When considering resistors, resistance is not the only important thing to think about. Like any other component there are a number of important things to consider. Microhm Electronics recommend resistors according to the resistance, tolerance, power, TCR, size and type demanded mainly. Frequency Response is also one of the key parameters to consider in certain special application, like radio frequency (RF).

 

Ideally, resistors should act as pure resistors, without any of the characteristics of other types of component and when they are used in DC circuits they do. In AC circuits however some resistors may have characteristics that make them unsuitable for a particular purpose. At high frequencies, some resistors also have characteristics of capacitance and/or inductance. Because of this they will have a property called reactance, similar to resistance but dependent on the frequency of AC signals passing through the component. The frequency response of a resistor tells us at what frequencies the resistor still acts as a pure resistor, without any significant effects associated with these other types of frequency dependent components. 

 

Carbon composition resistors although inferior to film type resistors in most other respects, act as pure resistors at frequencies in the Megahertz (MHz) range (at least those with a resistance below about 10KΩ) . Microhm provides carbon resistors NKT series, NTF series as well as film type including PMRY series, NTP series and more. Film type resistors having a spiral construction do tend to exhibit the properties of inductors (which are basically spirally wound coils of wire) but this is not usually a problem until used at frequencies in the MHz range. Film type resistors that do not have a spiral track, such as surface mount resistors remain purely resistive up to hundreds of MHz.
 

 

The resistors with the worst frequency response are not surprisingly wirewound types, as their construction is really a coil of wire - just like an inductor. Therefore the inductance and reactance effects must be considered when using wirewound resistors in any circuit operating at frequencies above a few hundred Hertz (Hz). Wirewound resistors are used for high power applications and are available in resistances up to a few KΩ. At higher resistances high power metal film resistors may be used, although they do not have as high a power rating as some wirewound types, they do have a much better frequency response, Microhm MPF series for instance.