a) Resistive shunts
Current flow may be determined by measuring the voltage drop across a resistor connected in series with a circuit. Resistors have a parasitic inductance and stray capacitance associated with them. The ratio of parasitic inductance to pure resistance determines the high frequency limit of the measurement. Increasing the value of resistance to reduce this ratio can cause dissipation and insertion loss problems.It is not easy to produce a low-resistance shunt capable of matching the high-frequency performance of broadband transformer, even with the aid of coaxial techniques. The resistor also suffers the disadvantage of not providing isolation from the circuit under test. If this is not important, then resistive shunts can prove useful for the measurement of low current for frequencies stretching from dc into the MHz region or, for higher currents, to a few hundred kHz.
Coaxial resistive shunts with very low values of parasitic inductance may be used at moderately high currents to frequencies up to about 20MHz. Arguably, the principal advantage of a current transformer is that it provides isolation from the circuit under test. It also imposes negligible burden on the circuit.Lilco terminated transformers can be operated to 100MHz and beyond and some unterminated transformers to about 10MHz
A transformer will not measure the dc component of a waveform nor, except in the special case of air-cored transformers, operate in the presence of high dc. However, some designs will operate with dc currents up to several hundred amps.