. A differential pair is any two transmission lines. Special case: symmetric lines. Differential driving has symmetric, opposite signal on each line. Differential impedance is the impedance the difference signal sees. With no coupling, current into one line depends on capacitance per length of the line.
Differential pair length matching for GigE/100M Ethernet
Differential pair length matching for GigE/100M Ethernet
There's a fairly common guideline for GigE/100M Ethernet PCB routing on the web (that I won't mention here, yet) that states to 'keep the differential pair trace lengths matched to 5 mils (0.005 in, 0.127 mm)'. Does anyone know the basis for that? It seems like a really nice design goal ('if you can hit that you'll have no problems'), but how much effort is it worth? It seems like overkill.
My thoughts:
125 MHz is the GigE carrier (on 4 pairs), so to keep the eye open I need to pass the 7th or eight harmonic (1 GHz).
FR4 has a propagation rate of 170 pS/in so 1 GHz has a wavelength on board of 5.88 inches. If I want my auto-transformer cancellation better than -40 dB (1%) then I need a length match better than 0.058 inches. Did they throw another 10x on that to minimize the error effect (-60 dB)?
It all seems a bit moot as the CM rejection on most GigE transformers drops from -50 dB at 1 MHz to 15 dB at 200 MHz so that extrapolates to negligible CM rejection (0 dB) around 1 GHz. The auto-transformer turns matching is only specified to +-2%, so worst case is only -34 dB. My EMI testing experience tells me you really aren't going to get much more attenuation than that with a single component.
So matching to 50 mils (1.25 mm) will not degrade the auto-transformer operation by more than -40 dB at 1 GHz (and 50 mils also happens to match another guideline on the web published by Intel). That seems more real-world.
Did I miss something?
Thanks,
Z
My thoughts:
125 MHz is the GigE carrier (on 4 pairs), so to keep the eye open I need to pass the 7th or eight harmonic (1 GHz).
FR4 has a propagation rate of 170 pS/in so 1 GHz has a wavelength on board of 5.88 inches. If I want my auto-transformer cancellation better than -40 dB (1%) then I need a length match better than 0.058 inches. Did they throw another 10x on that to minimize the error effect (-60 dB)?
It all seems a bit moot as the CM rejection on most GigE transformers drops from -50 dB at 1 MHz to 15 dB at 200 MHz so that extrapolates to negligible CM rejection (0 dB) around 1 GHz. The auto-transformer turns matching is only specified to +-2%, so worst case is only -34 dB. My EMI testing experience tells me you really aren't going to get much more attenuation than that with a single component.
So matching to 50 mils (1.25 mm) will not degrade the auto-transformer operation by more than -40 dB at 1 GHz (and 50 mils also happens to match another guideline on the web published by Intel). That seems more real-world.
Did I miss something?
Thanks,
Z
Differential pair length matching for GigE/100M Ethernet
Differential pair length matching for GigE/100M Ethernet
There's a fairly common guideline for GigE/100M Ethernet PCB routing on the web (that I won't mention here, yet) that states to 'keep the differential pair trace lengths matched to 5 mils (0.005 in, 0.127 mm)'. Does anyone know the basis for that? It seems like a really nice design goal ('if you can hit that you'll have no problems'), but how much effort is it worth? It seems like overkill.
My thoughts:
125 MHz is the GigE carrier (on 4 pairs), so to keep the eye open I need to pass the 7th or eight harmonic (1 GHz).
FR4 has a propagation rate of 170 pS/in so 1 GHz has a wavelength on board of 5.88 inches. If I want my auto-transformer cancellation better than -40 dB (1%) then I need a length match better than 0.058 inches. Did they throw another 10x on that to minimize the error effect (-60 dB)?
It all seems a bit moot as the CM rejection on most GigE transformers drops from -50 dB at 1 MHz to 15 dB at 200 MHz so that extrapolates to negligible CM rejection (0 dB) around 1 GHz. The auto-transformer turns matching is only specified to +-2%, so worst case is only -34 dB. My EMI testing experience tells me you really aren't going to get much more attenuation than that with a single component.
So matching to 50 mils (1.25 mm) will not degrade the auto-transformer operation by more than -40 dB at 1 GHz (and 50 mils also happens to match another guideline on the web published by Intel). That seems more real-world.
Did I miss something?
Thanks,
Z
My thoughts:
125 MHz is the GigE carrier (on 4 pairs), so to keep the eye open I need to pass the 7th or eight harmonic (1 GHz).
FR4 has a propagation rate of 170 pS/in so 1 GHz has a wavelength on board of 5.88 inches. If I want my auto-transformer cancellation better than -40 dB (1%) then I need a length match better than 0.058 inches. Did they throw another 10x on that to minimize the error effect (-60 dB)?
It all seems a bit moot as the CM rejection on most GigE transformers drops from -50 dB at 1 MHz to 15 dB at 200 MHz so that extrapolates to negligible CM rejection (0 dB) around 1 GHz. The auto-transformer turns matching is only specified to +-2%, so worst case is only -34 dB. My EMI testing experience tells me you really aren't going to get much more attenuation than that with a single component.
So matching to 50 mils (1.25 mm) will not degrade the auto-transformer operation by more than -40 dB at 1 GHz (and 50 mils also happens to match another guideline on the web published by Intel). That seems more real-world.
Did I miss something?
Thanks,
Z