Amplifier 12

original source1)

Amplifier PCB 12 should be used for all magnetic loop antennas and all ferrite rod antennas. It uses four single supply 5 Volt rail-to-rail op-amp ICs for each channel. The inputs of the amplifier are terminated by damping resistors of 2.2kΩ. This leads to a more realistic current measurement for loop antennas and will reduce self resonances. For larger loop antennas, the damping resistor can be lowered to 100Ω- 300Ωin parallel to the antenna. A coax cable antenna can be terminated by 75Ω. The first and third stage of the amplifier are realized by programmable gain amplifiers (PGAs) MCP6S91 from Microchip. The gain of these amplifiers is controlled by a serial peripheral interface (SPI) and can be adjusted between 1 and 32 for each MCP6S91. The second stage uses the dual opamp MCP6292 from Microchip. The two operational amplifiers of this IC realize a Besssel high-pass filter with a cut-off frequency of approximately 1kHz. This filter is realized by two active Sallen-Key filter circuits. The second operational amplifier of the MCP6292 additionally increases the gain by a factor of 4 on version 12.3. After this stage a Besssel low-pass filter with a cut off frequency of approximately 50kHz is placed. This filter is realized by a passive low-pass filter chain of order five. The last stage is realized by the output driver amplifier LMH6642 from Texas Instruments. This stage additionally increases the gain by an factor of 10. The maximal overall gain of the amplifier is about 76dB. The maximal output current is 75 mA. The block diagram, circuit diagram, printed circuit board, and an assembled circuit board of Amplifier PCB 12 Version 3 are shown in Figure 28, 29, 31, and 32, respectively.

The electronic parts necessary for Amplifier PCB 12 Version 3 are listed in Table 1.

The antenna is connected to the terminal block. If you are using ferrite rod antennas with varnished wires, you should solder the stripped wires to bigger copper wires that can be grasped by the terminals screws.

The serial peripheral interface (SPI) for setting the gain of the PGAs is controlled by the Atmel ATmega8 micro-controller. The gain can be adjusted either manually by a potentiometer or automatically by the controller board. It is a digital gain control in both cases. If the green LED named C is on, the gain is adjusted by the controller, if it is off, the gain is adjusted manually. The manual gain control operates as follows. A potentiometer is used for setting a variable voltage between 0 and 5 Volt measured by one of the A/D converters of the ATmega8 micro-controller. It determines from the adjusted voltage the corresponding digital gain values and sends them to the PGAs via the serial peripheral interface. In this case, the gain can be adjusted in 12 steps displayed by the yellow LED gain index as shown in Table 2.

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.

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.

operational amplifiers IC1, IC3, IC5, IC7 MCP6S91 SMD SOI8
IC2, IC6 MCP6292 SMD SOI8
IC4, IC8 LMH6642 SMD SOI8
IC9 ATMega8
LEDs Power LED LED 3mm (red)
Control LED LED 3mm (green)
Gain LEDs 1,2,4,8 LED 3mm (yellow)
Diodes D1, D2, D3, D4 1N4148
Resistors, metal 1%, 0207 R3, R13, R18, R28 100kΩ
R6, R21 68kΩ
R5, R7, R12, R20, R22, R27 22kΩ
R2, R4, R14, R17, R19, R29, R31 10kΩ
R8, R23 6.8kΩ
R1, R9, R16, R24, R32, R33 2.2kΩ
R10, R11, R25, R26 1kΩ
R34, R35 330Ω
R15, R30 47Ω
Resistor network, star, 6-1 RN1 1kΩ A102
Potentiometer P1 50kΩ
Capacitors, electrolytic, 5mm C33, C34 470µF
Capacitors, electrolytic, 2mm C15, C30 2.2µF
Capacitor, ceramic, 2.54mm C1, C2, C7, C11, C12, C13, C14, C16, C17, C22, C26, C27, C28, C29, C31, C32, C35 100 nF (104)
C3, C18 22 nF (223)
C4, C6, C19, C21 10 nF (103)
C10, C25 6.8 nF (682)
C5, C20 4.7 nF (472)
C9, C24 2.2 nF (222)
C8, C23 470 pF (471)
Inductors, ferrite, 5mm L5 470/330µH
Inductors, SMCC L2, L4 3.3mH
SMCC L1, L3 1.5mH

Table 1: The list of electronic parts for Amplifier 12 Version 3


Table 2: Gain index of Amplifier PCB 12

During normal operation, the controller powers the amplifier through the Ethernet cable. But for the first hardware check, you should not connect the controller but connect the USB jack on the amplifier. After connecting a 5V USB power supply, the red power indicator LED should glow and the four yellow LEDs of the gain index should start flashing four times. The green LED should be off. A few seconds later the four yellow LEDs should display the adjusted gain. If the potentiometer is turned then the new gain value should be displayed, see also Table 2. The green LED is only on if the amplifier is controlled by the controller board. In this case the yellow LEDs have no meaning. A manual gain adjustment is not possible if the gain is controlled by the controller.

12.2 First release
12.3 Added additional gain factor 4 in high pass
Changed IC2 and IC6 to SMD versions
More space between L1/L2 and L3/L4
12.3b Minor layout changes
12.3c 12.3c Changed IC1, IC3, IC5, IC7 to SMD

Table 3: Change log of Amplifier PCB 12


Figure 28: Block diagram of Amplifier PCB 12 Version 3


Figure 29: The complete circuit diagram of Amplifier PCB 12 Version 3


Figure 30: The Printed Circuit Board of Amplifier PCB 12 Version 3


Figure 31: Assembly Diagram of Amplifier PCB 12 Version 3


Figure 32: An assembled Amplifier PCB 12 Version 3