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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


    Installation costs and time are momentary…

But a bad install is forever.

Take the time to get everything as right as possible.

Understanding critical.

noise origins


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 



 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

other cables.

RF feedlines with  Better antenna

ground planes


 Better antenna




 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

talk over even high end grounded systems.

in noise and cross  That is


times better.

Audio Wiring  Grounded audio returns are highly subject to



 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


, as every audio line returned though ground becomes mixed with every other audio signal.

Ground Loops  The answer is to


audio lines from the airframe ground, and process them as floating signals.

 This improves both noise and cross-talk significantly.

Composites 


have these problems:    RF shielding

dramatically reduced .

Ground resistance

dramatically increased .

Static discharge problems

dramatically increased .

Ground Topology

Floating Topology

Victim Wiring  Wiring


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



Victim Modes  The only real cure for Inductive Coupling is


. Remember the inverse square 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



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.


      

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      

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short and sweet.

[email protected]


any time for audio

Available from Dallas Avionics .

Reference stuff:


See us at BOOTH 103 / 801

Thank you!