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AGO LF HF Reciever Instructions Manual

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    							AGO Field Manual
    Dartmouth College LF-HF Receiver
    May 10, 1996 
    						
    							1
    1  Introduction
    Many studies of radiowave propagation have been performed in the LF/MF/HF radio
    bands, but relatively few systematic surveys have been made of natural emissions in this
    part of the spectrum.  The predominance of man-made signals in this frequency range
    requires a remote location and a radio receiver of specialized capabilities in order to
    search for natural emissions.  For instance, a receiving system must be capable of both
    detecting very weak signals, and be able to step around or null out the known sources of
    interference, such as AM broadcast stations. Furthermore, a receiving system must be
    able to operate at remote locations with only limited human intervention.
    The Automatic Geophysical Observatory (AGO) receiver was designed to run
    unattended for periods as long as one year constrained by severe power and data
    acquisition limitations.
    2  Radio Receiving General Principles
    The basic components of a single-conversion superheterodyne receiver are shown in
    Figure 1.  An incoming signal is received by an antenna and amplified before reaching a
    mixing stage. At the mixer, the received signal is multiplied or heterodyned with a known
    local oscillator (LO) signal to establish an intermediate frequency (IF).  In essence, it is
    the LO frequency that tunes the receiver to the desired reception frequency.  The IF
    signal contains frequencies equal to the sum and difference of the frequencies of the LO
    signal and the input signal from the antenna.  The difference frequency is selected using a
    tuned IF crystal filter.  The resulting signal is amplified, detected, and digitized.  There is
    no need for an automatic gain control in our receivers since we are looking for absolute
    signal strength and are not interested in keeping a constant output, as is typically
    desirable in commercial receivers.  Furthermore, receivers with two mixer stages
    (double-conversion receivers) are also not desirable since the dynamic range of such a
    receiver is usually downgraded through the addition of the second mixer. 
    						
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    							3
    3  Antenna and Preamplifier
    The AGO LF/MF/HF receiver employs a magnetic loop antenna which is less susceptible
    to locally generated noise than an electric dipole, especially when oriented to null out the
    strongest local signal.  The loop consists of a single turn of wire arranged in a square
    between two vertical 12-foot-long 4 x4  posts placed 3 m apart.  One horizontal wire
    runs along the snow, and the other connects the tops of the posts, so that the area of the
    loop antenna is 10 square meters.  Figure 3 shows the antenna as deployed at AGO-P2.
    The preamplifier is buried in the snow at the base of the antenna, with a pole or
    flag installed to make it easy to retrieve.  A schematic of the most recent version of the
    preamplifier is included in the  schematics  portion of this manual.  A critical component
    of the preamplifier is the calibration circuit, which allows the absolute level of the
    received signals to be calibrated.  For this purpose, a broadband calibration signal
    designed to be near the top of the instrument s range is injected approximately hourly.
    This signal is detected by the receiver with its nominal gain, and then detected with the
    gain reduced by 20 dB.  Using both of these detections, the gain and offset of the
    instrument can be accounted for, and the signals from the various AGO s can be
    compared.  Figure 2 shows the effective calibration circuit.  (The 600-Ohm resistor
    represents the input impedance of the preamplifier.)
    The voltage at the antenna terminals of the loop antenna is related to the electric
    field of the impinging EM radiation:
    dt dE
    c A
    V= (1)
    where A is the antenna area (10 m
    2), E is the electric field strength, and c is the velocity
    of light.  Assuming that the antenna can be considered a perfect inductor at the
    frequencies of interest,
    dt dI
    L V=(2)
    where I is the current in the antenna, this leads to the following relation after integration
    E
    c A
    LI=(3) 
    						
    							4
    Figure 1: The 10 m2 loop antenna installed at AGO-P2, Antarctica.  The AGO facility,
    along with two Scott tents, can be seen in the background.  The preamplifier is buried
    several feet under the checkered flag in order to keep it at a constant temperature. 
    						
