LAN circuit Performance
Once all the parasitic parameters have been optimized, the transformer and the common mode choke are connected together and properly terminated as shown in the following typical 10/100 BaseT schematic.
The double ended model for this schematic is depicted below. The secondary parameters are reflected back to the primary side and analyzed from the primary perspective.
The single-ended model is shown below. Again, the secondary components are reflected back to the primary side for analysis.
Functional parameters:
The transformer total distributed capacitance is in parallel with the common mode choke interwinding capacitance Cww2, producing an equivalent shunt capacitance that is used in the equations for the Insertion Loss, the Return Loss and the Near End Crosstalk.
Insertion Loss – Low Frequency
The transformer primary inductance is the low frequency component that affects the insertion loss at the low frequency bandwidth.
Insertion Loss – High Frequency
The leakage inductance and the distributed capacitance are the high frequency components that affect the insertion loss at the high end of the bandwidth.
Where:
Return Loss – Low Frequency
The transformer primary inductance is the low frequency component that controls the return loss at the low end of the bandwidth.
dbwhere: LP = n2 (LS)
Return Loss – High Frequency
The leakage inductance and the distributed capacitance are the high frequency components that affect the return loss at the high end of the bandwidth.
Leakage Inductance effect
where
Distributed Capacitance effect
where
NEXT – Near End Crosstalk
Near end crosstalk exists when components or wires are very close to each other and the signal is coupled between channels by the accompanying capacitance between the two elements. The closer the elements are, the greater the crosstalk.
A spreadsheet has been created to provide an estimate of the magnitude of crosstalk that would be available when the size of the coils and their proximity to each other is input.
The first part of the calculation is performed with the dielectric constant K being equal to 1, or air. The summation of the capacitances between the transformers and the common mode choke is calculated for their existence in air alone. Then, the dielectric of the buffer is input into the attenuation formula for each frequency of interest. This affects the amount of capacitive reactance increase with frequency.
Capacitance formulas:
C1 = 0.224 ( K ) ( * ODXFMR ) (( 1 + 0.44 ( tXFMR ))2 ) ( 0.012 / tXFMR )
C2 = 0.224 ( K ) ( * ODCMI ) (( 1 + 0.44 ( tCMI ))2 ) ( 0.012 / tCMI )
CT = C1 + C2
Where K = 1, the dielectric of air.
Near End Crosstalk formula:
where: K = buffer dielectric at each frequency of interest