This is the filter described here:

https://turnislandsystems.com/filters-filters-and-more-filters/

and here:

https://turnislandsystems.com/test-results-shelving-low-pass-filter/

It’s a 30 MHz four section elliptic low-pass filter, combined with a bypassable two-section shelfing filter. The low-pass filter serves as an anti-aliasing filter for a SDR, especially one that is clocked at 66 MHz or so. The shelfing filter will provide moderate attenuation at the lower frequencies, where powerful signals can cause overload. Each of the two shelfing filters can be bypassed or enabled, using the provided jumper clips.

The filter is available in the in-line plug/jack configuration, or inside a small aluminum enclosure (which helps in a strong-RF environment)

Filter Performance:

LPF, Shelfing bypassed

With and without both shelf filters enabled:

Please note that the -3dB cutoff is 28.3 MHz without the shelf filters. If your receiver lacks adequate sensitivity a preamp may be desired between the filter and the receiver. Also, this is a receive filter — transmitting more than a few milliwatts through it will probably damage it.

Enabling or bypassing the shelf filters:

The jumper positions are indicated on the board silkscreen:

‘F’ is the filter-in position
‘B’ is the filter bypassed position

The jumper pair to the left is one filter, the pair to the right is for the other. The filters are identical. The jumper pairs must be in the same mode or the signal path will be interrupted.

Here, both shelf filters are enabled.

And these filters really work!  Here’s a report from one of the first to try it with the KiwiSDR and the RX888:

“The LPF is *great* at suppressing aliases!” – Phil Karn

See the User’s Guide for more details