[Elecraft] K-3 Roofing filter comment

[wa9fvp]

I understand that about the HAGC1.  Its purpose is to keep the level within
the range of the A to D.  If this signal is too big the A to D converter U23
will over range and the DSP is unable to perform the mathematical functions.   

The HAGC1 is fixed and is not adjustable by the user.  VIFGAIN1 on the other
hand is adjustable and ultimately controlled by the DSP.  The AGC loops are
nested that is the HAGC1 is the main loop and the VIFGAIN1 is a sub loop. 
If you look at the IF schematic you will see that the two loops are summed
at the same AGC buffer amplifier U3A.   HAGC1 is routed to the DSP board and
is actually an input to an A to D converter U8.  It’s then processed by the
DSP.  Without knowing the DSP code, I suspect there’s a PID (Proportional
Integral Derivative) function involved to maintain loop stability and data
to calibrate the S meter. 

The RF gain control is a potentiometer connected an analog port of the PIC
controller U1 (PIC18LF8722).  It’s converted to a digital signal that’s also
fed to the DSP to control the RF gain via VIFGAIN1.   

I guess where we differ is that you speak in terms of an analog signal and I
like to speak in terms in the digital world.  I was involved in a DSP
project in the late 80’s and early 90’s called “The Hamblaster a DSP for Ham
Radio”, where I wrote two articles in QEX.



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Jack WA9FVP
Willco Electronics
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