Device combinations for dielectric measurements
Typical parameters are:
- Frequency range: 1 mHz to 10 MHz
- impedance range: 1 kΩ to 1014 Ω
- tan(δ) accuracy < 3⋅10-4.
These systems are often used for dielectric and insulating samples where the charge transport is governed by molecular dipoles.
Most dielectric samples exhibit low conductivity; they thus behave in a first order approximation as a capacitor. Therefore, their impedance Z*(ω) at low frequencies generally becomes very high like e.g. 1014 Ω at 0.01 Hz. In addition, most dielectric samples have low loss factors tan(δ) below 10-2 down to 10-5
Due to their limited impedance range and tan(δ) accuracy, general impedance analyzers are hardly suitable for the characterisation of dielectric samples.
Instead most often special systems are used made up by an dielectric converter unit and a following frequency response analyzer (FRA) or lock-in amplifier.
Until 2001 such a system was offered by Novocontrol based on the Broadband Dielectric Converter BDC. This system is obsolete and was replaced by the Alpha analyzer, which extends the performance in many aspects at lower price.
The dielectric converter is a wide band electrometer amplifier which transforms the sample current into a voltage which is phase sensitive measured by the FRA or lock-in. For high phase accuracy, each sample measurement point is compared with a low loss reference capacitor.
The high phase accuracy applies only for capacitive samples with tan(δ) < 1 and if the sample capacity is close to the reference capacity. Somewhat conductive samples outside the range of the reference capacitor can still be measured within the system's impedance range. Anyhow, the Alpha analyzer or general impedance analyzers can measure these kind of samples typically with one decade better tan(δ) accuracy and down to about 4 decades lower impedance. Moreover, the Alpha analyzer offers about one order of magnitude improved phase accuracy.
As the measurement system is made up by two independent devices (dielectric converter and FRA or lock-in amplifier), they have to be controlled by a host computer which runs a special software. The software operates the both devices by low level functions so that they work as a single impedance system and in addition contains the user interface for experiment setup and data display.
This concept works fine as long as the software matches all user requirements. Problems arise, however, if the standard software can not be used for some reasons. This may happen if the user does not like the standard software, the impedance system has to be integrated in another system, or the user intends to use self-written software.