Bandwidth Limit Filters
Many Tektronix amplifier plug-ins contain a selectable low-pass filter usually called the "BWL Filter" or "Bandwidth Limit Filter". Selecting the BWL filter is useful when using a high-bandwidth amplifier to observe low-frequency signals. The BWL filter reduces the noise in the displayed signal. It also reduces the tendency for high frequency noise in the signal to cause false triggering of the sweep.
The 7A11 contains a selectable 20 MHz BWL filter. It is a third-order balanced pi-type LC filter or O filter. Third order filters are also known as 3 pole filters whose attenuation asymptotically reaches 18 dB/octave.