You may have noticed that the behavior of the VFO and filters is different with CTUN on vs. CTUN off.  The only difference in behavior that should happen is that with CTUN on, the spectrum (panadapter) should stay constant (i.e. stationary) and the VFO indicator should move around the display as you tune. With CTUN off, the spectrum moves and the VFO indicator stays in the center of the display.

However, there were some quirks in the CTUN behavior in 3.4.1 and earlier releases of PowerSDR/OpenHPSDR mRX.  These included:

• Changing modes in CW didn’t keep the signal centered in the passband when in CTUN mode, whereas it does this nicely when CTUN is off.
• Tuning the VFO indicator beyond the edge of the display was possible when zoomed in, and when not zoomed in, tuning would just halt at the display’s edges.
• Restoring a Quick-Memory frequency often would place the VFO outside the visible display range – same for VFO operations such as VFOB>A or A<>B.
• If you start with the VFO off center in CTUN mode, zooming in could position the VFO indicator outside the visible display range.

Most of these are fixed now in release 3.4.2, although tuning near the display edge still needs some work to make it smooth.

The following list describes how mode changes are handled now, whether CTUN is on or off, and are identical to the way they always worked with CTUN off in previous versions (examples are written for the case where CWPitch is set to 600Hz). For clarity in these descriptions, “receiver” refers to the actual received frequency, “VFO” or “VFOdisp” refers to the frequency displayed in the VFOA window, “VFO indicator” refers to the vertical line in the display corresponding to the frequency displayed in the VFO window, and “spectrum” refers to the panadapter display itself.

• Spectrum stays constant – filter adjusts – VFOdisp shifts down CWPitch – receiver doesn’t shift frequency – CWPitch offset taken into account in CW,
• e.g. in LSB, a 3510 CW signal reads as 3510.6 when tuned to 600Hz tone, then in CWL VFO shifts down to read as 3510 with the same 600Hz tone, as if the receiver is tuned 600Hz above what the VFO indicates

• Spectrum stays constant – filter adjusts – VFOdisp shifts up CWPitch – receiver doesn’t shift frequency – CWPitch offset is removed
• e.g. in CWL, a 3510 CW signal reads as 3510 when tuned to a 600Hz tone, as if receiver is tuned 600Hz above what the VFO indicates, then shifts up in LSB to read 3510.6 still with a 600Hz tone, meaning the VFO and receiver match

• Spectrum stays constant – filter adjusts – VFOdisp shifts up CWPitch – receiver doesn’t shift frequency – CWPitch offset taken into account in CW,
• e.g. in USB a 3510 CW signal reads as 3509.4 when tuned to a 600Hz tone, then in CWU the VFO shifts up to read as 3510 with the same 600Hz tone, as if the receiver is tuned 600Hz below what the VFO indicates

• Spectrum stays constant – filter adjusts – VFOdisp shifts down CWPitch – receiver doesn’t shift frequency – CWPitch offset is removed,
• e.g. in CWU, a 3510 CW signal reads as 3510 when tuned to a 600Hz tone, as if the receiver is tuned 600Hz below what the VFO indicates, then shifts down in USB to read as 3509.4 still with a 600Hz tone, meaning VFO and receiver match

• Spectrum shifts 1200Hz – filter flips from one side of VFO indicator to the opposite side – VFOdisp remains constant – receiver shifts frequency downward twice the CWPitch to maintain offset
• e.g. a 3510 CW signal reads 3510 at 600Hz tone both ways, as if receiver is tuned 600Hz above what the VFO indicates in CWL, and as if the receiver is tuned 600Hz below what the VFO indicates in CWU.

This means in CW modes the VFO always indicates the actual frequency of a CW signal when it is tuned to a note equal to CWPitch, and there is also a CWPitch offset when TUN is on or when transmitting. There is no offset when in a sideband mode, i.e. a CW station’s frequency is indicated in the VFO when tuned to actual zero beat. Switching between a sideband mode and its corresponding CW mode just changes the VFO display readout, the filter choice, and nothing else.

Behavior when tuning has also changed.  As the VFO approachs the edge of the display, instead of disappearing off the edge or stopping, the display re-centers itself so tuning is continuous, even in CTUN mode. The re-centering occurs as the edge of the passband hits the edge of the display, in order to keep any signals of interest visible even as it approaches the edge.