Amplifier 13

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The amplifier/pre-amplifier system PCB 13 and PCB 14 is designed for an electric field antenna. The system consists of an amplifier board (PCB 13) with three different filter systems and a pre-amplifier board (PCB 14) with a very high input impedance. Any vertically oriented wire or copper plate connected to the input of Pre-Amplifier PCB 14 can be used as e-field antenna.

The most important thing in building an e-field antenna is the search of the right site. Electric antennas work best on non-developed areas. The utility frequencies of the main electricity is in general between 50 Hz (Europe) or 60 Hz (America and parts of Asia). These frequencies, their harmonics, and other very low frequencies are well filtered out by the high pass filter of pre-amplifier PCB 14 and amplifier PCB 13. The combined high pass filter starts at 5 kHz and dims frequencies around 50 Hz /60 Hz with more than 150 dB, see Figure 33.

Figure 33: The high pass filters of Pre-Amplifier PCB 14 and Amplifier PCB 13

The signals have to be filtered, because at certain regions the very low frequency range is disturbed by various radio transmitters, see for These signals can considerably disturb our measurements, on the one hand, because the lightning signals can not be triggered and on the other hand, because the superposition can make it difficult to determine characteristic time stamps. The challenge is to find a filter that reduces most of the disturbances without destroying the measurements. Obviously, there is no ideal filter that suppresses all types of disturbances equally well. Amplifier PCB 13 has three different low pass filters, a 50KHz Bessel filter to keep compatible to the H-field amplifier PCB 12, a sharper 44KHz elliptic filter, and a very ’rough’ 18KHz elliptic filter, see Figure 34. The filtered signals can be used for different purposes. The signal after the 50KHz Bessel filter is optimal for computing characteristic time stamps. The signals after the 44KHz and 18KHz elliptic filters can be used to trigger in disturbed regions and for a more simple computaton of the polarity. This is a feature which is possible with e-field antennas.

Figure 34: The low pass filters of Pre-Amplifier PCB 14 and Amplifier PCB 13

Electronic parts

The electronic parts necessary for Amplifier PCB 13 Version 1 are listed in Table 4.

operational amplifiers IC1, IC3, IC5, IC7 MCP6S91 SMD SOI8
IC4, IC6, IC8 LMH6642 SMD SOI8
IC9 TLE2426
IC10 ATMega8
LEDs Power LED LED 3mm (red)
Control LED LED 3mm (green)
Gain LEDs 1,2,4,8 LED 3mm (yellow)
Resistors, metal 1%, 0207 R2, R11, R17, R23 100kΩ
R5, R7 39kΩ
R4 27kΩ
R10, R16, R22 22kΩ
R6 12kΩ
R3, R12, R18, R24, R62 10kΩ
R8, R9, R15, R21 1kΩ
R60, R61 330Ω
R14 220Ω
R20 180Ω
R1 75Ω
R13, R19, R25 47Ω
Resistor network, star, 6-1 RN1 1kΩ A102
Potentiometer P1 50kΩ
Capacitors, electrolytic, 5mm C62, C64 470 µF
Capacitors, electrolytic, 2mm C17, C27, C37 2.2 µF
Capacitor, ceramic, 2.54mm C1 220 nF (224)
C3, C4, C9, C13, C14, C15, C16, C23, C24, C25, C26, C33, C34, C35, C36, C60, C61, C63, C65 100 nF (104)
C30, C32 68 nF (683)
C20, C22, C29 33 nF (333)
C28 22 nF (223)
C18 15 nF (153)
C31 10 nF (103)
C12, C19 6.8 nF (682)
C7 4.7 nF (472)
C5 3.3 nF (332)
C11, C21 2.2 nF (222)
C8 1.5 nF (152)
C2, C6 1.0 nF (102)
C10 470 pF (471)
Inductors, radial, 5mm 09P L8 4.7 mH
L6 2.2 mH
L5 1.5 mH
L3/L4 1 mH
L7 470/330 H
Inductors, axial SMCC L2 3.3mH
SMCC L1 1.5mH

Table 4: The list of electronic parts for Amplifier PCB 13 Version 1

Figure 35: Block diagram of Amplifier PCB 13 Version 1 and Pre-Amplifier 14 version 1

Figure 36: The complete circuit diagram of Amplifier PCB 13 Version 1

Figure 37: The Printed Circuit Board of Amplifier PCB 13 Version 1

Figure 38: Assembly Diagram of Amplifier PCB 13 Version 1

Gain Control

The gain control is analogous to the gain control of amplifier PCB 12.

Connecting the controller

The RJ45 modular connector is used to connect the amplifier to the controller board. You should use a shielded one-to-one CAT 5e network cable. The length of the cable can be up to 30 meters. This allows us to place the amplifier near the antennas far away from noisy environments.

Output test pins

The output test pins can be used to monitor the output of the amplifier. This is useful if you want to connect an oscilloscope or a sound card to analyze the VLF output without to disconnect the amplifier from the controller board.

First hardware check

Figure 39: An assembled Amplifier PCB 13 Version 1