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参数资料
| 型号： | AD8316-EVAL |
| 厂商： | Analog Devices, Inc. |
| 英文描述： | Dual Output GSM PA Controller |
| 中文描述： | 双输出GSM功率放大器控制器 |
| 文件页数： | 12/20页 |
| 文件大小： | 497K |
| 代理商： | AD8316-EVAL |
REV. C
–12–
AD8316
The value of
V
OUT
is of interest, although it is a dependent param-
eter inside the loop. It depends on the characteristics of the
power amplifier, and the value of the carrier amplitude
V
CW
.
Using the control values derived above, that is,
G
O
= 0.316 and
V
GSC
= 1 V, and assuming that the applied power is fixed at
–7 dBm (so that
V
CW
= 100 mV rms), Equation 11 shows
V
max
V
V
V
OUT
SET
GSC
SLP
(
(
.
.
1 39
1 0 44
kG V
(
.
. /
0 1 199
V
V
V
CW
×
Z
)
=
(
)
)
=
×
×
=
=
log
log
.
.
.
.
10
10
0 0316
0 316
μ
3 2
0 7
2 5
(15)
V
min
(
V
V
V
kG V
(
.
. /
0 1 199
V
V
OUT
SET
GSC
SLP
CW
×
Z
)
=
(
)
)
=
(
×
=
=
log
.
.
log
.
.
.
10
10
0 31 1 0 44
0 0316
0 316
μ
0 7
0 7
0
(16)
Both results are consistent with the assumptions made about the
amplifier control function. Note that the second term is inde-
pendent of the delivered power and is a fixed function of the
drive power.
Finally, the loop time constant for these parameters, using an
illustrative value of 2 nF for the filter capacitor C
FLT
, evaluates to
=
(
=
(
/ .
.
1 0 44
3 07
2
T
V
V
T
)
s
nF
s
O
GSC
SLP
)
×
×
(
)
=
/
.
13 95
μ
μ
(17)
Practical Loop
At the present time, power amplifiers, or VGAs preceding such
amplifiers, do not provide an exponential gain characteristic. It
follows that the loop dynamics (the effective time constant) will
vary with the setpoint, since the exponential function is unique
in providing constant dynamics. The procedure must therefore
be as follows. Beginning with the curve usually provided for the
power output versus APC voltage, draw a tangent at the point
on this curve where the slope is highest (see Figure 5). Using
this line, calculate the effective minimum value of the variable
V
GSC
, and use it in Equation 17 to determine the time constant.
(Note that the minimum in V
GSC
corresponds to the maximum
rate of change in the output power versus V
OUT
.)
For example, suppose it is found that, for a given drive power,
the amplifier generates an output power of P
1
at V
OUT
= V
1
, and
P
2
at V
OUT
= V
2
. Then, it is readily shown that
(
20
2
1
2
1
–
/
–
This should be used to calculate the filter capacitance. The
response time at high and low power levels (on the “shoulders”
of the curve shown in Figure 5) will be slower. Note also that it
is sometimes useful to add a zero in the closed-loop response by
placing a resistor in series with C
FLT
.
A Note About Power Equivalency
Users of the AD8316 must understand that log amps funda-
mentally do not respond to power. For this reason, dBV
(decibels above 1 V rms) are included in addition to the com-
monly used metric dBm. The dBV scaling is fixed, independent
of termination impedance, while the corresponding power level
V
V
V
P
P
GSC
=
) (
)
(18)
is not. For example, 224 mV rms is always –13 dBV, with one
further condition of an assumed sinusoidal waveform; see the
AD640 data sheet for more information about the effect of wave-
form on logarithmic intercept. This corresponds to a power
of 0 dBm when the net impedance at the input is 50
. When
this impedance is altered to 200
, however, the same voltage
corresponds to a power level that is four times smaller (P = V
2
/R),
or –6 dBm. A dBV level may be converted to dBm in the special
case of a 50
system and a sinusoidal signal simply by adding
13 dB. 0 dBV is then, and only then, equivalent to 13 dBm.
–67dBm
–47dBm
–27dBm
–7dBm
+13dBm
33dBm
23dBm
13dBm
3dBm
–7dBm
P
RF
V
OUT1
V
2,
P
2
V
P
Figure 5. Typical Power Control Curve
Therefore, the external termination added ahead of the AD8316
determines the effective power scaling. This often takes the
form of a simple resistor (52.3
will provide a net 50
input),
but more elaborate matching networks may be used. The choice
of impedance determines the logarithmic intercept, that is, the
input power for which the V
SET
versus P
IN
function would cross
the baseline if that relationship were continuous for all values of
V
IN
. This is never the case for a practical log amp; the intercept
(so many dBV) refers to the value obtained by the minimum-error
straight-line fit to the actual graph of V
SET
versus P
IN
(more
generally, V
IN
). Where the modulation is complex, as in CDMA,
the calibration of the power response needs to be adjusted; the
intercept will remain stable for any given arbitrary waveform.
When a true power (waveform independent) response is needed,
a mean-responding detector, such as the AD8361, should be
considered.
The logarithmic slope, V
SLP
in Equation 1, which is the amount by
which the setpoint voltage needs to be changed for each decade
of input change (voltage or power) is, in principle, independent
of waveform or termination impedance. In practice, it usually
falls off somewhat at higher frequencies, because of the declining
gain of the amplifier stages and other effects in the detector
cells (see TPC 13).
Basic Connections
Figure 6 shows the basic connections for operating the AD8316
and Figure 7 shows a block diagram of a typical application.
The AD8316 is typically used in the RF power control loop of
dual mode and trimode mobile handsets where there is more
than one RF power control line.
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