Module 4 Flashcards
What is receiver sensitivity?
The minimum input RF signal a receiver needs to produce a specific intelligence signal at the output
What does the receiver do? (2 answers)
Detects and amplifies signals from an aircraft
Separates desired echoes from unwanted signals
What is noise
Undesired electrical signals
What are three types of external noise
Atmospheric
Human-made
Space
Interference is a type of external noise that is prevalent in ______ composed of undesired signals.
overcrowded airwaves
How can adjacent channel interference be eliminated
By allowing main equipment to produce triggers for all other systems
What does bandwidth do?
Helps determine receiver noise level and minimum sensitivity capability
A _______ IF results in higher noise from the mixer or LO, and a _______ IF results in higher noise from the IF amplifier.
lower, higher
The _______ the selected IF, the _______ the mixer or LO noise becomes
lower, higher
Bandwidth is a good approximation for a usable circuit because it tends to be _______ to account for instabilities in Doppler effects due to target velocity
higher in some systems
What causes adjacent system interference
Several stations operating in a frequency band
A lower noise factor indicates the receiver is introducing less noise which results in the ability to _________ the desired signal and noise.
differentiate between
The noise figure in a system can be determined by examining ___________.
known noise source
Transmitters have ________ power capabilities
limited
Radar performance is defined by the _____________ for a specified radar cross-section target.
maximum detection range
What is selectivity
The extent to which a receiver can differentiate between signals.
What is essential for maximum performance
Reducing front-end noise
What are some sources of an unwanted signal (3 answers)
Any object other than the desired target
Interference from other radars or communication systems
Signals coming from an aircraft
What are the major characteristics of a receiver? (2 answers)
sensitivity and selectivity
What are the types of internal noise (3 answers)
Shot
System
Thermal
The noise in a circuit is related to the input impedance’s temperature due to the relationship between ____________.
(3 answers)
bandwidth
Boltzmann’s constant
noise temperature
What are three sources of interference
Emissions from nearby radars
Intentionally generated signals in the radar’s frequency band
The radar’s other channel
What is Atmospheric noise
Caused by naturally occurring disturbances in the Earth’s atmosphere
What is Human-made noise
Generated by spark-producing mechanisms
What is SHOT noise
Introduced by carriers located in the pn-junctions of the semiconductors
What is Space noise
Composed of solar noise from the sun and cosmic noise from the stars
What is Thermal noise
Not limited to the receiver, generated within the receive path caused by thermal interaction between free electrons and vibrating ions within a conductor
What is Noise factor
Determines the amount of noise by relating the ratio of input signal power and input noise power to output signal power and output noise power
What is Noise figure
The increase in noise power of a network from input to output which is greater than signal gain
What is Probability of detection
The likelihood an input signal will be detected when deciding if a reading is the signal plus noise, or purely the signal
White noise
Frequency content is uniform across the spectrum
What is a large limiting factor to the minimum discernable signal that the radar can detect?
System noise
What is the correct order of Receiver components:
Attenuator
IF output
LNA
Mixer
Preselector
T/R limiter
- T/R limiter
- Attenuator
- LNA
- Preselector
- Mixer
- IF output
What component reduces the difference between the highest and lowest signals
STC (Sensitivity time control)
What component Prevents high-powered transmitter signals from entering the receiver and damaging front-end circuits
T/R Limiter
What component Measures the time needed for a receiver to recover to a given sensitivity
SRT
What component Improves signal-to-noise ratio of the receiver
LNA
What component Controls the attenuator and sends a signal telling it how much an incoming signal level should be reduced.
Gain Control Unit
What component Adds voltages together resulting in a voltage that changes at the rate of difference between two frequencies
Mixer
What component Determines the amount of signal phase shift
Hybrid coupler
What component Reduces the RF front-end’s bandwidth thereby reducing available noise power
Preselector
What component Separates sidebands and converts each one to the IF frequency
Image rejection mixer
What component Supplies most of the receiver’s gain and is a major determiner on receiver sensitivity and selectivity characteristics
IF amplifier
The system’s minimum range capabilities depend on the _______.
T/R limiter
What can produce strong returns that cause front-end amplifiers to go into saturation
Ground clutter
The signal from the _________ is developed based on receiver gain settings and STC (Sensitivity time control) curves.
gain control circuit
Receiver gain is essentially __________.
sensitivity
Which component changes attenuation automatically as radar range increases
STC (Sensitivity time control)
If the radar has two feedhorns, the ______ determines which beam to look at
attenuator
An LNA is _____ at improving the noise figure when it is _____ the receiver signal source
A. equally effective, any distance from
B. less effective, closer to
C. more effective, closer to
D. more effective, further from
more effective, closer to
What is normal video
The simplest type of radar video signal provided to an ATC display
What does an MTI (Moving Target Indicator) do
Cancels stationary targets not canceled by normal video
In legacy radar systems, one output from an IF amplifier supplies the ________. (3 answers)
amplifier
linear receiver
normal receiver
How does normal video affect the controller’s ability to differentiate between aircraft targets and clutter
Normal video makes it more difficult for the controller to differentiate aircraft targets from clutter.
