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TECHNISONIC Understanding Noise in Airframe installations By: Walter Shawlee 2 Technisonic products are available from Dallas Avionics. Noise What exactly is “noise”? Anything you didn’t want or expect to hear in the audio system. System Noise = Equipment Noise Floor + Cross-talk (de-selected audio) + Ground Loop noise + RF artifacts + Coupled noise + Acoustic Noise + Noise is the Enemy Installation costs and time are momentary… But a bad install is forever. Take the time to get everything as right as possible. Understanding noise origins is critical. Types of Noise In-band audio frequency noise. Cross-talk Signal contamination Inverter/Generator Contamination Types of Noise Out-of-Band RF Sources: AM Comms HF/SSB FM Comms RF Problems Ship Issues can be multi-modal: Radio to Radio Interference Harmonic Direct Radio to Audio line rectification Antenna Induced Standing Waves Antennas All simple monopole antennas assume they have a ground plane equal in radius to the height of the antenna. Anything less results in substantial reflected power,and high standing waves on the coax cable. This dramatically worsens interference. RF Solutions Triaxial Cable, shield grounded at one end only. Do not bundle RF feedlines with other cables. Better antenna ground planes. Better antenna spacing. Mechanical-RF The Main Rotor will modulate outgoing transmissions, and incoming reception, and change reflected power. Antenna PLACEMENT is everything to fix this. Antenna Issues Bottom Mounts work best for AM Comms on helicopters. RF Interference Rule Once the RF interference appears as AUDIO, it is too late to suppress or correct it. It has to be suppressed in the RF mode. Spacing & Proximity There’s rarely enough room in general aviation aircraft... 10 kg in a 5 kg box Sometimes it’s just not that easy… Audio Wiring Wiring topology and routing have a large bearing on audio system performance. No amount of shielding can overcome design limits. Audio Wiring Access/A (floating ground) technology can provide a 20-30dB improvement in noise and crosstalk over even high end grounded systems. That is 100-1000 times better. Audio Wiring Grounded audio returns are highly subject to GROUND LOOP noise. This style interconnect was an early method of saving weight, complexity and wire, but offers the worst possible performance. Ground Loops How we wish aircraft worked: Ground Loops What really happens: Ground Loops The “frame ground” is alive with voltages caused by different loads. Ground Loops Attaching audio returns to ground contaminates the audio signal with every load current flowing through the airframe. These induced voltages can be VERY large, and are a real problem especially for low level audio lines like microphones. Ground Loops Ground currents also inject CROSS-TALK, as every audio line returned though ground becomes mixed with every other audio signal. Ground Loops The answer is to LIFT audio lines from the airframe ground, and process them as floating signals. This improves both noise and cross-talk significantly. Composites Composites have these problems: RF shielding dramatically reduced. Ground resistance dramatically increased. Static discharge problems dramatically increased. Ground Topology Floating Topology Victim Wiring Wiring type determines susceptibility. Victim Wiring Un-shielded wiring, with a ground return is the MOST susceptible to both interference and ground loop noise. Shielding, and lifting the ground return dramatically improves this. Twisted pairs can be almost as effective as shielding, IF FLOATING. Victim Modes Inductive Coupling is CURRENT Driven Victim Modes The only real cure for Inductive Coupling is INCREASED SPACING. Remember the inversesquare law. Shields (unless co-netic material) are totally ineffective. Victim Modes Inductive coupling sources: Any high current line (AC or DC). Any inverter/magnetic switching element. Internal switching regulators. Victim Modes Capacitive coupling is VOLTAGE Driven. Victim Modes Shielding is effective for Capacitive Coupling, IF the shield is a true shield, and not carrying current. Spacing is also effective. Susceptibility Rules Most Susceptible Wiring: High Impedance Un-shielded/poorly shielded. Low Voltage Summary Use a system that minimizes audio problems for your specific task. Wiring has to be appropriate in terms of shielding and ground connections. Audio cable proximity to other systems is critical. Summary Never connect un-needed wiring. Comms to rear stations. Nav Inputs to non-flight stations. Unterminated inputs Finally Every system has a noise floor determined by these factors: Hardware design & topology Cabling Ground/composite issues. TECHNISONIC A711L Series 3rd Generation Advanced Analog audio. Full fail-passive capability. 7 Radios + PA, with 1-button transfer. Tactile, lighted controls. Cleaner layout. Tone alerting. Plug compatible. TECHNISONIC A711L series Cosmetic Options. NVG + flexible light. Powersonix PA capability. Nifty PA options. Latent features. Ideal mate to the TDFM-7300. Address integrated NAV/COM packages. ARTS-100 Audio Radio Test System Has it’s own high current power, can test or demo virtually all TIL products, all the way up to the TFM-7300. Rack based, or stand alone. TECHNISONIC http://til.ca short and sweet. [email protected] any time for audio questions. Available from Dallas Avionics. Reference stuff: http://www.sphere.bc.ca/test/data.html TECHNISONIC See us at BOOTH 103 / 801 Thank you!