How do HF HAM transceivers handle the USB and LSB?

Hi a question has come into mind related to these HF HAM commercial transceivers. How do these handle the SSB and LSB related to the oscillator frequency? I can think of two cases. It is better to present an example, to understand my question.
For both cases LOSC=10MHz and transmitter baseband modulating audio signal=1KHz tone.

Case 1:
When switching to USB setting, the LOSC remains at 10MHz and the LSB is filtered. Then the transmitter transmits at 10MHz+1KHz (10MHz carrier suppressed and LSB filtered).
Likewise, when switching to LSB setting, the LOSC remains at 10MHz and the USB is filtered. Then the transmitter transmits at 10MHz-1KHz (10MHz carrier suppressed and USB filtered).

Case2:
When switching to USB setting, the LOSC is shifted at 10MHz-1KHz and the LSB is filtered. Then the transmitter transmits at 10MHz (10MHz-1KHz carrier suppressed and LSB filtered).
Likewise, when switching to LSB setting, the LOSC is shifted at 10MHz+1KHz and the USB is filtered. Then the transmitter transmits at 10MHz (10MHz+1KHz carrier suppressed and USB filtered).

Which of the two cases is true, for commercial HF ham radio transceivers?

The IF filtering always stays constant.

The local oscillator is tuned to centre the received signal within the IF bandwidth (so that it can be heard).

But in order to correctly demodulate the SSB back into audio, a beat frequency oscillator is required to replace the missing carrier.
For USB the BFO will be placed at the low end of the IF bandwidth,
For LSB the BFO will be at the high end.

Now the actual frequency readout on the receiver dial corresponds to where the missing carrier would be. So the readout frequency on the receiver dial must be moved by the amount the BFO frequency changes when switching between sidebands.

In older equipment with a mechanical frequency readout, there is usually a movable graticule line that you can use to correct for this slight change in frequency when switching sidebands.

In modern equipment with digital readout there will be a microprocessor that adds or subtracts the slight frequency offset required automatically.

Warpspeed said:

The IF filtering always stays constant.

The local oscillator is tuned to centre the received signal within the IF bandwidth (so that it can be heard).

But in order to correctly demodulate the SSB back into audio, a beat frequency oscillator is required to replace the missing carrier.
For USB the BFO will be placed at the low end of the IF bandwidth,
For LSB the BFO will be at the high end.

Now the actual frequency readout on the receiver dial corresponds to where the missing carrier would be. So the readout frequency on the receiver dial must be moved by the amount the BFO frequency changes when switching between sidebands.

In older equipment with a mechanical frequency readout, there is usually a movable graticule line that you can use to correct for this slight change in frequency when switching sidebands.

In modern equipment with digital readout there will be a microprocessor that adds or subtracts the slight frequency offset required automatically.

Click to expand...

Ok, so the actual frequency shown in the display (in a modern HAM gear) is actually an offset of the LO (let's talk about transmit only, to simplify the example).
So the actual LO frequency does change, so that only one filter is used for both USB/LSB.
Am I getting it right?

Yes the LO stays the same to keep the signal centred in the IF filter, and that does not change.

The slight frequency offset comes in during demodulation, which depends on the operating mode selected. AM, CW, USB, LSB, (and sometimes even narrow band FM).

Warpspeed said:

Yes the LO stays the same to keep the signal centred in the IF filter, and that does not change.

Click to expand...

This is different of what I am saying, unless I have it all wrong. Maybe you wanted to say "Yes the LO does change..." ?

You tune the LO so you can hear the signal.
That places it into the middle of the IF passband.

If it sounds like Donald Duck, you switch sidebands.
No need to retune the LO.

SSB needs to have the missing carrier reinserted.
You can reinsert it above or below the signal coming out of the IF amplifier.

So you can switch between sidebands without touching the LO frequency.

Warpspeed said:

You tune the LO so you can hear the signal.
That places it into the middle of the IF passband.

If it sounds like Donald Duck, you switch sidebands.
No need to retune the LO.

Click to expand...

I see, you say that the offset is automatically handled by the transceiver circuit (you do not have to retune), when you have tuned the LO on a signal.
Right

Yes that is it.

The microprocessor measures the LO frequency, and measures the BFO frequency and does a simple addition or subtraction to drive the readout.

If the LO runs above the received frequency (as is usual) it will be subtraction.
If the LO runs below the received frequency it will be an addition.

When you swap sidebands, the BFO frequency is switched, and the digital readout changes accordingly.