What is a shortcoming of MTI
Valid aircraft targets can get lost in the cancellation process
Normal receivers tend to ________ the accepted signal through a nonlinear device to display ________.
A. down convert, intelligence
B. up convert, intelligence
C. up convert, targets
D. Misconstrue, noise
down convert, intelligence
What is one of the most effective nonlinear devices for detecting intelligence modulated in a high carrier frequency?
Diode detector
Which problem with diode detectors made transistor detectors popular
The inability to amplify
Which radar brought technological advances in radar signal processing that made using normal video obsolete in MTD systems?
ASR-9
A logarithmic receiver/amplifier amplifies ________ level signals more than _______ level signals.
A. low, high
B. high, low
C. high, moderate
D. moderate, low
low, high
What is a benefit of a wide dynamic range?
Limiting or overloading is not required.
The logarithmic receiver/amplifier _______.
A. has an output proportional to the logarithm IF input.
B. has an output proportional to the logarithm RF input.
C. has the same output as the normal receiver/amplifier.
D. produces logarithmic video for the processor’s weather circuits.
has an output proportional to the logarithm RF input.
produces logarithmic video for the processor’s weather circuits.
The logarithmic receiver’s effectiveness reaches the ______
maximum range
A logarithmic function is achieved with _____________ and _________ the outputs detected with each amplification stage.
A. cascading stages of limiting IF amplifiers, subtracting
B. cascading stages of limiting IF amplifiers, summing
C. two limiting IF amplifiers, subtracting
D. two limiting IF amplifiers, summing
cascading stages of limiting IF amplifiers, summing
The 3 dB bandwidth of a logarithmic receiver produces an output that is ________ by ________ times the transfer slope.
A. down converted, two
B. down converted, three
C. up converted, two
D. up converted, three
down converted, three
The amplifier’s signal-to-noise ratio __________ its drive level.
changes with
What is a logarithmic receiver used for?
Tracking aircraft through inclement weather
In a phase detector receiver, down conversion transforms:
RF into IF
WHat is a problem caused by sensitive phase detectors and radar systems transmitting as an antenna rotates seeing ground clutter differently each pulse
Antenna scanning
What Determines the system’s ability to detect a target
S/N ratio
what is it called when a moving target produces a phase shift between pulses such that the outputs from phase detectors have the same amplitude
Blind phase
what Mitigates the likelihood of target loss and reduce the effects of the blind phase
I & Q phase detectors
__________ will provide a reference phase for the phase detectors whose output is based on the difference between the reference phase and received echo IF.
COHO
Which target parameters are not considered by an MTI circuit? ( 2 answers)
Range and Size
In a phase detector receiver, modulated _______ is mixed with _________ for conversion to bipolar radar video intelligence.
A. IF, COHO
B. IF, STALO
C. RF, COHO
D. RF, STALO
IF, COHO
Transmitter frequency is the sum of _____.
STALO and COHO
The ____ is compared to the _______ to generate an IF.
A. COHO, receiver IF echo
B. COHO, receiver RF echo
C. STALO, receiver IF echo
D. STALO, receiver RF echo
STALO, receiver RF echo
What are the types of phase detector characteristics? ( 3 answers)
Sinusoidal
Square
Triangular
What does a phase detector’s output show? (2 answers)
Fixed and Moving targets
What compares the received echo from a target’s amplitude with its received echo in the next transmitter interval?
Canceller
A moving target’s ______ varies between transmitter intervals on a phase detector display.
amplitude
Maximum outputs happen at ___ phase difference.
0° and 180°
Normal and logarithmic video channels in a phase detector use a basic diode type detector circuit to remove the _____ frequency
IF
What is the first step in detecting a moving target?
Determining its relative phase returns
When do optimum phase conditions occur?
Maximum Positive and negative value
What determines the minimum range of a radar system?
System recovery time
Injecting a signal set below the limit into the waveguide and viewing the receiver output on an oscilloscope helps us determine __________.
System recovery time
What is the MDS power level used to determine?
Receiver sensitivity
For the _______ to be a valid input, the signal must represent the expected echo in terms of pulse width.
MDS
What does using an input signal with a pulse width greater than the transmitted burst indicate?
A false increase in receiver sensitivity
What is the time it takes a T/R limiter to come out of ionization and let received signals enter the receiver called?
A. Amplitude
B. Minimum range of a radar system
C. Pulse width
D. System recovery time
Minimum range of a radar system
System recovery time
What is a limiting factor for the minimum range of a system?
T/R recovery
What does an increase in SRT indicate?
Deterioration of a T/R device
What is determined by looking at the magnitude of thermal noise, but not realized in practice due to additional receiver noise?
Maximum possible sensitivity
A receiver’s __________ is measured by the weakest signal that produces a visible receiver output.
sensitivity
What is the receiver’s ability to pick up weak signals called?
Receiver sensitivity
What is a measurement of receiver sensitivity?
Minimum discernable signal
Systems measure the detection percentage a known signal produces through ______.
calculated thresholds
System noise displayed on an oscilloscope is referred to as ______.
the grass level
The _______ delivered to the receiver is _______ proportional to pulse width.
