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30 Cards in this Set
- Front
- Back
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What is a Superheterodyne Receiver |
- Type of radio receiver that uses frequency mixing. - Converts recieved signal into lower fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier wave. - Lower IF amplified then demodulated to get video signal. |
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Superheterodyne Receiver Components |
- Antenna. - Mixer <- local oscillator. - IF amplifier. - Demodulator -> local oscillator (AFC) - AF voltage amplifier. - AF power amplifier. |
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RF Amplifier |
- Amplifier increases amplitude of signal inputted. |
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Power Amplifier Key Design |
- Frequency response. - Noise. - Gain. - Distortion. |
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How Noise is Created |
- Ground loops and improper grounding. - Poor wiring. - Poorly designed in circuitry. - Close proximity to other electrical equipment. - Long wire leads picking up radio frequency. |
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Mixer |
- Received signal mixed with local oscillator to create IF, this is called heterodyning. - IF is difference between oscillator frequency and radio frequency. - IF allows amplifier circuit to operate with maximum stability, selectivity and sensitivity. |
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Local Oscillator |
- Constant frequency signal output. - IF signal easier to manipulate and amplify. - Frequency with HIGH STABILITY and LOW HARMONICS. - LO uses piezoelectric tuning fork with known period of oscillation. - Harmonics is where multiple waveform anti-nodes line up. - Piezoelectric crystal has LOW HARMONICS as frequency is so high. |
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Automatic Frequency Control |
- RADAR receiver requires limited tuning range to compensate for frequency changes. This is because of variations in temperature and loading of magnetron. - Method of circuitry automatically keep resonant circuit to frequency desired. |
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Why is an Automatic Frequency Control used |
- Used in circuits to accurately control frequency of an oscillator. - AFC senses difference between actual frequency and frequency desired. - Produces control voltage proportional to difference. |
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How does Automatic Frequency Control work |
- Sample of transmitted frequency fed to AFC, together with sample from Klystron. - Output from mixer fed to frequency discriminator circuit tuned to required IF. - If mixer output not at IF, discriminator will feed DC error voltage to Klystron. - Klystron adjusts frequency to make mixer output equal to desired IF. |
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IF Amplifier |
- Changes frequency levels in circuits that are too selective, difficult to tune and unstable. - Changing frequency levels also helps improve image display and tuning range. - Fixed frequency amplifiers that reject unwanted signals. - Process and enhances down converted or modulated signal. |
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Demodulator (Detector) |
- Converts IF pulses into video pulses. - Common detector is diode detector. |
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How does a Detector Convert IF into Video Pulses. |
- Detects pulse envelope. - Blocks IF frequency signals. - Leaving pulse envelope which is then digitized. |
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How does the Detector convert if the Target is moving |
- Form of additional FM introduced onto transmitted signal. - Movement introduces slight varying path length from transmission to reception. - If this can be detected, it gives instantaneous velocity measurement. - This can help detect if a target is wanted or not. |
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Characterstics of a Detector |
- Responds to low frequnecies contained within pulses envelope (<20MHz). - Rejects high IF oscillations, typically 60MHz. |
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What is Clutter |
- Unwanted echoes that are not required. |
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Natural Causes of Clutter |
- Ground. - Sea. - Precipitation. - Sand storms. - Animals. - Atmospheric turbulence. - Ionospheric reflections. - Meteor Trails. - Three body scatter spike. |
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What is Three Body Scatter Spike |
- Artefact on weather RADAR which is indicative of a hail storm. - Identified by spike of weak reflective echoes, extending from a thunderstorm. - Result of energy hitting hail or heavy rail. |
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Man-made Reasons for Clutter |
- Buildings. - RADAR countermeasures (chaff). |
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How to Reduce Clutter |
- Sea clutter reduced by horizontal polarisation. - Rain clutter reduced by circular polarisation. |
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What is Linear Polarisation |
- Horizontal: propagated electric lines of force lie in horizontal direction. - Veritical: propagated electric lines of force lie in a vertical direction. |
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What is Circular Polarisation |
- Propagated electric lines of force rotate through 360 degrees. - Electric field does not remain constant, field rotates. - Horizontal and vertical lines of the field exist. - Can be right-handed or left-handed. |
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How is Rain Clutter Reduced by Circular Polarisation |
- Reduced by using circular polarisation.
- Spherical raindrops causes circular polarised waves reflect in opposite sense.
- Minimises detection if the rain. - This enhances other reflections relative to rain reflections. |
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Improving RADAR Performance in Clutter Environments |
- Moving target indication: integrates successive pulses. - Doppler processing: Uses filters to separate clutter from desirable signals. |
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What is Noise |
- Noise is random, unwanted signal. - Results in erratic, random deflection or intensity of indicator sweep masking small return. |
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If Noise Didn't Exist |
- Maximum range at which object could be identified could be extended infinitely. - Without noise you could amplify signal to useable level without limitations. |
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Where is Noise most apparent |
- In regions with low signal level (wealth received echoes). |
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How can Noise be Expressed |
- Noise factor. - Noise figure. - Equivilent noise temperature. |
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What is FRUIT |
- False Replies Unsynchronised In Time. . - Echoes from A/C responding to other ATC. - Solution to this is to send lots of interrogator signals. |
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Garbling |
- Similar to FRUIT. - A/C in close proximity to each other. - Synchronous signals cannot be scrambled. - Non-synchronous, C2-SPI phantom bracket and closely spaced can be scrambled. |