    							5
    If a calibration resistor Rcal is placed in series with the antenna such that cal calR V I=, the
    calibration voltage (voltage at the antenna terminals) becomes
    E
    cL AR
    Vcal
    cal=(4)
    For our 10
    2 loop antenna, the electric field strength (V/m) and Vcal are related through the
    following equation,
    calV E01 . 0 =(5)
    which was obtained by substituting the appropriate measured quantities into the above
    equations.
    The least detectable signal of this receiving system corresponds to 
    V Vcalµ50 ≈.
    Therefore, the power spectral density of the received signal at the loop antenna is
    2 1/ 5mHz nV, assuming a 10 kHz bandwidth.
    4  Receiver
    The AGO LF/MF/HF receiver is located in the observatory, 300-500 feet distant from the
    antenna.  Figure 4 shows a block diagram of the AGO receiver.  Power from the DAU
    comes in on the specified connector and is converted to the required ±10 Volts and ±5
    Volts DC on the power supply board.  The mixing of the signals occurs on the receiver
    board; the LO signal used in the mixing is produced on the local oscillator board.  The
    signals are compressed and prepared for the DAU on the compression board.
    The receiver is tuned by adjusting the frequency of the LO signal. The sequence
    of frequencies to be tuned is programmed into an erasable programmable read only
    memory chip (EPROM), which is then clocked at the desired frequency-switching rate. In
    the AGO s, this EPROM is actually clocked at 20 Hz, twice the rate at which bytes are
    actually transferred to the DAU. The receiver compresses samples by a factor of two and
    passes one byte to the DAU each 0.1 s which most of the time represents two samples.
    The EPROM contains a sequence of 72000 steps, which are clocked at 20 Hz to control
    one hour of measurements. At the end of the hour, the EPROM is reset, and the cycle
    repeats. (The EPROM also contains a test program, to be used only for debugging, which
    is enabled by setting one address bit high using a DIP-switch.) 
    						
    							6 
    						
    							7
    The receiver as currently configured measures 116 frequencies from 30 kHz to
    4.5 MHz. The frequencies are not spaced linearly but are arranged to optimize reception
    of known natural signals such as auroral hiss and auroral roar. Furthermore, the
    frequencies are arranged in two sets of 58 frequencies with each of these subsweeps
    ranging from the low end of the frequency range to the high end but consisting of
    frequencies slightly offset. Using sub subsweeps provides higher effective time resolution
    for detection of relatively broadband signals. Table 1 at the end of this section gives the
    list of the 116 frequencies sampled. The 58 frequencies in the left column are sampled
    first, then the 58 frequencies in the right column are sampled. The 20-Hz data rate (with
    compression) implies that a full sweep of 116 frequencies is obtained each 5.8 s, and a
    subsweep of 58 frequencies is obtained each 2.9 s.
    Data compression is critical to the performance of the AGO LF/MF/HE receiver.
    The compression works by transmitting periodically a reference sweep of 120 bytes
    which provides 8-bit measurements of the logarithm of the received signal strength of
    each of the 116 frequencies (plus two sync-byte plus two  filler  bytes).  Following the
    reference sweep, 44 delta-sweeps are transmitted.  In these, only the change in each
    signal strength is transmitted, compressed to a 4-bit number.  Hence these 44 sweeps are
    require only 2640 bytes (44 times 60; two sync-bytes are attached to each delta-sweep).
    Following these delta sweeps, another reference is transmitted (which requires 120
    bytes), then 44 more delta-sweeps, and so on.  Once an hour, a calibration sweep is
    performed, consisting of 120 bytes: an 8-bit sample of the calibration signal at each of the
    116 frequencies, with the receiver at full gain for even samples and with gain reduced by
    20 dB for the odd samples; plus two unique sync-bytes, plus two  filler  bytes.  Thirteen
    blocks consisting of a reference and 44 delta-sweeps are transmitted between each
    calibration sweep; the result is a package of 36000 bytes transmitted to the DAU every
    hour, providing exactly the quantity of data (0.1 byte per second) allotted to the
    LF/MF/HF receiver.
    All of the numbers cited above — controlling the frequency of reference sweeps,
    calibration sweeps, etc. — can be changed by re-programming and replacing the EPROM.
    The number of sampled frequencies as well as the actual frequencies sampled may also
    be changed this way.  If in the future it becomes possible to store a larger quantity of
    data, the data rate can easily be increased by up to a factor of ten by adjusting DIP
    switches in the unit (without any change of hardware). 
    						
    							8
    The output of the receiver is available as an analog signal on the front of the
    receiver box, and instructions in this manual tell how a two-channel oscilloscope can be
    used to produce an image on the screen of power versus frequency.  Furthermore, a
    computer program has been written for DOS, which decodes the digital output of the
    receiver and produces a power-versus-frequency plot on the computer screen, which
    updates in real time. To use this program, the digital output of the receiver must be
    connected to the serial input of the computer. Figure 5 shows an example of a spectrum
    generated using this program in combination with one of the AGO LF/MF/HF receivers
    in the lab at Dartmouth. A version of this program is being prepared which will review a
    file of LF/MF/HF data extracted from the DAU. If a file can be produced which contains
    sequentially the bytes provided by the LF/MF/HF receiver, these bytes can be decoded
    and displayed on the screen as power versus frequency, updated either by the operator or
    at a rate fixed to the computer clock. All three of these tools will enable the operators in
    the field to determine whether the receiver is functioning.
    The AGO-based LF/MF/HF receivers should yield the most sensitive
    measurements to date in this frequency range. The capability of simultaneous
    measurements between several AGO sites also will enable the temporal and spatial
    effects of natural LF/MF/HF radio emissions to be studied. Finally, since each AGO will
    consist of a core group of synchronized geophysical experiments, correlations between
    data sets which are exactly co-located can be easily made. 
    						
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