[Elecraft] K3 / RF gain - S meter issue - Similar issue

[Lyle Johnson]

There are several things going on here.

1) The "hardware AGC" is derived at the 15 kHz IF and is designed to 
prevent overdrive to the DSP's IF Analog-to-Digital Converter (ADC).  It 
also serves to prevent the 15 kHz IF amplifier from being driven into 
non-linear operation.

2) The "hardware AGC" voltage is fed to the 8.215 MHz IF amplifier.  By 
placing the gain control here, it also helps to prevent overload of the 
2nd Mixer (8.215 MHz -> 15 kHz).

3) The "RF Gain" control voltage is fed to the same point at the 8.215 
MHz IF stage.

4) The RF ATTenuator (0 or 10 dB in the K3, more choices in the K3S) and 
PREamp (one choice in the K3, up to two choices in the K3S) are applied 
in the radio prior to the first mixer or crystal filter.  They thus have 
an impact on the "hardware" AGC."

All of the above is independent of the DSP subsystem.

5) The "RF Gan Cal" works by injecting known voltages to the 8.215 MHz 
IF amplifier control point and measuring the subsequent change in gain.  
The DSP measures this and creates a table.  Due to dynamic range 
limitations of the measuring system, the calibration procedure requires 
two signals levels to be injected. In the case of the higher level 
signal not being available, the factory default values are used.  Due to 
variations in the gain control led stage, there may be the change in S 
Meter indication that some of you have observed at high signal (or 
severely reduced RF Gain control) conditions.

In general, if you are going to do the RF Gain calibration, you really 
should use both signal levels.  The XG3 specifically provides the 
required levels.

6) The DSP regularly measures the control voltage to the 8.215 MHz IF 
amplifier. The RF Gain Cal information is used by the DSP when it then 
calculates the signal level in the IF passband.  It is not part of the 
signal control loop; it is strictly a measure and report function.

7) The DSP takes into account the PRE and ATT status provided by the MCU 
in the S Meter ABS mode; it ignores such information in the S Meter NOR 
mode.

8) The S Meter calculation is prior to the DSP's AGC algorithms so the S 
Meter is not affected by THR, SLP or other AGC-related settings.

9) The DSP presents the results of the S Meter calculation to the 
radio's MCU via a DAC voltage that is only routed to the MCU -- it has 
no other function in the radio.

10) The S Meter OFS and SCL values are used by the MCU (not the DSP) to 
manage the S Meter display of the DSP's DAC voltage on the K3's LCD.

Enjoy!

73,

Lyle KK7P