AGC look-up tables and error control loops]

[Andre' Kesteloot]

ROBERT EVANS wrote:

> There have been receiver designs where the AGC voltage drives an A/D and
>
> the D is the address to a look-up table ROM.  The contents of the memory
>
> is fed to a D/A to drive the front panel meter.  The ROM is burnt with
> the
> table to calibrate the meter to the correct S meter reading.  This was a
>
> discreet circuit built into the receiver back when receivers didn't even
> have
> microprocessors to meet military/commercial/industrial demands for
> accurate meter reading specifications.  I wonder how often this is
> correctly
> done in present day receivers were it would be an integral part of the
> display control microprocessor.
>
> But, on the subject of AGC...
>
> Many digital modems offer an output signal to control the gain ahead of
> the A/D.  Some technique is used to determine if the A/D is being fully
> filled without unacceptable compression thus offering the maximum
> dynamic range.   If not, a control signal increases or decreases the
> gain
> of the analogue stages ahead of the A/D.
>
> There are many ways of implementing this from simply processing the
> most significant bit to have a specified duty cycle to involved
> algorithms
> that take into account the degree of the distortion that the modulation
> and encoding scheme can handle.
>
> It would be useful, for instance, if the software controlled a pin (or
> pins)
> on the com port to describe how to adjust the audio level.  This
> would offer a much more appropriate method of implementing an AGC
> loop.
>
> For example, a PWM signal on a single pin could be integrated to produce
>
> a control voltage.  Two pins could offer a more straight foward error
> signal pair of too high / too low.  A receiver could make use of this
> signal to control its gain stages instead of a separate analog detector
> based AGC system.
>
> BCNU DE N2LO~>
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