Reference Section
•
83
Chapter 5
Filters
•
Low-pass and high-pass filters can be chosen to condition the Scaled Output and
Scope outputs. The -3 dB frequency is selectable from a list in the Output Signals
section of the main MultiClamp Commander window.
•
The type of low-pass filter (4-pole Bessel or Butterworth) is selected in the
General tab under the Options button (
).
•
The command stimulus can be low-pass filtered (with a 4-pole Bessel filter) at a
–3 dB frequency set in the General tab under
.
•
See also External Command Inputs, Headstage, and Noise.
The theory behind the design and choice of appropriate filters is very extensive, as you
will see from any book on signal processing. Here we provide just a few basic
principles that will assist you in choosing the filter type and cutoff frequency that are
most suited to your experiments.
-3 dB Frequency
The –3 dB, or cutoff, frequency (f
c
) of a filter is the frequency at which the output
signal
voltage
(or
current
) is reduced to 1/
√
2 (
i.e.
0.7071) of the input. Equivalently,
f
c
is the frequency at which the output signal
power
is reduced to half of the input.
These terms arise from the definition of decibel (dB):
Voltage: dB = 20 log(Vout/Vin)
Power: dB = 10 log(Pout/Pin)
For a low-pass filter, the frequency region below f
c
is called the pass band, while that
above f
c
is called the stop band. In the stop band, the signal attenuates (or ‘rolls off’)
with a characteristic steepness. (See Figure 4.10, noting the logarithmic frequency
axis.) The steepness of the roll-off at higher frequencies is determined both by the type
of filter (see below) and the number of poles of the filter: the larger the number of
poles, the faster the roll-off. The low-pass on the Scaled Output of the MultiClamp
700A are 4-pole filters. Filters with more poles can be constructed, but they are more
complex to implement and yield diminishing returns.
