AirspyHF: updated documentation

plugins/samplesource/airspyhf/readme.md

@@ -45,24 +45,36 @@ Record baseband I/Q stream toggle button
 
 Baseband I/Q sample rate in kS/s. This is the device to host sample rate (3) divided by the decimation factor (4). 
 
-<h3>2: Band select</h3>
+<h3>2: Lo ppm correction</h3>
+
+This is the correction factor in ppm applied to the local oscillator. The Airspy HF LO has 1 kHz increments so anything in between is obtained by mixing the signal with a Hz precision NCO. This is actually done in the AirspyHF library.
+
+On HF band the LO correction is not necessary because the LO is largely precise enough for the frequencies involved. You can disable the NCO in AirspyHF library by setting the value to zero. Since the LO control in SDRangel has a 1 kHz step the NCO correction will always be zero. In AirspyHF library (my fork) the NCO is not active (no extra complex multiplication) if the correction is zero. On HF band it is recommended not to use the LO correction (set it or leave it at 0). 
+
+You can reset the ppm value anytime by pressing on button (3)
+
+<h3>3: Reset LO ppm correction</h3>
+
+THis resets the LO ppm correction (zero the value). By doing so the LO trimming NCO in AirspyHF libray is disabled.
+
+<h3>4: Band select</h3>
 
 Use this combo box to select the HF or VHF range. This will set the limits of the frequency dial (1.1) appropriately and possibly move the current frequency inside the limits. Limits are given by the AirspyHF+ specifications:
 
   - HF: 9 kHz to 31 MHz
   - VHF: 60 to 260 MHz
 
-<h3>3: Device to hast sample rate</h3>
+<h3>5: Device to hast sample rate</h3>
 
 This is the device to host sample rate in samples per second (S/s).
 
 Although the combo box is there to present a choice of sample rates at present the AirspyHF+ deals only with 768 kS/s. However the support library has provision to get a list of sample rates from the device incase of future developments.
 
-<h3>4: Decimation factor</h3>
+<h3>6: Decimation factor</h3>
 
 The I/Q stream from the AirspyHF to host is downsampled by a power of two before being sent to the passband. Possible values are increasing powers of two: 1 (no decimation), 2, 4, 8, 16, 32, 64. When using audio channel plugins (AM, DSD, NFM, SSB...) please make sure that the sample rate is not less than 48 kHz (no decimation by 32 or 64).
 
-<h3>5: Transverter mode open dialog</h3>
+<h3>7: Transverter mode open dialog</h3>
 
 This button opens a dialog to set the transverter mode frequency translation options:
 
@@ -70,7 +82,7 @@ This button opens a dialog to set the transverter mode frequency translation opt
 
 Note that if you mouse over the button a tooltip appears that displays the translating frequency and if translation is enabled or disabled. When the frequency translation is enabled the button is lit.
 
-<h4>5a.1: Translating frequency</h4>
+<h4>7a.1: Translating frequency</h4>
 
 You can set the translating frequency in Hz with this dial. Use the wheels to adjust the sample rate. Left click on a digit sets the cursor position at this digit. Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position. Wheels are moved with the mousewheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arroews. Pressing shift simultanoeusly moves digit by 5 and pressing control moves it by 2.
 
@@ -84,10 +96,10 @@ For bands even higher in the frequency spectrum the GHz digits are not really si
 
 The Hz precision allows a fine tuning of the transverter LO offset
 
-<h4>5a.2: Translating frequency enable/disable</h4>
+<h4>7a.2: Translating frequency enable/disable</h4>
 
 Use this toggle button to activate or deactivate the frequency translation
 
-<h4>5a.3: Confirmation buttons</h4>
+<h4>7a.3: Confirmation buttons</h4>
 
 Use these buttons to confirm ("OK") or dismiss ("Cancel") your changes.