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John User

Centered Narrow Signals Recorded Have poorer quality

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I've noticed when recording with the WBT that when I'm trying to record a narrow signal (CW or low bandwidth) on the center of a tuner the playback quality seems worse.  If I move off the center of my signal just 5 kHz I no longer see that problem.  Larger signals (like 3.84 MHZ WCDMA) aren't showing any issue even when centered.  What's going on?

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Because the WBT uses direct conversion SDRs for recording and playback,the local oscillator (LO) is set to the center frequency. This is an artifact of the direct conversion process. In order to minimize the effects of this on recording and playback, there is a built-in filter to remove the LO contribution from the data.
 
While this filtering out of the DC offset does cause issues with narrowband signals that are located at the center frequency of the tuner, it has such a narrow bandwidth that it doesn’t generally affect wider signals.
 
The best way to work around this is to make sure you don’t center the tuner on narrow signals you wish to record, but offset it slight (The 5 kHz you mention above works, in general if it’s a narrow signal try to make sure none of it passes through the center of the tuner to avoid distortions).  For wider signals, especially the modern signals like WCDMA, LTE, WiFi they’re all designed to work with 0IF radios and can be centered without concern.  

Edited by Joey P
clarification

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The WBT uses a direct-conversion downconverter.  Tuners with this design may have a DC component that acts as an interfering signal at the center of the band.  It is narrow and has very little affect on most modulation types.  For CW and other narrow-band modulation types, just tune a little off the carrier center frequency and, as you noted, it works very well.

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