Passive radar receivers are widely used in electronic support and reconnaissance as well as in anti-radiation passive radar systems. A passive radar receiver is a key component in anti-radiation missiles. It captures and tracks emitters' signals, extracts and recognizes radar signal features, performs threat assessment, and reports detected signals' angular information so the missile can track the target in real time until impact. In wideband passive guidance systems, the main tasks of a passive radar digital receiver include the following.
Main tasks of a digital passive radar receiver
(1) Signal detection. Detect radar pulse signals in a wideband system, addressing challenges such as low signal-to-noise ratio detection and adaptive, dynamic-threshold detection suitable for varying signals and reception environments.
(2) Signal parameter measurement. Measure parameters for each radar pulse to form pulse descriptor words (PDW). Key parameters include carrier frequency (CF), direction of arrival (DOA), pulse width (PW), pulse repetition interval (PRI), pulse amplitude (PA), time of arrival (TOA), signal bandwidth (BW), start frequency, stop frequency, and so on. Modern receivers also face intrapulse parameter measurement challenges for LPI signals, such as modulation slope and relative coding forms.
(3) Signal identification. This divides into inter-pulse modulation identification and intrapulse modulation identification. Inter-pulse identification clusters PDWs to separate signals emitted by the same radar (emitter). Intrapulse identification analyzes individual pulses to identify intentional intrapulse modulations such as FM or phase modulation. With advancing technology and reconnaissance needs, unintentional modulation recognition for specific emitters has also become an important receiver capability.
(4) Signal tracking. Track signals of interest or high-threat signals and output azimuth and elevation information to provide real-time signal tracking for reconnaissance systems or seeker heads.
From analog to digital receivers
Early electronic systems were analog, so traditional passive radar receivers were built from analog components. Those receivers tended to be large, inflexible, power-hungry, and low in integration, with limited dynamic tuning and high initial cost. With technological progress and digital advances, the development of analog-to-digital converters (ADCs) and digital integrated circuits has driven receivers into the digital era. Digital receivers have become a major research focus for modern radar receivers.
Common receiver architectures
Traditional electronic warfare receivers were mainly analog, especially analog channelized receivers. Achieving wideband coverage required many separate receivers, and channel matching and sensitivity were often poor. Analog receivers can be categorized into six major classes by structure: crystal video receivers, superheterodyne receivers, instantaneous frequency measurement (IFM) receivers, channelized receivers, compression receivers, and Bragg receivers. In addition, microwave photonic receivers have emerged as an important class of receivers.
1. Crystal video receiver
The crystal video receiver is the simplest reconnaissance receiver. It can be as simple as a crystal detector diode and a video amplifier within a given frequency band. When a radar signal in that band exceeds a set threshold, the video amplifier output surpasses a prescribed voltage, indicating signal detection and performing detection functions.
Crystal video receivers have wide frequency coverage but relatively low sensitivity and small dynamic range, and they cannot handle simultaneous incoming signals.
2. Superheterodyne receiver
Superheterodyne receivers use a locally generated oscillator mixed with the input signal to convert the input frequency to a predetermined intermediate frequency. The superheterodyne architecture addresses the weak output and poor stability of high-frequency amplifier receivers, offering higher selectivity and better frequency characteristics with easier tuning.
Superheterodyne receivers generally offer high sensitivity, capable of better than -70 dBmW, and large dynamic range that allows reception of multiple signals simultaneously. However, their input bandwidth is typically narrow. A key characteristic is the design trade-off between receive sensitivity and coverage bandwidth, making them suitable for separating continuous-wave and narrowband signals.
3. Instantaneous frequency measurement (IFM) receiver
Instantaneous frequency measurement (IFM) is a frequency measurement technique based on phase comparison and is suitable for ELINT, radar warning, and related applications. The correlator is the core unit of an IFM receiver: a delayed version of the signal correlates with the input signal to determine the input frequency. IFM reconnaissance receivers are structurally simple, cover wide reconnaissance bands, and offer high resolution, making them widely used in various electronic warfare devices.
IFM receivers have very wide instantaneous bandwidths, covering up to 2–18 GHz, and can measure signal frequency in extremely short time intervals with high frequency resolution. They handle narrow pulses well; for a 0.1 μs pulse an IFM receiver can reach approximately 1 MHz precision. However, IFM can produce incorrect frequency information when multiple signals arrive simultaneously.
4. Channelized receiver
A channelized receiver divides the received signal bandwidth into multiple channels, typically using filter banks. Early channelized receivers were analog and required multiple analog bandpass filters to form different receive channels. This led to poor channel balance, large hardware and volume as channel count increased, and high cost. Narrowband filter transient effects caused "rabbit ear" artifacts before and after pulses, making them unsuitable where high frequency resolution is required.
5. Compression receiver
A compression receiver is a superheterodyne receiver designed for fast frequency search. The trade-off between sweep speed and frequency resolution limits search speed in conventional superheterodyne receivers. Compression receivers use compression filters to compress wideband linear FM (chirp) signals into narrow pulses, alleviating the sweep speed versus frequency resolution conflict. Compression receivers can rapidly scan wide bands, detect frequencies and signal strengths of simultaneous arrivals, and provide good sensitivity, though identifying modulation types can be difficult.
