When not using power planes, in order to prevent >100 MHz noise from travelling excessively around the board via the power distribution network, one can place filters just outside the decoupling caps nearest to the consumer, inline with the power supply connections. The classical topologies are T-Filters and Pi-Filters, both of which are available in small integrated form factors.
But when to use which filter topology ?
When simulating this naively with Spice, it looks like T-Filters would be almost always better to prevent noise entering the power distribution, using the same values for the L and C. At the same time, they are better to prevent noise from reaching the consumer. However, Pi-Filters seem to be much more popular.
So what am I missing ? And assuming my sims are wrong, what would be a scenario for T-filters ?
To simulate this, I set either V1 or the IC as a voltage source and observed the rejection on the other end, respectively. Typical values for the L and C of the filter are 100 nH and 100 pF, respectively. For both parts I included some typical parasitics, e.g. 0.1 nH for the C and 0.3 pF for the L, aswell as some ESR. I don't model the Network itself as a transmission line, but a inductor with parallel capacitance and parallel resistor of 10 Ohm. It is not an extended plane, but large polys over GND plane, so the impedance will be rather low. However, I am not sure what Rser to assume for the voltage source (when the IC is the voltage source). Putting a high Rser of e.g. 100 Ohms there makes the Pi Filter better, but I am not sure what Rser would be realistic.
As can be seen below, the only advantage of the Pi (red curves) in my sims, is less noise reaching the consumer between 1 and 10 GHz.