Impedance analyzer spectrum analyzers

S. Doerner, T. Schenieder and PR. Hauptmann, Wideband impedance spectrum analyzer for process automation... 

The capacitance and the series resistance have values which are not constant over the frequency spectrum. The performances may be determined with an impedance spectrum analyzer [70], To take into account the voltage, the temperature, and the frequency dependencies, a simple equivalent electrical circuit has been developed (Figure 11.10). It is a combination of de Levie frequency model and Zubieta voltage model with the addition of a function to consider the temperature dependency. 

Impedance measurements can be made in either the frequency domain with a frequency response analyzer (FRA) or in the time domain using Fourier transformation with a spectrum analyzer. Commercial instrumentation and software is available for these measurements and the analysis of the data. 

A disadvantage common to both classes of spectrum analyzer discussed so far is that they do not measure absolute amplitudes accurately, and they do not measure phase at all. Although this last limitation can be circumvented by the use of KK transformations (see Section 3.1.2.9), these instruments generally are poor choices for linear circuit (ac impedance) analysis. 

Electrochemical impedance measurement systems used for the analysis of the ac properties of electrochemical cells typically consist of a potentiostat (sometimes called an electrochemical interface) together with a frequency response analyzer (FRA) or a spectrum analyzer, or even a combination of the two. The potentiostat provides buffered connections to the cell under investigation together with circuitry for applying a controlled voltage or current stimulus and for the measurement of the dc properties of the cell. The FRA is connected through the potentiostat to the cell and therefore the bandwidth of the potentiostat is a very important consideration for accurate high frequency analysis. 

To achieve high sensitivity of impedance measurements it is recommended to consider frequencies (v) of the alternating electric field applied where the capacitance (C) of the sorbent/sorbate system is changing a lot, as is the case in the vicinity of resonance frequencies of the system. As these frequencies normally are unknown one has to take a spectrum C = C(v) in as broad a range of frequencies as possible and to select an appropriate characteristic frequency afterwards. This is laborious and requires an efficient impedance analyzer which could be fairly expensive. 

The electrical impedance, conductivity, real and imaginary part of dielectric permittivity and magnetic permeability have been determined using an Agilent E4991A RF Spectrum Analyzer. The used frequency was in the range from 1 MHz to 3 GHz. The sample thickness varied from 0.5 to 1 mm. The frequency step was 20 Hz. The tests were performed at room temperature (=25°C)... 

The flat surfaces of the samples were potished, cleaned and coated with silver electrode to get a better Ohmic contact for electric properties measurement. There are total five different electrode size used for electrode size study, as tist in Table-1. After electrode coating, the samples were poled under 3.0 kV/mm electric field at elevated temperature lOO C for Ih in the silicone oil bath. After polarization, the piezoelectric properties were surveyed with an HP4194A Impedance/ Gain-Phase Analyzer and Berlincourt ds3 meter based on the IEEE standards [IEEE, 1987]. Resonant spectrum of the samples was determined in the frequency range from 100 kHz to 1500 kHz, and the results were recorded by a PC base data acquisition system. 

Doemer, S., Schneider, T., Schroder,)., and Hauptmann, P. (2003) Universal impedance spectrum analyzer for sensor applications. Sensors, 2003. Proceedings of IEEE, Bd. 1, pp. 596-599. 

Proton conductivity is one of the key properties for predicting the PEM suitability, since a high conductivity is necessary for their effective utilization in fuel cell devices [21]. Conductivity measurements are performed on the acid form of the membranes using the special cells. This cell geometry is chosen to ensure that the membrane resistance dominates the response of the system. An impedance spectrum is recorded from 10 MHz to 10 Hz using an impedance/gain-phase analyzer. The resistance of the film is taken at a frequency that produces the minimum imaginary response. All the impedance measurements are performed at various temperatures and various RHs. 

Film thicknesses of the Glassclad-RC adhesion layers were measured with a Rudolph Research Auto-El ellipsometer. Conductance spectra (10,11) were obtained with a Hewlett-Packard 4192A impedance analyzer. Real-time measurements of the oscillation frequency of the QCM were made with a broadband oscillator circuit [10] built at UW and powered by a HP dual output power supply, a Philips PM6654 frequency counter, and a Kipp 2 nen XYY recorder. Note that the broadband oscillator circuit is designed to track the series resonant frequency of the QCM resonator in real time as its mass changes due to metal binding, while the impedance analyzer is used to characterize the entire resonance spectrum of the resonator. 

In studies of these and other items, the impedance method is often invoked because of the diagnostic value of complex impedance or admittance plots, determined in an extremely wide frequency range (typically from 104 Hz down to 10 2 or 10 3 Hz). The data contained in these plots are analyzed by fitting them to equivalent circuits constructed of simple elements like resistances, capacitors, Warburg impedances or transmission line networks [101, 102]. Frequently, the complete equivalent circuit is a network made of sub-circuits, each with its own characteristic relaxation time or its own frequency spectrum. 

Fig. 5. Complex-plane plot of impedance spectrum for a polycrystalline diamond film between two ohmic contacts. Frequency/kHz shown on the figure. Solid circles data obtained with ac bridge. Open circles data obtained with phase-sensitive analyzer. Top equivalent circuit [30].

The advantage of network analysers is the possibility of impedance measurement near resonance with evaluation of the parameters R, L, C and C0 and test of the equivalent electrical circuit. However frequency response and network analysers are relatively slow with 1-10 s per measurement in typical experiments. A new generation of faster instruments has come to the market like the HP E5100 Network Analyzer with 40 (is per point in the impedance spectrum which allows us to obtain the impedance of the system in less than 10 ms. 

More detailed information can be obtained from noise data analyzed in the frequency domain. Both -> Fourier transformation (FFT) and the Maximum Entropy Method (MEM) have been used to obtain the power spectral density (PSD) of the current and potential noise data [iv]. An advantage of the MEM is that it gives smooth curves, rather than the noisy spectra obtained with the Fourier transform. Taking the square root of the ratio of the PSD of the potential noise to that of the current noise generates the noise impedance spectrum, ZN(f), equivalent to the impedance spectrum obtained by conventional - electrochemical impedance spectroscopy (EIS) for the same frequency bandwidth. The noise impedance can be interpreted using methods common to EIS. A critical comparison of the FFT and MEM methods has been published [iv]. 

Another interesting photoelectroanalytical method for the characterization of polymer films is a method which might be called photoimpedance spectrum. A small-amplitude sine-wave signal is applied to the working electrode and the resulting absorbance response is recorded at different frequencies. Alternatively, several frequencies are applied simultaneously and the response analyzed by using Fourier transform. The main advantage compared with the conventional electrical impedance measurements is naturally that only faradaic current... 

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