18-05-2012, 11:24 AM
RF Mixers
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RF Mixers are 3-port active or passive devices. They are designed to yield both, a sum and a difference frequency at a single output port when two distinct input frequencies are inserted into the other two ports.
In addition to this, a Mixer can be used as a phase detector or as a demodulator.
The two signals inserted into the two input ports are usually the Local Oscillator signal, and the incoming (for a receiver) or outgoing (for a transmitter) signal.
To produce a new frequency (or new frequencies) requires a nonlinear device.
In a mixing process if we want to produce an output frequency that is lower than the input signal frequency, then it is called down-conversion and if we want to produce an output signal that is at a higher frequency than the input signal, it is referred to as up-conversion.
Important Mixer properties are:
Conversion Gain or Loss, Intercept point, Isolation, Noise Figure, High-order spurious response rejection and Image noise suppression.
1. Conversion Gain or Loss of the RF Mixer is dependent by the type of the mixer (active or passive), but is also dependent by the load of the input RF circuit as well the output impedance at the RF port. Also is dependent by the level of the LO.
The typical conversion gain of an active Mixer is approximately +10dB when the conversion loss of a typical diode mixer is approximately -6dB.
The Conversion Gain or Loss of the RF Mixer measured in dB is given by:
Conversion[dB] = Output IF power delivered to the load[dBm] – Available RF input signal power[dBm]
2. Input Intercept Point (IIP3) is the RF input power at which the output power levels of the unwanted intermodulation products and the desired IF output would be equal.
• From an RF System point of view, a Mixer linearity is more critical than Noise Figure.
The Third-Order intercept point (IP3) in a Mixer is defined by the extrapolated intersection of the primary IF response with the two-tone third-order intermodulation IF product that results when two RF signals are applied to the RF port of the Mixer.