Re: automatic gain control

On Thu, 14 Feb 2008 16:10:13 +0000, Vladimir Vassilevsky wrote:

Tim Wescott wrote:

On Thu, 14 Feb 2008 09:44:33 -0600, nwa wrote:


Hi,

i'm new in this area. I'm now using ADSP 21061 SHARC digital signal
processing. I want to ask about automatic gain control, how to compute
the signal power from the received signal? I want to use this signal
power to scalling my signal.

Cheers..


Square and average. AGC loops are multiplicative, so it's usually a
good idea to take the logarithm of the average power to linearize the
loop.

BTW they always recommend using a squaring detector which implies the
double numeric precision and requires the square root or log operation
which is quite expensive. In many cases you don't really need the exact
power but some sort of approximate measure of the signal. The averaged
absolute value works just as well as RMS for many practical cases. If
the peak detection is required, then the averager can have the different
fall and raise time constants.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant http://www.abvolt.com

A logarithm good enough for an AGC is cheap, particularly if you have a
chip with a normalizer. You take the log to the base 2: shift up until
the MSB is set -- that's the mantissa of your log. Then you take the
remainder and linearly interpolate it between where you are and the next
log up.

Averaging the magnitude works well and is less expensive unless you have
a DSP chip that can square quicker than it can take an absolute value.
For a wide-range AGC you can still do better with a log.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.

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