Modelling of TPM noise problems

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

2. 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= m0(length) to within an order of magnitude and m0=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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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

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

11. 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