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Al's picture

Hello,

From your experiments it does sound like there's an issue with interference, probably from the dongle or the USB bus, these dongles are prone to it. Some people have tried shielding them or adding USB filtering to lower the noise.

The software attempts to get a flat frequency response from the dongle by taking 2 regions from the captured spectrum and averaging them.
To set this, go back to the 'Scan Offset' window, disconnect the antenna and replace it with your 50ohm load. Then click refresh and adjust the offset so the green areas cover the flattest parts of the spectrum. Typically there will be a spike in the middle the the response will roll off at the edges, 250kHz was chosen as it seems to work well with most tuners.
My FC0012 has a few spikes in this range so I find a 360KHz offset is better.

Setting the offset has no effect on bandwidth as this stays constant, it only affects which part of the spectrum is used for generating the plot.

Hope that clears things up a bit.