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 critical.

noise origins

is

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

other cables.

RF feedlines with  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

talk over even high end grounded systems.

in noise and cross  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 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

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

      

3 rd 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]

questions.

any time for audio



Available from Dallas Avionics .



Reference stuff: http://www.sphere.bc.ca/test/data.html

TECHNISONIC



See us at BOOTH 103 / 801



Thank you!