Radar Receiver Mixers

Radar Receiver Mixers Many radar receivers do not use the RF amplifier stage; instead, they use a crystal mixer stage as the receiver front end. If an RF amplifier is used, the design is critical, because the weak signal levels may easily be masked by noise generated by components in the RF amplifier. LNAs have solved some of the noise problems, but they are not in wide use in radar applications. The simplest type of radar mixer is the single- ended, or unbalanced, crystal mixer. The mixer uses a tuned section of coaxial transmission line that is one-half wavelength long and matches the crystal to the signal echo and the local oscillator (LO) inputs. Local oscillator injection is accomplished by a probe, while the signal is injected by a slot in the coaxial assembly. This slot is normally inserted in the du- plexer waveguide assembly and properly oriented to provide coupling of the returned signal. In this appli- cation, the unwanted signals at the output of the mixer (the carrier, the local oscillator, and the sum of these two signals) are effectively eliminated by a resonant circuit tuned to the intermediate, or difference, fre- quency. One advantage of the unbalanced crystal mixer is its simplicity. It has, however, one major disadvantage —its inability to cancel LO noise. Since a klystron generates a high degree of noise, it makes it difficult to detect weak signals if that noise is allowed to pass through the mixer along with the signal. One type of mixer that cancels LO noise is the balanced hybrid mixer (sometimes called the magic T), shown in figure 2-14. In hybrid mixers, crystals are inserted directly into the waveguide one-quarter wavelength from their respective short-circuited waveguide ends. This is a point of maximum voltage along a tuned line. The crystals are also connected to a balanced transformer, the secondary of which is tuned to the desired IF. Figure 2-14.—Balanced hybrid crystal mixer. Since there is a difference in phase between echo signals applied across the two crystals, and because the signal applied to the crystals from the LO is in phase, there will be a condition when both signals ap- plied to crystal number 1 will be in phase, and the signals applied to crystal number 2 will be out of phase. This means that an IF signal of one polarity will be produced across crystal number 1, and an IF signal of the opposite polarity will be produced across crystal number 2. When these two signals are applied to the balanced output transformer, they will add. Outputs of the same polarity will cancel across the balanced transformer. It is this action that eliminates the LO noise. Noise components that are introduced from the LO are in phase across the crystals and are canceled in the balanced transformer. It is necessary that the RF ad- mittances of the crystals be nearly equal or the LO noise will not completely cancel. Only the noise pro- duced by the LO is canceled; noise arriving with the echo signal is not affected. 2-25