Extending low frequency range limit down to 10 kHz
The SARK-110 lower frequency range is limited by the hardware and the software of the device to 100 kHz. This limit is not an issue for most use cases, but for some applications such as eddy current measurements it is required operation at even lower frequencies.
Note: these Hardware Modifications described below are already implemented in units sold starting September 2014
However, it is possible to overcome this limitation and prepare the unit for a low frequency range limit of 5 kHz by replacing six capacitors and downloading a variant of the firmware.
Due to the small size of the components (0402 size), you will need excellent soldering skills and proper tools to perform this operation safely.
You will need a complete set of calibration loads
The hardware modification consists in replacing the following capacitors: C62, C64, C67, C79, C81, and C84. The changed capacitors have a capacity of 220nF, dielectric X7R, and 0402 size. Following is the description of a capacitor available at DigiKey:
DigiKey P/N: 1276-1176-1-ND
Manufacturer P/N: CL05B224KO5NNNC
Description: CAP CER 0.22UF 16V 10% X7R 0402
Note that it is convenient purchasing additional spares because they are very easy to lose during the assembly.
The location of the capacitors in the board is illustrated in the Figure 1.
Figure 1, Changed capacitors
Updating the Firmware
There is a specific variant of the firmware supporting this lower frequency range. In order to differentiate from the standard firmware, the version identifier includes the suffix “b”, example “SARK110-VAA-APP0.8.5.3b.dfu.zip”.
After installing the new firmware, it is necessary performing the detector calibration first and then the OSL calibration.
One of the possible applications for the extended lower frequency range is eddy current measurement. In this application note I will not enter in the theory and details of eddy current measurements, but just in a practical measurement example.
Following reference is an excellent source of information about eddy current testing: An introduction to eddy current testing
For this example, I built the probe coil shown in Figure 2. This is built of 0.35mm copper, and four layers of about 25 windings per layer. The radius of the coil is 7.4mm and height 9.6mm.
As measurement targets, I used a FR4 board (Figure 3) and aluminium board (Figure 4).
Figure 2, Coil
Figure 3, FR4 Board
Figure 4, Aluminium plate
Before performing the measurements, it is necessary doing OSL calibration. For the measurements, select Scalar Chart mode and <Ls> for the left axis and <Zs> for the right axis. Then select the desired frequency range.
Following screenshots show the measurements in free space (Figure 5), FR4 board (Figure 6) and aluminium plate (Figure 7). You will easily observe the variations in the inductance of the probe coil due to the different conductivity of the measurement targets.
Figure 5, Measurements in free space
Figure 6, Measurements with FR4 board
Figure 7, Measurements with aluminium plate