Stanford Research Systems SR860 Operation Manual page 185

Appendix B
Press the [Filter Slope] key once to increase the
filter slope to 12 dB/oct.
The FFT Display
The peak amplitude at 99.000 kHz is
diminished even further. This is the effect of the
steeper time constant filter.
At the lock-in output, the 1 kHz output is
attenuated further by this filter.
This example has provided a quick overview of
the FFT display in the SR860.
SR860 DSP Lock-in Amplifier
167