Embed Size (px)
Citation preview
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.
Audio WiringAudio Wiring
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.
Audio WiringAudio 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 LoopsGround Loops
How we wish aircraft worked:
Ground LoopsGround Loops
What really happens:
Ground LoopsGround Loops
The “frame ground” is alive with voltages caused by different loads.
Ground LoopsGround 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 LoopsGround Loops
Ground currents also inject CROSS-TALK, as every audio line returned though ground becomes mixed with every other audio signal.
Ground LoopsGround 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.
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
Victim ModesVictim 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 ModesVictim Modes
Inductive coupling sources:
Any high current line (AC or DC).Any inverter/magnetic switching
element.Internal switching regulators.
Victim ModesVictim Modes
Capacitive coupling is VOLTAGE Driven.
Victim ModesVictim Modes
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.
TECHNISONICTECHNISONIC
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
TECHNISONICTECHNISONIC
See us at BOOTH 103 / 801
Thank you!
