TPM System Work On Sept. 13 and 14,2005
D. Cussans, M. Johnson, G. Heath
Much progress had been made in the previous few weeks of work. The 4 terminal DC-toDC converter had been converted to a 3 terminal device by connecting the input and
output ground pins together. This reduced the common mode voltage on the system
ground by nearly an order of magnitude. What appears to be happening is that switching
noise in this high current regulator is coupled through the intrinsic capacitance in the
device onto the output terminals including the output ground. This power must get back
to the transformer in the regulator. When the converter was wired as a 4 pin device, this
path was quite long and complicated. The current had to travel through the signal ground
plane on the board, then through the signal ground on the backplane to single connection
to chassis ground. From there it probably flowed to the 48 volt filter or the local 48 volt
ground connection and then back to the regulator. Since this path is long and has a high
impedance, it is not surprising that there were 3 volt spikes on the signal ground relative
to chassis ground. Connecting the 2 ground terminals together substantially reduced the
impedance of this path and thus the voltage on the ground. Unfortunately, the wire
connecting the two grounds together is rather inductive. A simple calculation gives about
150 nH for this wire, which, in turn, gives 19 ohms impedance for a 20 MHz signal. We
replaced the wire with a 100 µ thick copper sheet that is slightly larger than the converter
(3 by 4 cm or so). The converter case was also fastened to this sheet. This change
reduced the noise on the signal ground to around 280 mV and also nearly doubled its
frequency. This indicates that the converter and its associated filter form a resonant
circuit. We also observed that the noise pulses looked like those from a shock excited,
under damped circuit. Note that there are probably other methods that could achieve the
same results.
Next, we connected a scope probe to the signal ground on the TPM module referenced to
chassis ground and one on the signal ground of the input module also referenced to
chassis ground. We also checked that the AC voltage difference between the 2 chassis
ground points was small. When we plugged in the cable from the TPM into the input
module we saw the voltage rise on the signal ground on the input module to around 300
mV. We then put a high frequency ferrite core on the cable and observed no change in
the signal ground scope trace. Since the ferrite core must add substantial impedance for
common mode currents, the only explanation is that there is little or no common mode
current flowing. This can only be true if there is high impedance between the signal
ground of the input module and the signal ground of the TPM module. The ground path
of the signal ground follows a path similar to the TPM module. The signal must go from
the IM to its backplane, then to the TPM backplane and finally to the TPM module. Thus,
it is not surprising that this path has high impedance. To fix this, we connected a 1.5 cm
wide ground braid between the signal ground on the input module and the signal ground
on the TPM module. No additional noise was observed on the IM signal ground when
this connection was made. Noise did appear when the signal wires were connected to the
IM and it was very similar to that on the TPM board. Since this noise only appears when
the signal wires are connected, it must be associated with transmitting signals between
the boards. Further evidence for this is that the noise on the IM signal ground was
reduced when the copper braid was put next to the signal cables. Moving the copper
braid next to the signal cables reduces the inductive loop for current traveling down the
signal cables and back through the copper braid. The noise on the IM ground was further
reduced when a ferrite core was placed on the signal wires. This indicates that there is a
common mode signal flowing on the signal cables.
The above evidence suggests that there is a small imbalance in the differential signals
between the boards. The differential transmitter on the TPM board is referenced to the
TPM ground so the imbalance voltage at the IM would also follow the ground. The braid
of the signal cables was attached to chassis ground which is poorly connected to signal
ground. The drain wire and Al foil shield is too inductive and resistive to provide a good
ground return. Thus, when the copper braid was installed, it provided a good ground
return which reduced the noise. The ferrite core increased the impedance for common
mode signals so it reduced the amplitude of the imbalance and, thus, the noise.
We then started operating the system and checking for bit errors. After 20 hours of
running (6 10^14 bits) there were no errors.
The LVDS signal standard has a high common mode rejection for voltages approaching 1
volt. This common mode rejection decreases with signal frequency and appears to be
about 0.5 volts for the TPM system. Above this threshold, the noise immunity is reduced
to nearly 0. It appears that the signal ground on the input module is moving with respect
to the signal ground on the TPM module. When this voltage gets to the LVDS threshold,
the noise immunity falls dramatically and bit errors occur. We triggered the scope on a
loss of lock signal and observed that the noise pulse that caused the loss of lock was
about 350 mV.
We also tried 2 TPM modules in adjacent slots. We observed that the signal ground
noise increased by 75 mV. We also noticed that the signal on the signal ground planes of
the 2 boards (referenced to chassis ground) was nearly identical and 180 degrees out of
phase. Separating the boards by one slot reduced the coupling substantially. Separating
the boards by half a crate did not reduce the coupling by much more. Thus, it appears
that the two boards are coupled by capacitance and that there is a circuit on the boards
that rings. The obvious candidate is the return circuit around the DC-to-DC converter.
To check this, we put in a 100 micron thick sheet of copper between the boards. We
observed that the signal in the second board decreased a bit in amplitude but switched to
being in phase with the driving signal. Although we tried to ground the copper sheet, we
did not have good access to the signal ground plane so the resistive and inductive
impedance between the copper and the ground plane is probably comparable to the
capacitive coupling between the copper shield and the two TPM’s. Thus it introduces a
roughly 180 degree phase shift in the coupling between the 2 boards. This is enough to
cause the phase to shift to a second stable point. Note that this shows that the coupling is
not through the magnetic field. An ungrounded sheet of copper will stop magnetic field
penetration as long as the copper is thicker than the skin depth. The skin depth for copper
at 1 MHz is 66 microns.
In summary, the board requires 3 modifications. Converting the 4 terminal regulator to a
3 terminal one with a good ground plane connecting the input and output ground appears
to work well. Adding one filter to each of the regulators may reduce the overall board
noise. Several application notes from various converter manufacturers use capacitors to
connect to the ground plane. Given the large bandwidth requirements, I would choose a
DC connection. I would also move all 3 converters to a rear corner of the board and
provide a ground mote between the converters and the serializers for the 3 GB link. An
even better approach would be to put the converters on a little daughter board that is
separated from the main board by standoffs. This would significantly improve the
isolation for the board. It would preserve the advantages of a 48 volt system and allow
for substantially better thermal management but it would require going to a 2 wide
module.
The signal and chassis ground need to be converted to one ground plane.
The drain wire and outer shield from the coax need to be attached to the signal ground on
both modules.