF signal from one frequency to another, without varying the signal properties for both modulated or demodulated RF signals.
RF mixer will have basically 3 ports ( 2 input and 1 output port), one port is to feed the input signal ( IF or RF) and the second port is the feed the other signal from the local oscillator. In the third port, we will get the combined or the difference of 2 input frequencies. Users can select the suitable required frequency by adding a filter circuit at the output port of the RF mixer.
RF mixers can be designed using passive components for mixing two signals and generating the third signal, which is either the sum frequency of the input signals or the difference of it, and if we need to improve the signal strength, active components can be used.
Active RF mixers offer better isolation between the input ports, but they may cause to produce a higher noise in the circuit. Even though active RF mixers consume more power, they are less tolerant of overload.
The essential characteristic of an RF mixer is that it produces an output signal, with a non-linear device, which is the product of the two input signals. RF Mixers are developed using discrete components, either as a part of integrated circuits or in a module package.
RF Mixers can be classified into 3 types on the base of the topology. The selection of an RF mixer topology based on the particular application.
In unbalanced RF mixed, both the input signals to the RF mixer will pass through it and appear as components in the output, in addition to the mixed signal. This type of RF mixer is easy to design and very commonly available.
In a single balanced RF mixer, either the local oscillator (LO) signal or the original input RF signal will appear in the output by suppressing other signals at the output, but not both. This is achieved by applying one of the inputs signals to the balanced/differential circuit.
In a double-balanced RF mixer, only the product signal appears at the output of the mixer. Suppression of both the input signal is achieved by feeding both the signal to differential circuits of the mixer. Double balanced RF mixers circuits are more complex than unbalanced and single balanced designs and require higher drive levels.
Triple Balanced RF Mixers is a type of double-balanced mixer with 2 double-balanced mixers connected together, it is also called Double Doubly-Balanced Mixers. These RF mixers will have three separate baluns for RF, Local oscillator, and IF ports. The triple balanced mixer offers good isolation between LO and RF, LO and IF, and RF and IF ports, in addition to the better return loss, and IP3.
All RF mixer circuit characterizations include conversion gain/loss and noise figure, it will vary for each of the topologies of the above filters.
RF Mixers are used in a variety of applications in the field of RF and microwave communication systems in military radar, SATCOM systems, cellular base stations, and more.
A superheterodyne RF receiver is a type of radio receiver that uses an RF mixer to convert an applied received signal to an intermediate frequency signal ( IF), for the convenient additional process of the applied signal. In the superheterodyne receiver, an RF mixer is also used to modulate the processed IF signal to the carrier for radio signal transmission.
In the superheterodyne receiver, the RF signal is converted to IF before processing because it is easier for a designer to make sharp cut-off filters at lower fixed frequencies and it will help to improve the signal quality.
RF mixer can be used as a downconverter if it is part of a receiver chain and used as an upconverter if it is used as a part of a transmitter chain.
Depending upon the circuit on which the RF mixer is used, the input signal can be fed either from the IF port or to the RF port. LO port will be driven with either a sinusoidal continuous wave or a square signal wave.
In an RF receiver, if the LO frequency is less than the RF frequency, it is called a low-side injection and for the high-side injection, the LO frequency will be above the RF frequency.
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Selection of an RF mixer based on the properties such as operating frequency, conversion gain, and noise figure. Below are the few factors which customers need to consider in the selection process of the RF mixer for any particular application. A wide specification range of duplexers is available from different manufacturers. The designer needs to select an RF mixer as per the RF design requirement of the system.
All the ports of the RF mixer need to meet the frequency operation requirement of RF signal, IF signal, and output signal. The Narrowband of frequency covering for all the ports will be more suitable for the operation.
Input intercept point (IIP3) of RF mixer, is the RF input power at which the output power of the unwanted intermodulation output signal and the desired IF output signal become equal. The high IP3 value for an RF mixer specifies that the mixer will have better linearity.
Isolation is defined as the amount of power leakage from one port of the mixer to another. When the isolation of the RF mixer is higher, the signal leakage between the ports will be smaller. It is ideal to select a mixer that offers higher isolation among the ports.
Spurious of the RF mixer is the undesired frequencies generated inside the mixer that may fall into the IF-band. Lower the spurious for the mixer shows the better specification.
Power handling of all the ports should be higher than the expected power applied to it for safe operation. The dynamic range of the RF mixer is the signal power range over which an RF mixer generates the desired output signal. Need to select a mixer which have the dynamic range of the LO and RF signal for down-conversion and LO and IF range for the upconversion.
For the passive mixers, signal strength is degraded inside the mixer, it is called the conversion loss. For the active mixer, it will generate the gain in the signal output, it is called the conversion gain. Conversion gain or loss is defined as the ratio of the IF output power of the RF mixer to the RF input power.
The typical conversion loss range for an RF mixer is between 4.5 to 9 dB. RF mixer with lower insertion loss within the frequency bands operation needs to select.
Designers need to select an RF mixer that can offer higher return loss, in all the ports to avoid the RF reflection back to the system.
Noise figure (NF) of the RF mixer is defined as the added noise generated within the mixer. RF mixer with lower value needs to select for the design.
The RF designer needs to select a mixer that matches the load and source impedance to avoid any internal reflection of the circuit. Impedance mismatch will cause a reduction in the output signal quality.
RF mixers are available with different types of connectors like SMA, N-type, 3.92 type, etc, and also with the SMD and waveguide packages. Designers can select a suitable mixer on the basis of required connector options.
An RF mixer is a passive or active device that enables to mix of different signals to generate a third signal in the sum or difference frequency.
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