The radar receiver is an intricate system designed to capture the extremely weak electromagnetic echoes reflected from targets, amplify them, separate them from noise and interference, and convert them into a format suitable for detection, processing, and display. The efficiency and sensitivity of the receiver are paramount for the overall performance of a radar system.

The superheterodyne receiver architecture is the almost universally adopted design for modern radar receivers (and most high-performance radio receivers). Its superiority stems from its ability to provide high gain, excellent selectivity (ability to reject unwanted signals), and stable performance across various operating conditions.

The main functional blocks of a superheterodyne receiver are:
1. Antenna: The antenna serves as the interface between the electromagnetic waves in free space and the electrical signals in the receiver. It collects the weak echo signals.
2. RF Amplifier (Low Noise Amplifier - LNA): The first active stage in the receiver chain. Its primary role is to amplify the extremely weak incoming RF signals while adding as little noise as possible. The noise figure of the LNA is critical, as any noise introduced here will be amplified by all subsequent stages. A high-quality LNA significantly improves the radar's sensitivity and maximum detection range.
3. Mixer: This is the heart of the superheterodyne principle. The mixer is a non-linear device that combines two input signals: the amplified RF signal from the antenna and a stable, high-frequency signal generated by the Local Oscillator (LO). The output of the mixer contains sum and difference frequencies of its inputs.
4. Local Oscillator (LO): A highly stable, tunable oscillator that generates a continuous wave signal. Its frequency is precisely controlled relative to the expected RF signal such that their difference results in the desired IF.
5. IF Filter: This filter is placed immediately after the mixer to select only the desired IF signal (i.e., ∣fRF −fLO ∣) and reject the other mixer products and unwanted signals. Its bandwidth is carefully chosen to pass the echo pulse while minimizing noise.
6. IF Amplifier: This stage provides the bulk of the receiver's overall gain. Because the IF is a fixed frequency, the IF amplifier can be highly optimized for gain, linearity, and bandwidth. Multiple stages of amplification are often used. This fixed frequency operation allows for very stable and repeatable performance.
7. Detector (Demodulator): The detector extracts the information (amplitude and timing) from the IF signal. For pulsed radar, this typically involves converting the modulated IF signal (which is a pulse of RF at IF) into a baseband video signal. This often involves rectification (e.g., using a diode) to convert the AC IF signal into a varying DC voltage representing the envelope of the pulse, followed by low-pass filtering to smooth out the IF ripple.
8. Video Amplifier: Amplifies the low-frequency video signal from the detector to a level suitable for display on a screen (like an A-scope or PPI) or for further digital processing.

The Intermediate Frequency (IF) amplification stage is paramount for the overall performance of the radar receiver. It is where the vast majority of the signal gain is achieved, boosting the very weak IF signal (which is still orders of magnitude smaller than the noise floor) to a usable level for detection.

Key aspects of IF amplification:
● Gain: IF amplifiers are designed to provide significant gain, often tens of decibels (dB), required to lift the target echo above the noise floor.
● Bandwidth Matching: The bandwidth of the IF amplifier is critically chosen to match the spectral characteristics of the received radar pulse. For a rectangular pulse of width τ, the optimal bandwidth (BWIF ) for maximizing the signal-to-noise ratio (SNR) is approximately 1/τ. This ensures that the pulse energy is captured efficiently while limiting the amount of noise that enters the system. A wider bandwidth would admit more noise, while a narrower bandwidth would distort the pulse shape and reduce signal energy.
● Stability and Selectivity: Because the IF is a fixed frequency, the amplifier stages can be designed with high Q-factor resonant circuits, leading to excellent frequency stability and sharp selectivity. This helps in rejecting interference and noise outside the desired signal band.
● Automatic Gain Control (AGC): Many IF amplifiers incorporate AGC circuits. AGC automatically adjusts the gain of the amplifier based on the strength of the received signal. This prevents the amplifier from saturating (clipping) when strong echoes (e.g., from close-range targets) are received, and increases gain for weaker, distant echoes, optimizing dynamic range.
● Logarithmic Amplifiers: In some radar receivers, logarithmic IF amplifiers are used. These provide an output voltage proportional to the logarithm of the input signal power, which compresses a wide dynamic range of signals into a manageable range, making detection of both strong and weak targets easier.

After the IF amplification stage, the processed IF signal needs to be converted into a format that allows the radar to determine the presence of a target and its range (by measuring the time of arrival of the echo). This process is known as signal detection.
For simple pulsed radar, basic signal detection typically involves:
1. Envelope Detection (Rectification): The IF signal is a high-frequency alternating current (AC) signal. The first step in detection is usually to extract the "envelope" of this signal, which corresponds to the amplitude modulation of the pulse. This is commonly done using a diode detector. A diode allows current to flow in only one direction, effectively rectifying the AC signal. A capacitor often accompanies the diode to smooth out the rectified signal, producing a varying DC voltage that mirrors the shape of the radar pulse. This output is often called the "video signal."
2. Video Amplification: The output of the detector (the video signal) is often still quite weak. A video amplifier boosts this signal to a level suitable for display or for input into an Analog-to-Digital Converter (ADC) for further digital processing.
3. Thresholding: The amplified video signal contains not only target echoes but also noise. To decide whether a detected pulse is a genuine target echo or just a random noise fluctuation, the signal is compared against a pre-defined detection threshold.
○ If the video signal amplitude exceeds the threshold, it is declared a "detection" or "target."
○ If it falls below the threshold, it is considered noise and is discarded.
○ The choice of this threshold is critical.
■ Setting it too low will lead to a high Probability of False Alarm (Pfa), where noise spikes are incorrectly identified as targets (nuisance alarms).
■ Setting it too high will lead to a low Probability of Detection (Pd), where weak but legitimate target echoes are missed.
○ The optimal threshold is usually determined based on the desired Pfa and the statistical properties of the noise (often assumed to be Gaussian noise).