HERE

Report
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!

similar documents