The (effective) low-pass is only applied after the signals have been multiplied. This filters out the signal with the same frequency (and phase) as the reference. I have a more detailed comment on how this works in response to another person here in this comment section.
This is useful for applications, where the signal can be drowned out by the noise, but you know what to look for, and where the phase information is important. (e. g. a DC signal that is chopped at a selected frequency or an AM-signal).
The (effective) low-pass is only applied after the signals have been multiplied. This filters out the signal with the same frequency (and phase) as the reference. I have a more detailed comment on how this works in response to another person here in this comment section. This is useful for applications, where the signal can be drowned out by the noise, but you know what to look for, and where the phase information is important. (e. g. a DC signal that is chopped at a selected frequency or an AM-signal).