Nominal impedance
The term is applied in a number of different fields, most often being encountered in respect of: The actual impedance may vary quite considerably from the nominal figure with changes in frequency.
It is usual practice to speak of nominal impedance as if it were a constant resistance,[1] that is, it is invariant with frequency and has a zero reactive component, despite this often being far from the case.
Depending on the field of application, nominal impedance is implicitly referring to a specific point on the frequency response of the circuit under consideration.
It also became apparent that reflections on the transmission line would severely limit the bandwidth that could be used or the distance that it was practicable to transmit.
Each leg of the pair is connected to a relay coil which detects the signalling on the line (dialling, handset off-hook etc.).
It is possible to manufacture this kind of cable to have a 600 Ω characteristic impedance but it will only be this value at one specific frequency.
On the other hand, at high frequency in the MHz region, the characteristic impedance flattens out to something almost constant.
[4] Local area networks (LANs) commonly use a similar kind of twisted pair cable, but screened and manufactured to tighter tolerances than is necessary for telephony.
This is because the LAN data is in a higher frequency band where the characteristic impedance is substantially flat and mostly resistive.
50 Ω first arose as a nominal impedance during World War II work on radar and is a compromise between two requirements.
This standard was the work of the wartime US joint Army-Navy RF Cable Coordinating Committee.
This is not the same condition as minimum loss, because power handling is usually limited by the breakdown voltage of the dielectric.
The ideal ratio, Dr, for maximum power handling, is 1.65 and corresponds to a characteristic impedance of 30 Ω in air.
[8][9] While 30 Ω cable is highly desirable for its power handling capabilities, it has never been in commercial production because the large size of inner conductor makes it difficult to manufacture.
Cable with 75 Ω nominal impedance has been in use from an early period in telecommunications for its low loss characteristic.
For coax video cables and interfaces, 75 Ω is now the near universal standard nominal impedance.
The technique used is to carefully adjust the cable termination to obtain as close a match as possible and then to measure the variation of return loss with frequency.
The reason for this is that equally spaced errors introduced by the manufacturing process will cancel and be invisible, or at least much reduced, at certain frequencies due to quarter wave impedance transformer action.
[18] This is a common interconnection scheme, not just for audio, but for electronic units in general which form part of a larger equipment or are only connected over a short distance.
In a typical loudspeaker, the impedance will rise with increasing frequency from its DC value, as shown in the diagram, until it reaches a point of its mechanical resonance.
[30][failed verification] Oscilloscope inputs are usually high impedance so that they only minimally affect the circuit being measured when connected.
However, the input impedance is made a specific nominal value, rather than arbitrarily high, because of the common use of X10 probes.
A common value for oscilloscope nominal impedance is 1 MΩ resistance and 20 pF capacitance.
