Modelling of TPM noise problems Greg, following discussions and measurements with David and Senerath

Ground rules (ha-ha) Try to understand the behaviour of our setups for high frequencies >1MHz –Model signal ground and all power planes as a single point in all circuits Should be a good approximation –Similarly model the 48V supply and return lines as a single point Probably a less good approximation? –Model “short circuit” wires as inductance Crudely: L=  0  (length) to within an order of magnitude and  0 =10nH per cm This corresponds to 1  per cm at 100 MHz –Ignore unwanted stray capacitance for now Since  0 is 0.1pF per cm, giving impedances >k  at 100MHz unless overlap area is very large Look at observed effects in GCT modules, with and without modification, and in various test setups Work out what to do to verify the model and design power supply layout for production GCT

The unmodified design Noise generators modelled as current sources Noise voltage measurements observed between signal and chassis grounds –3-5 Volt swing Common mode caps provide loop for the return of internal noise currents –Lower impedance than internal stray capacitance (or perhaps internal bypass caps) –If one set of caps is removed, currents can flow through earth connections at the back of the crate –Observed voltage swing implies currents are O(Amps) External noise currents flow mainly through the caps at the input –Not seen on signal planes

Modelling (I) Here’s a model of the circuit that has some of the right behaviour –Most components can be seen on the sketch –Added resistors to damp resonances –Tuned component values to match observed behaviour 5V

What does this do to the links? The IM looks similar to the TPM –Same chassis ground –It doesn’t have the common mode caps to chassis Signal planes are connected through the drain wires –This is correct according to National design note –Required to provide low impedance return path for common mode currents –Nevertheless: common mode rejection is limited and if voltages are too large the links will be unreliable Connecting the IM significantly reduces observed noise amplitudes on TPM –Factors of order 2 –Implies an alternative path for noise currents with impedance comparable to our common mode caps Through the drain wires (eight per cable assembly) to the IM signal ground Back through the rack metalwork to the supply input

First modification Direct connection from supply to signal ground –Initially suggested as a capacitor –But L dominates impedance Perhaps 3-5 times lower than paths to the back of the crate Observed noise voltage reduces by 2-3 –Impedance of direct connection is lower than common mode caps, but still comparable –Some further improvements with “fatter” connection (Cu tape) External noise now has a lower impedance path through the signal planes –Not observed in this configuration, internally generated noise dominates –But we didn’t look for the external noise

Modelling (II) Adding a small inductor to the model reduces the size of noise pulses –Note change of vertical scale on the plot 1V

Second modification Remove common mode caps between supply and chassis Internal noise voltage reduces by a further factor 2-3 –Return path through direct connection is now much lower impedance than loop through chassis ground Noise voltage seen on scope now dominated by external noise –This has got larger than before –The path through signal planes is still there, now with higher impedance

External noise source? Adding filtering to the 48V supply has little effect –Noise from this source is already quite low –Our input filtering is doing its job here The source of the external noise turns out to be the 5V crate supply on the VME backplane –Injects noise directly into the signal ground with no filtering Design error; the CM has a filter –The common mode caps were doing a job here Not so sure what the loop path is, perhaps something like this

Modelling (III) Removing the caps reduces the noise amplitude further The unwanted currents now flow through stray capacitance to the supply input 200mV

Conclusion We have reduced the noise amplitude in the existing TPMs by more than a factor 10 –Provide low impedance path between input and output supply A solid plane is needed –Break loops through the “chassis ground” Input filters should achieve this as long as there is no path to chassis between the filter and the DC-DC converter Do we understand what the path is in the modified TPMs? –The second problem comes from the common mode caps Or rather: would be much less obvious without them Perhaps if less obvious it would just have come to bite us later –It is particularly bad because one supply has a filter and the others don’t So noise generated by the 3V3 converter hits the filter and meets a high impedance in the loop through chassis But the common mode caps act on all supply planes (since they are capacitatively coupled together) And the noise generated by the other converters doesn’t meet any large series impedance Modelling reproduces qualitative features of the observed behaviour