Radar systems use a wide variety of frequencies and signal shapes. In many environments, it is desired to be able to determine that a radar source is operational, and to identify the type of radar system which is transmitting energy into the environment. For example, aircraft traveling in a region may wish to be able to identify the type of radar systems operational in an area.
Radar detectors have been produced which are tuned for a specific, relatively narrow energy band. Other radar detectors can receive signals at a few discrete frequencies. For example, U.S. Pat. No. 5,606,736 to Hasler et al. discloses a receiver for receiving signals from both GPS satellites and GLOSNASS satellite, each of which transmits at a different frequency.
Commonly assigned U.S. Pat. No. 6,029,054 to Lemley, the disclosure of which is incorporated by reference herein in its entirety, discloses a receiver that is suitable for receiving energy over a band of frequencies from 0.5 GHz to 18 GHz and converts the received frequency to an intermediate frequency of either 160 MHz or 400 MHz. Lemley discloses using image rejection mixers and filters at the input to reduce image problems. Attention is also drawn to an example of a superheterodyne receiver discussed in Microwave Receivers with Electronic Warfare Applications, James Bao-Yen Tsui, 1992, which discloses using filters before the first mixer stages to reduce image problems.