This chapter is the introduction to the book 'Microwave Receivers with Electronic Warfare Applications' and covers the following: historical reviews; basic units of an electronic warfare receiving system; classification of electronic warfare receivers through frequency range; classification of electronic warfare receivers through applications; classification of receivers through structures; and future trend of electronic warfare receivers. It also provides an overview on the organization of the book.

This chapter discusses the quantities that an EW microwave receiver measures. The digital words a receiver generates and passes to a digital processor usually include five quantities. They are frequency, pulse amplitude (PA), pulse width (PW), time of arrival (TOA), and angle of arrival (AOA), as shown in the paper. Except for the frequency measurements, all these quantities are measured with similar schemes, even in different kinds of receivers. Therefore, they will be discussed in this chapter. The frequency information is very important for both sorting and jamming. By comparing the frequency of the pulses received, pulse trains of various radars can be sorted out. Knowing the frequency of the victim radar, the jammer can concentrate its energy in the desired frequency range. The theoretical aspect of frequency measurements will be discussed here; the actual measurement schemes will be discussed separately, in the chapters devoted to the particular receiver.

This chapter discusses superheterodyne and homodyne receivers for electronic warfare applications. It covers the following: intermodulation generated in a mixer; preselector (tracking RF filters); YIG filters; RF amplifiers in front of mixer; logarithmic amplifiers; operating principle and characteristics of mixers; single-diode mixers; single balanced mixers; double balanced mixers; image rejection mixers; image-enhanced mixers; harmonically pumped mixers; stability of oscillators; YIG-tuned oscillators; voltage-controlled oscillators; oscillators with phase-locked loops; direct frequency synthesizers; and crystal bandpass filters.

A channelized receiver can be considered to be many fix tuned superheterodyne receivers operating in parallel. It has all the good characteristics of a superhet receiver: high sensitivity, wide dynamic range, and fine-frequency resolution. The bandwidth of the receiver is proportional to the amount of hardware used in the receiver. The more channels built, the more bandwidth the receiver can cover. But the large number of parallel channels also make the receiver bulky and expensive. The most critical portion of a channelized receiver is to determine which slot contains the input signal. Although several approaches have been presented in this chapter, a thoroughly theoretical study may improve the frequency-determining scheme. Using SAW filters in a channelized receiver may reduce the size and possibly the cost, but the SAW filters also seem to degrade the performance of the receiver. Further research in filter technology is required to improve receiver performance. Finally, reducing the receiver size is another research topic to be investigated.

A Bragg cell receiver has the potential of being used as an EW receiver. It can cover a wide instantaneous bandwidth of possibly over 1 GHz, and the frequency resolution can be very fine. However, to process short pulses, the frequency resolution is governed by the minimum PW of the input signals rather than by the performance of the Bragg cell. At present, the sensitivity and dynamic range of the Bragg cell receiver is limited by the photodetectors. With present detector technology, the dynamic range is rather low for EW receiver applications. Theoretically, the dynamic range of the Bragg cell receiver can be improved with the interferometric approach. However, further development is required to demonstrate the feasibility. A Bragg cell receiver can be considered a channelized receiver where the channelization is accomplished through optical means. One Bragg cell is equivalent to several hundred filters connected in parallel. In a power Bragg cell receiver, RF amplifiers cannot be added after the frequency channelization to improve the sensitivity, but light amplification may be added in front of the photodetectors to accomplish this. In an interferometric Bragg cell receiver, RF amplifiers can be added after channelization to improve the sensitivity of the receiver. Therefore, the interferometric Bragg cell receiver is very much like the channelized receiver discussed in Chapter 7. A Bragg cell receiver has potential performance characteristics that are very desirable for EW applications. Although the feasibility of the receiver has been demonstrated, extensive research and development, especially in the digitizing circuits, is critically needed to realize its full capability. To take the advantage of the small size of the optical bench, the digitizing circuits must be made small in size using VLSI circuit technology.

Advances in millimeter wave components have revived research and development in EHF receivers. There are presently two general approaches to fabricate EHF receivers: the crystal video receiver and wide-band channelization. In both approaches, the small size of the receiver is very much emphasized. The miniaturization includes not only the receivers but also the antennas. The advances in MMIC will have a big impact on EHF receiver development. In the future, EHF receivers may be made in integrated circuit forms.