Impedance Spectroscopy
525
2.1.7 Automated Frequency Response Analyzers (FRA)
The impedance in FRAs is determined by correlating the response of cell with two
synchronous reference signals [29–31]. One of the reference signals is in phase
with the sine wave stimuli while another is phase shifted by 90º. FRAs can determine harmonics present within the output by multiplying reference signal with the
harmonic coefficient. These characteristics of FRAs prove to be a powerful tool for
exploring nonlinear systems [32]. This is a very convenient, precise and wide bandwidth technique for determining impedance. Commercially available instruments
are precise upto 4 and ½ digits in the range of frequencies from 10
–4 to 10
6 Hz.
Measurements are done directly so they are not vulnerable to limitations suffered by
potentiostat control. Since these instruments are very precise, considerable knowledge about electrical systems is important to obtain accurate impedance data, mostly
at higher frequencies. It is often preferred to use an oscilloscope in parallel with
the input channels of an FRA so that the form of input and output signals can be
monitored continuously. But these analyzers are quite expensive.
2.1.8 Automated Impedance Analyzers
The operation of these types of analyzers involves an autobalance bridge, a range
resistor and null detector. These analyzers are usually not used for measurements
in electrochemical systems. High speed and precision are some of the advantages
of these instruments. Effects of stray capacitances are greatly reduced since these
analyzers use the null detection technique. This method can measure impedance at
very high frequencies such as tens to hundreds of MHz. So, it finds application in
network component analysis. It suffers from limitations such as two terminal nature
i.e. potential across sample changes during the balance procedure and dc voltage
cannot be applied to electrode with respect to reference. These devices are restricted
to operate at low frequencies (up to 1 Hz) because of the instability of analog filters.
2.1.9 Spectrum Analyzers
Spectrum analyzers characterize signals in the frequency domain. These analyzers
can be utilized to measure impedance quickly at audio or higher frequencies. Basic
function of such type of analyzers is to measure amplitude or power of signal at
numerous discrete points within a definite range of frequencies. This is accomplished
by applying an unknown signal to filters arranged parallelly; with each filter adjusted
to pass a narrow band of frequencies. Speed and simplicity are the benefits of this
technique. There are two types of spectrum analyzers: (1) parallel filter and (2) swept
filter analyzer. The swept filter analyzer has higher resolution and is generally cheap.
It cannot be utilized to analyze transient events because all frequencies cannot be
seen simultaneously. The parallel filter analyzer has limited resolution and is costly.
Dynamic signal analyzers have been developed by combining the best properties of
525
2.1.7 Automated Frequency Response Analyzers (FRA)
The impedance in FRAs is determined by correlating the response of cell with two
synchronous reference signals [29–31]. One of the reference signals is in phase
with the sine wave stimuli while another is phase shifted by 90º. FRAs can determine harmonics present within the output by multiplying reference signal with the
harmonic coefficient. These characteristics of FRAs prove to be a powerful tool for
exploring nonlinear systems [32]. This is a very convenient, precise and wide bandwidth technique for determining impedance. Commercially available instruments
are precise upto 4 and ½ digits in the range of frequencies from 10
–4 to 10
6 Hz.
Measurements are done directly so they are not vulnerable to limitations suffered by
potentiostat control. Since these instruments are very precise, considerable knowledge about electrical systems is important to obtain accurate impedance data, mostly
at higher frequencies. It is often preferred to use an oscilloscope in parallel with
the input channels of an FRA so that the form of input and output signals can be
monitored continuously. But these analyzers are quite expensive.
2.1.8 Automated Impedance Analyzers
The operation of these types of analyzers involves an autobalance bridge, a range
resistor and null detector. These analyzers are usually not used for measurements
in electrochemical systems. High speed and precision are some of the advantages
of these instruments. Effects of stray capacitances are greatly reduced since these
analyzers use the null detection technique. This method can measure impedance at
very high frequencies such as tens to hundreds of MHz. So, it finds application in
network component analysis. It suffers from limitations such as two terminal nature
i.e. potential across sample changes during the balance procedure and dc voltage
cannot be applied to electrode with respect to reference. These devices are restricted
to operate at low frequencies (up to 1 Hz) because of the instability of analog filters.
2.1.9 Spectrum Analyzers
Spectrum analyzers characterize signals in the frequency domain. These analyzers
can be utilized to measure impedance quickly at audio or higher frequencies. Basic
function of such type of analyzers is to measure amplitude or power of signal at
numerous discrete points within a definite range of frequencies. This is accomplished
by applying an unknown signal to filters arranged parallelly; with each filter adjusted
to pass a narrow band of frequencies. Speed and simplicity are the benefits of this
technique. There are two types of spectrum analyzers: (1) parallel filter and (2) swept
filter analyzer. The swept filter analyzer has higher resolution and is generally cheap.
It cannot be utilized to analyze transient events because all frequencies cannot be
seen simultaneously. The parallel filter analyzer has limited resolution and is costly.
Dynamic signal analyzers have been developed by combining the best properties of