TECHNISONIC Understanding Noise in Airframe installations By: Walter Shawlee 2 Technisonic products...

TECHNISONICTECHNISONIC

Understanding Noise in Airframe installations

By: Walter Shawlee 2

Technisonic products are available from Dallas Avionics.

NoiseNoise

What exactly is “noise”?

Anything you didn’t want or expect to hear in the audio system.

System Noise =System Noise =

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

Noise is the Noise is the EnemyEnemy

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 NoiseTypes of Noise

In-band audio frequency noise.Cross-talk

Signal contaminationInverter/Generator Contamination

Types of NoiseTypes of Noise

Out-of-Band RF Sources:AM Comms

HF/SSBFM Comms

RF ProblemsRF Problems

Ship Issues can be multi-modal:Radio to Radio Interference

HarmonicDirect

Radio to Audio line rectificationAntenna Induced Standing Waves

AntennasAntennas

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 SolutionsRF 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-RFMechanical-RF

The Main Rotor will modulate outgoing transmissions, and incoming reception, and change reflected power.

Antenna PLACEMENT is everything to fix this.

Antenna IssuesAntenna Issues

Bottom Mounts work best for AM Comms on helicopters.

RF Interference RuleRF 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 & ProximitySpacing & Proximity

There’s rarely enough room in general aviation aircraft...

10 kg in a 10 kg in a 5 kg5 kg box box

Sometimes it’s just not that easy…

Audio WiringAudio Wiring

Wiring topology and routing have a large bearing on audio system performance.

No amount of shielding can overcome design limits.


Access/A (floating ground) technology can provide a 20-30dB improvement in noise and cross-talk over even high end grounded systems.

That is 100-1000 times better.


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 LoopsGround Loops

How we wish aircraft worked:


What really happens:


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


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


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.

CompositesComposites

Composites have these problems:

RF shielding dramatically reduced.

Ground resistance dramatically increased.

Static discharge problems dramatically increased.

Ground TopologyGround Topology

Floating TopologyFloating Topology

Victim WiringVictim Wiring

Wiring type determines susceptibility.

Victim WiringVictim 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 ModesVictim Modes

Inductive Coupling is CURRENT Driven


The only real cure for Inductive Coupling is INCREASED SPACING. Remember the inverse-square law.

Shields (unless co-netic material) are totally ineffective.


Inductive coupling sources:

Any high current line (AC or DC).Any inverter/magnetic switching

element.Internal switching regulators.


Capacitive coupling is VOLTAGE Driven.


Shielding is effective for Capacitive Coupling, IF the shield is a true shield, and not carrying current.

Spacing is also effective.

Susceptibility RulesSusceptibility Rules

Most Susceptible Wiring:

High ImpedanceUn-shielded/poorly shielded.

Low Voltage

SummarySummary

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.

SummarySummary

Never connect un-needed wiring.

Comms to rear stations.Nav Inputs to non-flight stations.Unterminated inputs

FinallyFinally

Every system has a noise floor determined by these factors:

Hardware design & topologyCablingGround/composite issues.

TECHNISONIC A711L SeriesTECHNISONIC 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 seriesTECHNISONIC 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-100ARTS-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.


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


See us at BOOTH 103 / 801

Thank you!

TECHNISONIC Understanding Noise in Airframe installations By: Walter Shawlee 2 Technisonic products...

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