Datasheet

LM97593
www.ti.com SNWS019B –JULY 2007–REVISED APRIL 2013
AGC Theory of Operation
A block diagram of the AGC is shown in Figure 81. The DVGA interface comprises four pins for each of the
channels. The first three pins of this interface are a 3-bit binary word that controls the DVGA gain in 6dB steps
(AGAIN). The final pin is ASTROBE which allows the AGAIN bits to be latched into the DVGA’s register. A key
feature of the ASTROBE, illustrated Figure 82, is that it toggles only if the data on AGAIN has changed from the
previous cycle. Not shown is that ASTROBE and BSTROBE are independent. For example, ASTROBE only
toggles when AGAIN has changed. BSTROBE will not toggle because AGAIN has changed. This is done to
minimize unnecessary digital noise on the sensitive analog path through the DVGA. ASTROBE and BSTROBE
are asserted during MR and SI to properly initialize the DVGAs.
The absolute value circuit and the 2-stage, decimate-by-8 CIC filter comprise the power detection part of the
AGC. The power detector bandwidth is set by the CIC filter to F
CK
/8. The absolute value circuit doubles the
effective input frequency. This has the effect of reducing the power detector bandwidth from F
CK
/8 to F
CK
/16.
For a full-scale sinusoidal input, the absolute value circuit output is a dc value of 511*(2/π). Because the absolute
value circuit also generates undesired even harmonic terms, the CIC filter (response shown in Figure 80) is
required to remove these harmonics. The first response null occurs at F
CK
/8, where F
CK
is the clock frequency,
and the response magnitude is at least 25dB below the dc value from F
CK
/10 to 9F
CK
/10. Because the 2
nd
harmonic from the absolute value circuit is about 10dB below the dc this means that the ripple in the detected
level is about 0.7dB or less for input frequencies between F
CK
/20 to 19F
CK
/20. Setting the AGC_COMB_ORD
register to either 1 or 2 will narrow the power detector’s bandwidth as shown in Figure 80.
Figure 80. Power detector filter response, 52 MHz
Figure 81. LM97593 AGC circuit, Channel A
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