than 6 dB per octave and a correspondingimprovement in phase distortion (deviation from linear phase). Although a transformer cannot have response to 0 Hz or dc, it canhave much less phase distortion than a coupling capacitor chosen for the same cutoff frequency. Or, as a salesperson might say“it’s not a defect, it’s a feature.”The simplified equivalent schematic of Figure 21shows the parasitic elements which limit and control high-frequency response. Except in bi-filar wound types discussed below, leakage inductance LL and load capacitance are the major limiting factors. This isespecially true when Faraday shields because of the increase in leakage inductance. Note that a low-pass filter is formed by seriesleakage inductance LL with shunt winding capacitance CS plus external load capacitance CL. Since this filter has two reactiveelements, it is a two-pole filter subject to response variations caused by damping. Resistive elements in a filter provide damping,Ddissipating energy when the inductive and capacitive elements resonate. As shown in the figure, if damping resistance Ris toohigh, response will rise before it falls and if damping resistance is too low, response falls too early. Optimum damping results inthe widest bandwidth with no response peak. It shouldbe noted that placing capacitive loads CL ontransformers with high leakage inductance not onlylowers their bandwidth but changes the resistancerequired for optimum damping. For most transformers,RL controls damping. In the time domain, under-damping manifests itself as ringing on square-waves asshown in Figure 22. When loaded by its specified loadresistance, the same transformer responds as shown inFigure 23. In some transformers, source impedancealso provides significant damping.In bi-filar wound transformers, leakage inductance LLis very low but inter-winding capacitance CW and winding capacitances CP and CS are quite high. Leakage inductance must bekept very small in applications such as line drivers because large cable capacitances CL would otherwise be disastrous to high-frequency response. Also note that a low-pass filter is formed by series RG and shunt CP plus CS. Therefore, driving sources maylimit high-frequency response if their source impedance RG is too high. In normal 1:1 bi-filar output transformer designs, CWactually works to capacitivelycouple very high frequencies between windings. Depending on the application, this can be either adefect or a feature.1.3.3Insertion LossThe power output from a transformer will always be slightly less than power input to it. As current flows in its windings, their dcresistance causes additional voltage drops and power loss as heat. Broadly defined, insertion loss(or gain) is that caused byinserting a device into the signal path. But, because even an ideal lossless transformer can increase or decrease signal level byvirtue of its turns ratio, the term insertion loss is usually defined as the difference in output signal level between the realtransformer and an ideal one with the same turns ratio.
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