Appendix I. CS110 as a slow
antenna
The CS110 can sample the external electric field at a maximum rate of 5 Hz (200 ms) using the
CS110() instruction. Faster sampling of the rapid electric field changes associated with lightning
discharges is desirable in some applications, and can be accomplished with the CS110 electric
field meter configured as a slow antenna, also known as a field change meter.
I.1 Slow antenna response in frequency domain
The CS110 as a slow antenna with the 250 μs integration responds to events as shown in (Figure I-
1 (p. 62)). The lower frequency limit is due to the measurement circuitry and the upper frequency
limit is a function of the integration time. Both are explained in the following sections.
The CS110Shutter() instruction can be used to fully open the shutter, indefinitely exposing
the sense electrode to external fields. Execution of the CS110Shutter() instruction with the
open command changes the CS110 panel board charge amplifier circuitry to a slow antenna by
switching in a 200 MΩ resistor in parallel with the 330 pF feedback capacitor, resulting in a (330
pF)(200 MΩ) = 66 ms decay time constant. In this slow antenna configuration, the charge
amplifier has a high-pass filter frequency response with the lower cutoff frequency defined by the
decay time constant such that f 3 dB = (2πRC) – 1 = 2.4 Hz. This means that events with
frequencies higher than 2.4 Hz (shorter than 417 ms) are passed through, while lower frequency
events are cut off . The –3dB point for voltage is:
The CS110 can measure the slow antenna output at rates up to 50 Hz (100 Hz may be possible but
it has not been tested), using the fast integration (250 μs integration) for the VoltDiff()
instruction. Voltage measurements using the 250 μs integration duration for the analog
integrator, result in an upper 3 dB bandwidth of 1.8 kHz (0.555 ms). The following shows the
combined effect of both filters.
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