Q: Why there are two different frequencies (LO / Tune) ? A: The LO frequency represents the SDR hardware's oscillator frequency. With an I/Q receiver this is at the centre of the waterfall/spectrum display. [ For IF receivers, e.g. 12 kHz, this is the leftmost displayed frequency in the waterfall/spectrum display. ] Many SDRs do not work quite so well close to the LO frequency, there may be a band of noise there. Soundcard SDRs may show a spike and some noise from ground loop effects. HDSDR's DC removal (Options - Input Channel Calibration for RX) will notch out the spike, normally "Auto" is effective. The tune frequency represents the software oscillator (NCO - Numerically Controlled Oscillator), which is used to tune the signal one wants to hear. Usually LO is set, so that a desired band with multiple signals can be seen and tuned simultaneously. This allows quick tuning to a signal with just single mouse click - without the need to move the LO - provided you can see all the desired band. RF (full spectrum) recordings may be made. You may later replay and tune to every visible signal - one by one. With wideband receivers this allows you to record all the AM (MW) band for example.
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