This is the  schematic of the ELFRAD Receiver which is used at our monitoring sites.

   The signal from the antenna inputs are channeled through F1 and F2 fuses which are used in combination with the Zenor diodes, D1 and D2. This combination is used to prevent damage to equipment during transients caused by electrical storms and other causes. U7 sections A,B,C,D, are configured as a signal input conditioner which passes signals from the input with low attenuation through 45 hertz. The low noise very high impedance field effect op-amps configured as voltage followers, give a high impedance input to the signal, while causing the filter circuits to see a virtual ground to the input voltage, which is necessary for active filter operation.  This signal goes to U6B, which is a 2 pole low pass filter with a corner frequency of 40 hertz. The passband gain at this point is X3.5.  Stage U6A  is a buffer amp with the output feeding U5 which is an 8 pole low pass Butterworth clocked filter with a cutoff frequency of 22 hertz.  The clock frequency is 2200 hz. The clock is generated internally with the frequency determined by R11 and C7. C10 is a non-polar capacitor which is used if desired to block background DC voltage from the earth. Switch S1 is normally in the closed position to pass all signals. U4D is configured as a buffer which passes the filtered signal to U4C which is a X10 amplifier. Again the signal is buffered by U4B and then goes to U3 which is another 8 pole low pass filter.  The clock frequency is again the same as U5 and obtains the clock signal from Pin 4 at U5. R1b is a voltage divider which supplies DC bias to pin 8 on U3. This balances the background DC voltage which is found between the antenna array probes. This is usually a one time adjustment for conditions found at each individual site.

U4A buffers the filtered signal to U2 which is an isolation amplifier. This stage is necessary to completely eliminate any ground loops and isolates your recording equipment from the antenna array. After leaving U1D the isolated and filtered signal is again filtered to remove any clocking noise which is generated by U5, U3, and U2. The gain of this stage is X1.5. U1B is a X5 amplifier and U1A is the final buffer. The total signal passband gain is 262.5.

The output will deliver a clean distortion free signal.   The processed signal at the output may be viewed with an oscilloscope, fed directly into a chart recorder, or passed along to an analog to digital converter card which is directly connected to a computer.  PC based data acquisition is recommended in order to continually detect, monitor, and record data on a 24 hour basis. ©1998