Allan Isaacs

I'm looking at signals below the broadcast band so I made a low pass filter which is set to around 110KHz to avoid cross modulation etc. from broadcasts.
It gives me an attenuation of 40-50dB at 190KHz and broadcast stations are so strong they are easily audible even with 50dB attenuation from the low pass filter..
I now notice my Andrus behaves a little differently with the filter in place. I cannot see baseline noise, just a flat line if I increase the gain to 0dB or -12dB..
Previously, any gain setting of greater than -24dB resulted in cross modulation of VLF signals so the filter is doing its job.

My question:  Is it possible to reduce the display cut off  below -150dBm which is position of the flat line?
I can add -160dBm using View, Scale, Low but the flat line stays at -150dBm
If I set the gain to say -24dB or -30dB from 0dB the baseline noise becomes visible but signal to noise is poor. Increasing the gain to -12dB improves S to N and to 0dB improves S to N even more but both settings give me a flat line as in the attached picture. Am I missing a setting or is it a limitation of an AD converter?
Allan G3PIY

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