6. Receivers, Integration, Pulse Compression Flashcards
Explain the components of the Superheterodyne Receiver
Duplexer: Switches between transmitter and receiver
RF Attenuator: Attenuates return from clutter that is very close to the antenna.
LNA: Low Noise Amplifier
Local Oscillator: Allows you to downconvert to a different frequency, which allows your receiver to use and put out the same frequency with a dynamic range of input frequencies. This is important because some equipment performs optimally at certain frequencies.
IF Preamp: Intermediate Frequency Preamp
IF to Video Conversion: Converts radar data into digital signals in a video format.
Explain Dynamic Range
The range of power which the receiver can handle. It varies significantly due to differences in RCS and distance.
Describe Sensitivity Time Control
As the return time decreases, the receiver attenuation increases IOT handle high power returns from clutter at close range.
Draw a Superheterodyne Receiver
Logarithmic Receiver
Compresses the values of large returns relative to small returns IOT increase the Dynamic Range.
Gain Control
Allows operator to set the gain of the overall receiver. Can be manual or automatic.
Instantaneous AGC (IAGC)
Automatic Gain Control based on the strength of target returns IOT avoid receiver saturation. It adjusts for changes in background clutter. It will only change the gain over the cells which have a high return.
The difference between this and STC is that this is with power not time.
Difference from CFAR is that it does not chagne the detection threshold, it changes the gain.
Fast TIme Constant
Eliminates saturation caused by rain. Return signals from rain are much longer and weaker than those of a solid object.
Gain is reduced in the area. However, when this also reduces the
Heterodyne vs. Superheterodyne
Superheterodyne has tunable Frequency Downconversion, allowing it to handle a wide range of input frequencies.
Explain this shit
Frequency Downconversion.
Tunes the input frequency to desired freq for later equipment.
Matched Filters
Used to maximize SNR by comparing receive signal to transmitted waveform.
The correlation of the transmitted waveform and received signal provides ability to find common elements within the signal.
Pulse Compression
Two types: Linear Frequency Modulation and Phase Coded Waveforms
The pulse frequency (or phase) is increased along the pulse. This increases bandwidth while maintaining pulse width.
A messy modulated pulse is sent and the receiver is looking for that exact modulation match. It also helps discerning targets from noise because the noise will distort the signal significantly.
This significantly improves the range resolution. the target must be perfectly centered over the pulse to give a return.
This solves the problem that pulse width increases power but degrades range resolution.
Linear Frequency Modulation
Frequency is modulated within the pulse.
Phase Coded Waveforms
Phase is modulated within the pulse.
Done using something such as a Barker Code where the phase is changed in a pattern.
Barker Codes
The patters of phase changes for Phase Coded Waveforms. Different codes have different qualities due to different amounts of uniqueness.
Integration
By combining multiple samples of the same signal, we can average don the noise, resulting a higher SNR. This is because the noise is random so it will average closer to zero the more you do this.
Types of Integration
Coherent: Part of threshold testing.
Noncoherent: Part of threshold testing.
Binary (m of n): Performed after threshold testing.
Systems can use none, one, or multiple types.
Coherent Integration:
Combining complex (I/Q) data samples using FFT.
Noise components experience destructive interference
Signal components experience constructive interference in phase which improves SNR.
Shifts the mean of Noise signal.
Noncoherent Integration
Magnitude of data is integrated.
Does not move the mean but reduces the variance of Noise.
How is Doppler calculated?
