RF Mixers Information
RF mixers convert RF power at one frequency into power at another frequency to make signal processing easier and more efficient. The frequency that is to be shifted is applied at the RF input and the frequency shifting power or voltage (from a local oscillator, LO) is applied to the RF mixers LO port, resulting in two outputs at the mixer Intermediate Frequency (IF) port. For a given RF signal, an ideal mixer would produce only two IF outputs: one at the frequency sum of the RF and LO, and another at the frequency difference between the RF and LO. Filtering can be used to select the desired IF output and reject the unwanted one, which is generally called the IF image. Configurations available for RF mixers can be surface mount, flat pack, through hole or plug-in, connectorized, and waveguide assembly.
Important performance specifications to consider when searching for RF mixers include RF frequency range, LO frequency range, IF frequency range, conversion loss, and LO power. RF frequency range is the range of input (RF) frequencies that the mixer was designed to work with. A LO (Local Oscillator) is an oscillator used in superheterodyne receivers which when mixed with an incoming signal results in a sum or difference frequency equal to the intermediate frequency of the receiver. IF (Intermediate Frequency) is the frequency to which all selected signals are converted for additional amplification, filtering, and direction. Conversion loss is defined as the loss of power between the input RF signal and the output IF signal during the frequency translation. It is calculated as the ratio (in dB) of the IF output power to the RF input power. It is a measure of the efficiency of the mixer in providing frequency translation between the input RF signal and the output IF signal. All conversion loss measurements are normally based on the mixer being installed in a 50 Ohms system and a stated LO signal power specified according with the appropriate mixer type. LO power is the power level (in dBm) that must be supplied by the local oscillator to properly drive the mixer diodes. The LO power is a very important factor in determining the values of IP3, 1 dB Compression Point and Dynamic Range of a mixer.
Isolation is also important to consider when specifying RF mixers. Isolation is a measure of the circuit balance within the mixer. It is the level of attenuation of a signal injected into a specific port when measured at a different port. When the isolation is high, the amount of "leakage" or "feed thru" between the mixer ports will be very small. Isolation is formally defined as the ratio (in dB) of the power level applied at one port of the mixer to the resulting power level at the same frequency appearing at another port. LO isolation is the isolation from the LO port to the RF port. It is the degree of attenuation of the LO signal measured at the RF port with the IF port properly terminated. LO-IF isolation is the isolation from the LO port to the IF port. It is the degree of attenuation of the LO signal measured at the IF port with the RF port properly terminated. Common mixer types available include single balanced mixer, double balanced mixer, triple balanced mixer, quadrature IF mixer, and image reject mixer. An important environmental parameter to consider when searching for RF mixers includes the operating temperature.