component B has no local connection to ground but still has some local parasitic
capacitance to ground. The latter condition (switch open) is in the star-grounded
configuration. At low frequencies, the star-grounded configuration has the advantage of
having no extra ground loop since the potentially large loop between the local ground
connections has been cut. Due to this, differential-mode emissions will be reduced. The star-
grounded condition also has the advantage of having only one 0-V reference voltage and is
immune to differences in the local 0-V potential and induced differences from the large loop
that would otherwise be present. All this works fine at low frequencies. However, recall that
the differential emission envelope (
), decreases above a certain frequency for
large loop size. Thus, the large loop that was avoided by using star grounding will radiate in
the differential mode primarily at lower frequencies, which are not usually the most difficult
emission problem in a high-speed digital system. Furthermore, for smaller systems in which
the ground loop avoided by star grounding may not be so large as to be purely a low-
frequency emitter, the capacitance to chassis ground at remote points (component B) can
contribute to a resonance that can create an efficient radiator at high frequency. Even with
no resonance, the capacitance closes the loop anyway, leaving no benefit of star grounding
at high frequency. Finally, a distant ground, such as necessary for star grounding, will quite
likely make the power delivery impedance too large for high-frequency current distribution.
Thus, although star grounding has several advantages at low frequency, it should generally
not be done on a high-speed digital system unless there is a compelling reason. A
motherboard, for instance, should have many connections to chassis ground and should
have low-impedance ground and power paths throughout. This is true particularly near high-
frequency regions because without the dc path to chassis ground, a capacitive path to
chassis may still be found and may excite a resonance. To summarize, in high-speed
systems, avoid star grounding.
Figure 10.19: Star grounding. If switch is open, the systems are star grounded.
The emission effects of star grounding can be seen graphically in
. At low
frequencies it can be seen that emission is reduced significantly; however, at chassis
resonances, there may be large peaks. Not evident in
is the fact that star
grounding could also increase the impedance in the power delivery path and cause large
common-mode voltages due to voltage drops in the power delivery system. This could
negate any emissive gains as well as ruin signal integrity.
component B has no local connection to ground but still has some local parasitic capacitance to ground. Furthermore, for smaller systems in which the ground loop avoided by star grounding may not be so large as to be purely a low- frequency emitter, the capacitance to chassis ground at remote points (component B) can contribute to a resonance that can create an efficient radiator at high frequency.