Inductance increases as the frequency approaches the SRF (self-resonant frequency) level. Therefore, the inductance level measured at low frequency is typically lower than that at higher frequencies. Along the same lines, the measured reading on most commercial-grade LCR test equipment is lower than on RCD’s military-grade test equipment since parts are generally tested at lower frequencies when using LCR meters. The difference is typically 2% – 15% but can be greater. Therefore, it is recommended that customers correlate sample readings with RCD or specify the required test frequency.
Another reason for correlation is due to the fact that there can be a fairly wide variation in the values measured between different set-ups resulting from variations in lead wire length and shape, as well as the intrinsic accuracy, capacitance & inductance of the test equipment and fixtures. Low values, such as 1uH or below are especially sensitive to the parasitics in test fixturing as well as the otherwise minor differences in inductance attributable to lead length, etc.
When testing inductors, there can be a fairly wide variation in the values measured between different set-ups, primarily due to test frequency, lead length, leadwire shape, test fixture capacitance & inductance, and test instrument intrinsic accuracy and self-inductance. Low values, such as 1uH or below are especially sensitive to the parasitics in test fixturing as well as the otherwise minor differences in inductance attributable to lead length, etc.
More commonly, the differences are attributable to the test frequency. Most of RCD’s inductors are measured at Mil-specified test frequencies (refer to data sheets). These frequencies (7.9MHz, 2.5MHz, 0.79MHz, 0.25MHz, etc) generally do not match the pre-set frequencies on most LCR meters (1MHz, 100KHz, 1KHz, etc.). Since inductance increases as the frequency approaches the SRF (self-resonant frequency) level, the measured reading on most LCR test equipment is lower than on RCD’s test equipment since parts are generally tested at lower frequencies when using LCR meters. The difference is typically 2% – 15% but can be greater. Therefore, it is recommended that customers “correlate” sample readings with RCD or specify the required test frequency.
Inductor amperage ratings are based on DC current. The AC current ratings are much lower, especially at higher frequencies. Designing AC inductors involves more factors. For instance, instead of dealing with resistance, AC inductor designs focus on impedance because the impedance is dominant over the DC resistance and varies with frequency. Material losses must also be accounted for. Another issue is the skin effect. When RF voltage is used the alternating current causes the current to migrate to the surface of the copper conductor. As a result, the center (core) of the conductor is barely utilized for carrying current. The best method to minimize the uneven heating associated with the skin effect is to utilize stranded instead of solid wire. For the same reason, high-frequency AC inductors are typically much larger in size than DC counterparts. RCD’s engineering group would need to know the voltage, current, frequency, and size constraints in order to determine the most appropriate design.
