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ISL5216
The loop gain mantissas and exponents are set in IWA
register *010h, with IWA register *013h selecting loop gain 0
or 1 and the settling mode.
In the ISL5216, a SYNCI signal will clear the AGC loop filter
accumulator if GWA register F802h bit 27 is set. This sets
the AGC to unity gain or to the lower gain limit (IWA *011h
bits 15:0) if it is larger than unity.
The settling mode of the AGC forces either the mean or the
median of the signal magnitude error to zero, as selected by
IWA register *013h bit 8. For mean mode, the gain error is
scaled and used to adjust the gain up or down. This
proportional scaling mode causes the AGC to settle to the
final gain value asymptotically. This AGC settling mode is
preferred in many applications because the loop gain
adjustments get smaller and smaller as the loop settles,
reducing any AM distortion caused by the AGC.
With this AGC settling mode, the proportional gain error
causes the loop to settle more slowly if the threshold is
small. This is because the maximum value of the threshold
minus the magnitude is smaller. Also, the settling can be
asymmetric, where the loop may settle faster for “over
range” signals than for “under range” signals (or vice versa).
In some applications, such as burst signals or TDMA signals,
a very fast settling time and/or a more predictable settling
time is desired. The AGC may be turned off or slowed down
after an initial AGC settling period.
The median mode minimizes the settling time. This mode
uses a fixed gain adjustment with only the direction of the
adjustment controlled by the gain error. This makes the
settling time independent of the signal level.
For example, if the loop is set to adjust 0.5dB per output
In the median mode, the maximum gain step is
approximately 3dB/output. The step is fixed (it does not
decrease as the error decreases) so a large gain will cause
AM on the output at least that large. The fixed gain step is
set by the programmable AGC loop gain register
IWA *010h.
The AGC gain limits register sets the minimum and
maximum limits on the AGC gain. The total AGC gain range
is 96dB, but only a portion of the range should be needed for
most applications. For example, with a 16-bit output to a
processor, the 16 bits may be sufficient for all but 24dB of
the total input range possible. The AGC would only need to
have a range of 24dB. This allows faster settling and the
AGC would be at its maximum gain limit except when a high
power signal was received. The AGC may be disabled by
setting both limits to the same value.
The median settling mode is enabled by setting IWA register
*013h bit 8 to 0 while the mean loop settling mode is
selected by setting bit 8 to 1.
Cartesian to Polar Converter
The Cartesian to Polar converter computes the magnitude
and phase of the I/Q vector. The I and Q inputs are 24 bits
wide. The converter phase output is 18 bits wide and is
routed to the output formatter and frequency discriminator.
This 18-bit output phase can be interpreted either as two’s
complement (-0.5 to approximately 0.5) or unsigned (0.0 to
approximately 1.0), as shown in Figure 5. The phase
conversion gain is 1/2 π . The 24-bit magnitude is unsigned
binary format with a range from 0 to 2.32. The magnitude
conversion gain is 1.64676. The MSB of the magnitude (the
sign bit) is always zero.
sample, the loop gain can slew up or down by 16dB in 16
symbol times, assuming a 2-samples-per-symbol output
+ π /2
400000 3fffff
Q
π /2
400000 3fffff
Q
sample rate. This is called a median settling mode because
the loop settles to where there is an equal number of
magnitude samples above and below the threshold. The
7fffff
± π
800000
I 000000
0
ffffff
7fffff
π
800000
I
000000
0
ffffff
disadvantage of this mode is that the loop will have a wander
(dither) equal to the programmed step size. For this reason,
bfffff c00000
- π /2
bfffff c00000
3 π /2
it is advisable to set one loop gain for fast settling at the
beginning of the burst and the second loop gain for small
adjustments during tracking.
26
FIGURE 5. PHASE BIT MAPPING OF COORDINATE
CONVERTER OUTPUT
FN6013.3
July 13, 2007
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