morris cohen stanford...
TRANSCRIPT
What is a VLF receiver?
Antenna StorageMedia
SignalsTo Be
Studied
VLF Receiver
Frequency range: 100Hz – 50kHz
The AWESOME Receiver
Designed by Morris Cohen, Justin TanUltra sensitiveMedium cost (~$3000)Narrowband amplitude/phaseBroadband 100kHz dataDeployed worldwideAuto-calibration
AWESOME receiver overview
B-Field Antenna
Preamp Long Cable Line Receiver
Analog to Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Antenna Design
B-Field, magnetic loopOrthogonal pair1.0 Ω, 1.0 mH (high pass cutoff at ~159Hz)Various sizes (larger more sensitive)
Preamp
E/W Antenna
N/S Antenna
North
Antenna
Maxwell’s Equations show that magnetic field (B) perpendicular to loop of wire produces a voltage,
and current (I).
kE
B
I ∫ ∫ •=• dlEdABdtd
BA
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Preamplifier
Impedence matchedPaschal amplifier3 cutoff modesGain selectableCalibration circuitWeatherproof
Pre-Amplifier Card
LOW-NOISE PASCHAL AMPLIFIER
Voltage induced in antenna loop: V = (ω)(N)(A)(B) = (ω)(12)(1.69m2)(pT) / jωl
Voltage is amplified and noise is attenuated: V2 = αV1 + noise
Pre-Amplifier Card
CALIBRATION TONE GENERATOR
Generates special calibration tone that is used to convert digitally recorded values to magnetic field values
Pre-Amplifier Card
OPTIONAL FILTERING
Allows filtering of signals to obtain data at desired frequencies
-Ex: Remove noise from power lines (collect only f > 7kHz)
-Ex: Extend data range for quiet sites (collect as low as f > 80Hz)
Pre-Amplifier Box
E/W CHANNEL
N/S CHANNEL
TRANSFORMERS
Match the impedance of output driver of pre-amplifier to impedance of cable
CABLE TO LINE RECEIVER
AWESOME receiver overview
B-Field Antenna
Preamp Long Cable
Line Receiver
Analog to Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Preamp to line receiver
Long cablingBelden 1217BFour twisted pairs, shieldedProvides +/- 15VCalibration enable
Line TransformerCustom builtBetter low frequency responseMatches to 75-Ohm transmission line
Belden Cable
BELDEN 1217B CABLE
- Takes signal from pre-amplifier to line receiver
- Four twisted pairs, shielded
- Provides +/- 15V
- Calibration enable
- Impedance Z = 75Ω
- Max 3500 ft length from pre-amplifier to line receiver
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Anti-Aliasing Filter
LTC156212th order lowpassElliptical filter47 kHz cutoff90dB attenuation by 55 kHz<50us group delay
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Line Receiver GPS
Motorola M12+ OnCoreProvides 1 PPS signal
200 ns accuracyFPGA generates 100 kPPS
Divides down 10 MHzFeedback trims blockTime synched to 1 PPS100 ns maximum draft
Phase error < 2.7° at 25 kHzMore accurate TrueTime also available
Timing feedback
Motorola GPS unit supplies 1 PPS signalNeed PRECISE 100 kHz signal for sampling
Phase coherenceInterferometry
10 MHz Oscillator
1 PPS Signal
Counter 100x Divider
16-bit DAC
100 kPPSOutput
FPGA
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Ni-DAQ Card
NATIONAL INSTRUMENTS DATA ACQUSITION CARD
-Inserted into computer
-Data enters at 200 samples/s, N/S and E/W interleaved 100kHz per channel
-16 bit resolution
- +/- 5V
AWESOME receiver overview
B-Field Antenna
Preamp Long CableLine
ReceiverAnalog to
Digital
ComputerGPS
Antenna
A tmosphericW eatherE lectromagneticS ystem forO bservation,M odeling, andE ducation
Software
VLF_DAQ designed by Robb Moore, Eddie KimFlexible recording scheduleBroadband and narrowbandLive spectrogramsData sent over ftp
Data management
Data sent over FTP daily“Synoptic” data
Capture small amounts of data periodicallyOnline spectrogramsAutomatic transfer to external hard drive
500 GB hard drive ~350 hours of data“Pac-man mode”
Archive most recent few days continuous dataUseful for major events know shortly afterward