AGO ELF / VLF Receiver System

The following is a block diagram of the ELF / VLF Receiver instrument which is part of the Automatic Geophysical Observatories.  AGO sites P1-P4 contain the full system, P5 has only the narrowband portion, and currently there is no VLF system installed at the sixth AGO site.  The diagram shows the 1998 configuration.  Before 1998, both the N/S antenna and the E/W antenna where 1.70 m. sq, and also the narrowband system had a different channel configuration.
 

ELF / VLF Broadband Snaphot

The Broadband System records 2 seconds of broadband data three times each hour ( at  5, 20, 35 minutes and 30 seconds after the hour) in the frequency range of 1 to 3 kHz.  The broadband data is recorded on channel 16.  The following block diagram details the processing of the broadband snapshots.  The broadband data is recorded from the N/S antenna, it is filtered and then sampled at 25kHz and digitized. Next, the signal is multiplied with a complex exponential at the frequency w.  This has the effect of shifting the frequency range of interest down to the baseband. Currently, the mixer frequency is set at 2 kHz.  The now complex signal is lowpass filtered and decimated by a factor of ten.  The resulting signal has an effective sampling rate of 2.5kHz.  The processed broadband data is then sent serially by alternating the 8-bit real and imaginary components to the AGO data acquisition unit.  The AGO Data Format section details the format of the broadband data on the AGO CD-ROMs.  Here are a few examples of processed ELF/VLF Broadband Data.
 

ELF / VLF Narrowband System

The narrowband portion of the VLF receiver records continously in the frequency bands listed in the tables below.  In 1998, two new channels of data were added and some of the frequency ranges where changed as well.  For details on exactly when the channel configuration was changed at each site, see the Calibration page.  There are two different types of filters within the narrowband system.  The first are the 'Hiss' filters which are tuned to a range of frequencies and have a width of .5 to 16kHz.  The second type of filters are tuned to the specific frequencies of VLF Navy Transmitters.  After the channels are filtered, the signals pass through a detector / integrator circuit and then are sampled (12 bits) at a rate of  1 or 2 Hz before being set to the AGO data acquisition unit.  The AGO Data Format section details the format of the narrowband data on the AGO CD-ROMs.
 
P1-P4 Pre-1998 and P5 Pre-1999 Channel Configuration
Channel
Antenna
Frequency
9
N/S
 1 - 2 kHz
10
N/S
 2 - 4 kHz
11
N/S
 30 - 40 kHz
12
E/W
 1 - 2 kHz
13
N/S
 23.4 kHz (NPM)
14
N/S
 24.0 kHz (NAA)
15
N/S
 24.8 kHz (NLK)

 
P1-P4 1998 and P5 1999 Channel Configuration
Channel
Antenna
Frequency
9
N/S
0.5 - 1 kHz
10
N/S
1 - 2 kHz
11
N/S
2 - 4 kHz
12
N/S
4 - 8 kHz
13
N/S
8 - 16 kHz
14
N/S
16 - 32 kHz
15
N/S
30 - 40 kHz
19
E/W
1 - 2 kHz
20
N/S
24.0 kHz (NAA)

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