RF Mixer basics tutorial
This page of RF tutorial section covers RF mixer tutorial.It covers RF mixer basics, its use in rf circuit design, terminologies, RF mixer types etc.
The RF mixer has 3 ports RF, IF and LO. It has two input ports and one output port. For Up frequency conversion, IF and LO are used as input ports and output will be available at RF port. For down frequency conversion, RF and LO are used as input ports and output will be available at IF port.
Actually it is a non linear device and used for shifting signal from one frequency to the other point in the spectrum. It is considered a linear device because it keeps the properties of input signal intact while doing frequency translation.
Spectral components of the RF mixer are mentioned in the figure-1. As shown it produces +/-m(Input1)+/-n(Input2) where in Input1 and Input2 are input frequencies. m and n range from 0,1,2,3,..and so on. Figure-1 mentions upto third harmonics of inputs.
RF mixer device is used in up conversion and down conversion modules of RF Transceiver. It converts RF to lower IF frequency or to baseband to make signal processing easier in the RF receivers. This frequency translation process is known as down conversion.
It converts lower IF frequency or baseband frequency to a higher IF or RF frequency to provide efficient power transmission in the RF transmitter part. This frequency translation process is known as up conversion.
If LO frequency is lower than RF frequency, then image frequency will be lower than LO. Here image frequency will exist at (fLO-fIF) in frequency spectrum. This is referred as low-side LO system. When LO frequency is higher than RF frequency, then image fequency will be higher than both LO and RF. Here image frequency will exist at (fLO+fIF) in frequency spectrum. This is referred as high-side LO system.
In down-convert mixers, image frequency will be converted directly to IF position along with IF frequency itself. In up-convert mixers, image frequency will be unwanted sideband, which has amplitude at level same as wanted desired signal. This image frequency need to be filtered out in up conversion. There are specially designed image reject mixers which does the function of removal of these image frequencies.
RF Mixer Terminologies
Let us understand RF Mixer terminologies. These terminologies are the mixer specifications used while selecting the mixer
for rf circuit design as described above.
Frequency Range: These have been specified for all the ports of the RF mixer device. It include RF, LO and IF ports. This is the range for which mixer has been designed to provide optimum performance as desired.
Power Level: This is the power level of the signal to be fed at the mixer ports. usually LO power level is specified. This is required as part of rule of thumb for using mixers in Rf circuit design. LO power level should be 15-20 dB higher than RF Power for optimum performance. This help us to decide power level of other input port(either RF or IF) compare to LO input port.
Conversion Loss: It is the ratio of output signal to the input signal. In other words, conversion loss is the difference between input RF power and output power level. For example, if we are converting from IF to RF frequency then,
Conversion Loss in dB = Power at output RF frequency in dBm - Power at input IF frequency in dBm
For PRF = -10 dBm and PIF = -17 dBm
conversion loss of RF mixer = 7 dB
Typical values of conversion loss is between 5 to 10 dB.
1dB compression: As we know under normal mixer working, conversion loss will be constant irrespective of input power. If input power increases by 1dB, output power will accordingly increase by 1 dB. However when input power becomes too large in magnitude, 1dB to 1dB relation ship will not exist and output vary in nonlinear relation with the input.
It is defined as input power needed to increase conversion loss by value of 1dB from ideal. 1 dB compression point is considered as a measure of linearity of RF mixer. While designing power levels in the rf circuit line up, output power should be less than output 1dB compression point of mixer device to certain considerable amount. If this is not taken care, it will drive the device into saturation.
Isolation: It is measure of amount of power which leaks from one mixer port to the another. The desired port isolation is schieved using mixer balance circuit and use of hybrid junctions at appropriate places. Usually LO/RF or LO to RF isolation is specified. Typical values of isolation are between 15-25dB.
Practically, there will be some amount of power leakage exists at LO, RF and IF ports.
Isolation is measured as difference in input power signal and leakage power available at other ports. For example, if we feed input signal at LO port and obtain output at RF port, then isolation(dB) between these two ports is expressed as:
Isolation between LO and RF port = PinLO-PoutRF
The isolation between ports is reciprocal and hence it is measured only in one direction. Mixer Isolation from port 1 to port 2 is same as from port 2 to port 1. The three types of mixer isolation is specified as L-I isolation, L-R isolation and R-I isolation.
VSWR: If mixer VSWR is perfect there will be minimum reflections. Though VSWR is least concern in the rf circuit design, it is used as matching parameter for circuits at the three ports of the mixer device.
Intermodulation distortion (TOI or Input and output IP3): Refer second order and third order intercept points and TOI basics for more information.
Noise Figure: It is the ratio of SNR at input to the SNR at output. It is approximated by value of mixer conversion loss. For low power applications, connversion loss and noise figure should be of low values.
Image rejection: For I/Q mixers and image reject mixers this rejection value is specified in the mixer data sheet.
RF Mixer Types
As mentioned depending upon devices and configurations there are various RF mixer types. The two main types are passive(using diode) and active(using BJTs or FETs). Further based on configurations and number of devices in use there are different types. They are unbalanced, single balanced, double balanced, triple balanced etc.
RF Mixer types page which covers difference between Single balanced, double balanced and doubly double balanced mixer types.
This RF Mixer tutorial is very useful to understand the RF mixer basics for beginners as well as experts in using mixers for Microwave transceiver circuit design.
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