In a compression receiver, the input signal is mixed with a local sweep source to convert it into a linear FM (chirp) signal. The chirped signal is compressed by a dispersive delay line into short pulses. After detection, these short pulses become video signals. Each output pulse's time position relative to the local oscillator sweep start indicates the corresponding input signal frequency.
6. Bragg receiver
The Bragg receiver uses an optical Bragg cell for frequency separation. The input RF signal is converted to an acoustic wave propagating through the Bragg cell. The Bragg cell deflects an incident laser beam, and the deflected beam position is a function of input frequency. An imaging detector converts the optical output to a time-frequency signal for detection. The main advantage of a Bragg receiver is simplicity: a small number of components—laser, deflector, two lenses, a Bragg cell, and an imaging detector—can realize many channels.
Comparing these architectures shows that Bragg, compression, and channelized receivers can provide excellent overall performance. The Bragg receiver uses optical Bragg cells for spectral separation, but the system complexity is high and dynamic range is limited. Compression receivers use dispersive delay lines to compress incoming RF signals into narrow pulses; their high data processing rate, sidelobes from compression, and loss of intrapulse modulation information can affect detection performance. Channelized receivers separate received signals using analog or digital filter banks and can separate signals at different frequencies, receive time-overlapping signals, and achieve high sensitivity and frequency resolution with near-100% intercept probability. They offer strong selectivity and anti-jam capability with fidelity comparable to superheterodyne receivers, making channelized structures currently practical for wideband electronic warfare. Their main drawbacks are complexity, large size, high mass, power consumption, and cost, which limit development to some extent. However, ongoing improvements in ADCs, digital integrated circuits, and digital signal processing are driving receivers toward digital implementations that mitigate the limitations of analog channelized architectures.
7. Microwave photonic receiver
Conventional microwave-based receivers face technical bottlenecks in instantaneous bandwidth, system sensitivity, miniaturization, and low power. Photonic technologies inherently provide large bandwidth, low transmission loss, and immunity to electromagnetic interference; photonic systems are also lightweight, compact, and integrable. These characteristics offer new approaches to overcome traditional limitations.
The microwave photonic receiver is essentially an optoelectronic hybrid system. The front-end antenna reception and backend intermediate-frequency processing remain in the electrical domain, while functions such as microwave mixing, microwave interference, and beamforming can be performed optically. As photonic processing advances, more signal processing can be implemented in the optical domain to increase processing speed.
Advantages of microwave photonic receivers include:
(1) Low loss. Compared with microwave systems, microwave photonic receivers process signals in optical fiber, producing much lower loss. RF coaxial cable attenuation is typically 0.2–1 dB/m and requires multi-stage amplification for long-distance transmission, introducing nonlinearity and noise and increasing energy consumption. Ultra-low-loss optical fiber (transmission loss on the order of 0.0002 dB/m) can replace bulky, heavy, lossy, and EMI-vulnerable coaxial cable.
(2) High sensitivity. Optical processing links have lower phase noise and system noise compared with microwave links, enabling improved receiver sensitivity. For example, optoelectronic oscillators (OEOs) implemented with microwave photonics can generate spectrally pure microwave or millimeter-wave signals from megahertz to hundreds of megahertz with phase noise approaching quantum limits (e.g., -163 dBc/Hz @ 10 kHz), making them ideal high-performance microwave sources.
(3) Very large instantaneous bandwidth. Traditional surface acoustic wave channelizer filter banks and acousto-optic channelized receivers are limited by acoustic modulation bandwidth, with instantaneous bandwidths not exceeding a few gigahertz. While ADC and digital signal-processing progress allow generation and processing of signals up to a few gigahertz, existing electronic ADCs struggle to directly sample tens of gigahertz of bandwidth. Photonic technology offers very large bandwidths, enabling radar signals with tens of gigahertz of bandwidth. For example, optical frequency-to-time mapping can generate signals with bandwidths up to 50 GHz.
Research programs have supported core microwave photonic components such as optoelectronic oscillators, optical arbitrary waveform generation (OAWG), optical ADCs (OADC), analog photonic signal processing, photonic front ends, and photonic integration.
For example, optical ADC technology currently uses semiconductor and fiber mode-locked lasers to generate optical sampling pulses with sampling rates of 40–100 GHz. Optical sampling pulses can have picosecond or even femtosecond widths, enabling direct optical sampling of RF signals from 0.2 to 40 GHz. Optical ADCs have two main operation modes: fully optical analog-to-digital converters (OADC) and electro-optical ADCs (E-OADC). Both remain under research.
Microwave photonic technology can implement photonic channelized receivers by splitting a wideband received signal in the optical domain into multiple narrowband processing channels, followed by optoelectronic detection and processing per channel. Compared with conventional channelized receivers, photonic channelization offers stronger EMI resistance, much larger carried and instantaneous bandwidth, and extremely low transmission loss. Channelization is inherently a multi-channel parallel processing system, and the optical domain's rich spectral resources and flexible multiplexing (e.g., wavelength-division multiplexing) match this paradigm. Therefore, microwave photonic channelization has attracted broad attention.
Challenges for photonic channelization include the design of suitable filter banks: narrowband filters with flat passbands, high stopband suppression, and steep roll-off edges are difficult to realize, whether with integrated photonic technologies or discrete components. Additionally, photodetection typically loses phase information, so optoelectronic channelization often only provides presence/absence detection of signals and cannot recover full signal content in its basic form.
