Appendix e. cs110 as a slow antenna – Campbell Scientific CS110 Electric Field Meter User Manual
Page 63
Appendix E. CS110 as a Slow Antenna
As previously mentioned 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 which is sometimes called a
field change meter.
E.1 Response of the CS110 Slow Antenna in the
Frequency Domain
The CS110 as a Slow Antenna with the 250 µs integration responds to events as
shown in Figure E-1. 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 below.
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 M
Ω resistor in
parallel with the 330 pF feedback capacitor, resulting in a (330pF)
⋅(200MΩ) =
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
3dB
= (2
⋅π⋅R⋅C)
-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”
(search “cutoff frequency” in Wikipedia). The -3dB point for voltage is:
true
of
0.708
=
=
−
− 10
/
3
10
3dB
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). Figure E-1 shows the combined effect of
both filters.
E-1