The schematic has been designed using the CadSoft EAGLE Layout Editor.

The RF pre-amplifier section is built around one non-inverting amplifier (IC3A) with a gain of 200 and an inverting amplifier (IC3B) with a gain adjustable between 0 and 25 by R7.

The signal is then filtered in the tuning section. It feeds a MAX275 (IC4), which is a continuous-time 4th-order active filter. The MAC275 is configured as a bandpass filter; center frequency and gain are adjusted by R101/R201, R102+R100/R202+R200, R103/R203 and R104/R204.
A table listing resistor values for some VLF stations can be downloaded here. It has been defined for a quality factor of each 2nd-order section of 170 and a gain of 4.

MAX275 tuning resistor values

(application/pdf, 11Kb, 02 Jul 2007)

For calculating tuning resistor values for other frequencies, refer to the MAX275 datasheet that contains all formulas.

The next section is the RF post amplifier. The first stage is an inverting amplifier (IC6A) with a gain selectable from 2, 5 or 20. The second stage is a full-wave rectifier and averaging filter peak detector build around IC7A and IC7B. A strength signal is available at the output of this stage.

In the analog output section, IC5A and IC5B filter out and amplify the signal.

Finally, the analog to digital conversion is performed by the 12-bits ADC MAX187 (IC10). Interface to RS232 signal levels is done through a MAX220 (IC8).

Eagle 4.14 Schematics		(application/octet-stream, 326Kb, 21 Jul 2007)
Schematics in PDF format		(application/pdf, 157Kb, 21 Jul 2007)
Components List		(application/pdf, 9Kb, 02 Jul 2007)

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