| 327 / | HD | MISCELLANEOUS ACTIVE ELECTRICAL NONLINEAR DEVICES, CIRCUITS, AND SYSTEMS |
| 524 | DF | SPECIFIC IDENTIFIABLE DEVICE, CIRCUIT, OR SYSTEM {22} |
| 551 | DF | .~ Unwanted signal suppression {1} |
| 552 | DF | .~.~ Active filter {5} |
| 553 | | .~.~.~ Adjustable {2} |
| 554 | DF | .~.~.~.~> Switched capacitor filter |
| 555 | DF | .~.~.~.~> Selective type signal filtering (e.g., from low pass to high pass, etc.) |