Transcript Noise Reduction for an Amateur Radio Repeater

Amateur Radio Repeater
Daniel Harkenrider
ECE 499 – Capstone Design
Advisor: Professor Hedrick
March 1, 2008
What is a Repeater?
 Receives a low-power
signal and retransmits
at a offset frequency
 Higher power and
greater sensitivity
allow for extended
range
 Especially necessary
for UHF
• Transmission must be
by line of sight
Reasons for Project
 Interest in communications
 Study elements of RF communications
• Circuits
• Transmission range
 Receiver design factors
• Sensitivity
• Noise
Uses of Repeater
 Study of RF communications
• Propagation, noise
 Relay amateur radio transmissions
 Modify for digital data transmission
 Emergency services
Factors affecting Radio Range
 Antenna height
• Especially for line of sight propagation
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Antenna gain
Receiver sensitivity
Transmitter power
Loss in transmission line
Repeater design objectives and
Criteria
 Tasks to make repeater operational
• Antenna in place
• CW ID installed and operational
 Design improvements
• Range – does not cover entire campus
• Increase to minimum 10 mile radius
• Improve receiver sensitivity
Antenna
 11.5 dB gain
 Installed on roof of
Science &
Engineering
• increased antenna
height
• Line of sight
Receiver Sensitivity
 Minimum signal power detected
 Signal to noise ratio (SNR)
 Amplification can boost weak signal, but
also noise
• How to maximize SNR in receiver
Noise
 From transmission or receiver components
• In receiver: thermal, shot, etc.
 Noise Factor: Each stage of receiver adds
noise, SNR decreases
 F = (SNRi/SNRo)
 Noise Figure = 10 log F (dB)
• Provides a parameter for analyzing noise
characteristics of component
Noise Source
 Based around
noise diode
• noise power is
proportional to the
diode current.
 Calibrated using
existing calibrated
noise source
 Produces 15.6 dB
ENR
Measurement and Calculation
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Noise power measured with spectrum analyzer
Noise power (noise source on): NON
Noise power (source off): NOFF
Excess noise ratio (ENR): (NON – NOFF ) / NOFF
Calculate noise factor NF
• Y = NON / NOFF = log10 (NON) – log10 (NOFF)
• NF (dB) = 10 log10 (10(ENR/10) / (10(Y/10) – 1))
Noise source calibration
 Calibrated noise source (26.2 dB)
 Add attenuators to source until power ratio for
calibrated source = ratio for unknown source
 ENRcalibrated – attenuation = ENRunknown
20.0V
DC
25.0V
DC
Noise
Source
Calibrated
Noise
Source
3 dB
attenuator
Attenuator
Spectrum
Analyzer
Spectrum
Analyzer
NON/NOFF
NON/NOFF
Preamplifier
 Improve signal to noise ratio of receiver
 Amplify signal close to receiver front end
• Signal power increases relative to internal
noise
 Operates in 70 cm band
 GaAs FET used for low noise
 Necessary to look at noise figure and
change in receiver sensitivity
SINAD measurement
• Signal-to-noise plus distortion
• Gives best measure of sensitivity
• At SINAD meter, signal is filtered from
noise, and the power levels are compared
Signal
generator
447.55 MHz
1 kHz tone
Preamp
Receiver
Audio
Output
SINAD
Meter
Receiver Sensitivity Results
 Without preamplifier
• 5.9uV for 20 dB
SINAD
 With preamplifier
• 0.8uV for 20 dB
SINAD
Preamplifier Noise Figure
 NON = -104.7 dBm
 NOFF = -114.0 dBm
 Noise figure = 6.84
dB
Completed
 Increased Range: Preliminary testing 12 miles
• Antenna placement
• Low-noise, high gain preamplifier installed
 Receiver design
•
•
•
•
Constructed test setup to measure noise figure
NF of preamplifier = 6.84 dB
Test setup to measure SINAD
Receiver sensitivity increased from 5.9uV to 0.8uV
 Legal / regulatory
• Obtained amateur radio license
• CW ID
Remaining tasks
 Measure loss in feed line from transmitter
to antenna
 More complete measure of range of
repeater
 Adjust preamplifier to reduce noise figure
Acknowledgements
• Professor Hedrick
• Jules Madey
• Robin Stevenson, EE 2006
• Slide 3:The ARRL Handbook. American Radio
Relay League, 1997. p. 23.4
• Slide 15: Jules Madey