TECHNISONIC l Understanding Noise in Airframe installations l

TECHNISONIC l Understanding Noise in Airframe installations l l By: Walter Shawlee 2 Technisonic products are available from Dallas Avionics.

Noise l What exactly is “noise”? l Anything you didn’t want or expect to hear in the audio system.

System Noise = l Equipment Noise Floor + l Cross-talk (de-selected audio) + l Ground Loop noise + l RF artifacts + l Coupled noise + l Acoustic Noise +

Noise is the Enemy l Installation costs and time are momentary… l But a bad install is forever. l Take the time to get everything as right as possible. l Understanding noise origins is critical.

Types of Noise l In-band audio frequency noise. l Cross-talk l Signal contamination l Inverter/Generator Contamination

Types of Noise l Out-of-Band RF Sources: l AM Comms l HF/SSB l FM Comms

RF Problems l Ship Issues can be multi-modal: l Radio to Radio Interference l Harmonic l Direct l Radio to Audio line rectification l Antenna Induced Standing Waves

Antennas l All simple monopole antennas assume they have a ground plane equal in radius to the height of the antenna. l Anything less results in substantial reflected power, and high standing waves on the coax cable. This dramatically worsens interference.

RF Solutions l Triaxial Cable, shield grounded at one end only. l Do not bundle RF feedlines with other cables. l Better antenna ground planes. l Better antenna spacing.

Mechanical-RF l The Main Rotor will modulate outgoing transmissions, and incoming reception, and change reflected power. l Antenna PLACEMENT is everything to fix this.

Antenna Issues l Bottom Mounts work best for AM Comms on helicopters.

RF Interference Rule l 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 l There’s rarely enough room in general aviation aircraft. . .

10 kg in a 5 kg box l Sometimes it’s just not that easy…

Audio Wiring l Wiring topology and routing have a large bearing on audio system performance. l No amount of shielding can overcome design limits.

Audio Wiring l Access/A (floating ground) technology can provide a 20 -30 d. B improvement in noise and crosstalk over even high end grounded systems. l That is 100 -1000 times better.

Audio Wiring l Grounded audio returns are highly subject to GROUND LOOP noise. l This style interconnect was an early method of saving weight, complexity and wire, but offers the worst possible performance.

Ground Loops l How we wish aircraft worked:

Ground Loops l What really happens:

Ground Loops l The “frame ground” is alive with voltages caused by different loads.

Ground Loops l Attaching audio returns to ground contaminates the audio signal with every load current flowing through the airframe. l These induced voltages can be VERY large, and are a real problem especially for low level audio lines like microphones.

Ground Loops l Ground currents also inject CROSS -TALK, as every audio line returned though ground becomes mixed with every other audio signal.

Ground Loops l The answer is to LIFT audio lines from the airframe ground, and process them as floating signals. l This improves both noise and cross -talk significantly.

Composites l RF have these problems: shielding dramatically reduced. l Ground resistance dramatically increased. l Static discharge problems dramatically increased.

Ground Topology

Floating Topology

Victim Wiring l Wiring type determines susceptibility.

Victim Wiring l Un-shielded wiring, with a ground return is the MOST susceptible to both interference and ground loop noise. l Shielding, and lifting the ground return dramatically improves this. l Twisted pairs can be almost as effective as shielding, IF FLOATING.

Victim Modes l Inductive Coupling is CURRENT Driven

Victim Modes l The only real cure for Inductive Coupling is INCREASED SPACING. Remember the inversesquare law. l Shields (unless co-netic material) are totally ineffective.

Victim Modes l Inductive l Any coupling sources: high current line (AC or DC). l Any inverter/magnetic switching element. l Internal switching regulators.

Victim Modes l Capacitive coupling is VOLTAGE Driven.

Victim Modes l Shielding is effective for Capacitive Coupling, IF the shield is a true shield, and not carrying current. l Spacing is also effective.

Susceptibility Rules l Most Susceptible Wiring: l High Impedance l Un-shielded/poorly shielded. l Low Voltage

Summary l Use a system that minimizes audio problems for your specific task. l Wiring has to be appropriate in terms of shielding and ground connections. l Audio cable proximity to other systems is critical.

Summary l Never connect un-needed wiring. l Comms to rear stations. l Nav Inputs to non-flight stations. l Unterminated inputs

Finally l Every system has a noise floor determined by these factors: l Hardware design & topology l Cabling l Ground/composite issues.

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TECHNISONIC l http: //til. ca short and sweet. l Walter 2@til. ca any time for audio questions. l Available from Dallas Avionics. l Reference stuff: http: //www. sphere. bc. ca/test/data. html

TECHNISONIC l See us at BOOTH 103 / 801 l Thank you!