Download Automatic gain control

GSM Power Amplifier Control Loop Design
)Automatic Gain Control)
1. Introduction:
Power amplifier control (PAC) for a Global System for Mobile communications
(GSM) compatible radio is one of the more challenging aspects of the GSM-based
system design. Not only must the radio meet all output radio frequency (RF) spectrum
specifications, but the Power Amplifier (PA) control loop must also be stable under
varying environmental conditions.
Automatic gain control (AGC) was implemented in first radios for the reason of
fading propagation (defined as slow variations in the amplitude of the received
signals) which required continuing adjustments in the receiver’s gain in order to
maintain a relative constant output signal.
Such situation led to the design of circuits, which primary ideal function was to
maintain a constant signal level at the output, regardless of the signal’s variations at
the input of the system. Originally, those circuits were described as automatic volume
control circuits, a few years later they were generalized under the name of Automatic
Gain Control circuits (AGC).
With the huge development of communication systems during the second half of the
XX century, the need for selectivity and good control of the output signal’s level
became a fundamental issue in the design of any communication system. Nowadays,
AGC circuits can be found in any device or system where wide amplitude variations
in the output signal could lead to a lost of information or to an unacceptable
performance of the system.
AGC systems are part of any wireless communication system where a constant output
signal is desired. The complexity of the AGC system is determined by the
requirements of the communication system, therefore the analysis, design and
implementation can become quite difficult. AGC systems and circuits will continue to
evolve as long as wireless technology becomes faster, smaller and more complex.
New devices, circuits and techniques must be studied, developed
and implemented.
2. Objectives of using AGC circuits:
Automatic Gain Control (AGC) circuits are employed in many systems where the
amplitude of an incoming signal can vary over a wide dynamic range. The role of the
AGC circuit is to provide relatively constant output amplitude so that circuits
following the AGC circuit require less dynamic range. If the signal level changes are
much slower than the information rate contained in the signal, then an AGC circuit
can be used to provide a signal with a well defined average level to downstream
circuits. In most system applications, the time to adjust the gain in response to an
input amplitude change should remain constant, independent of the input amplitude
level and hence gain setting of the amplifier.
Why we need to control the output power level of mobile phone signal booster?
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To prevent intermodulation in base station receivers.
To prevent interference with other mobile phones using the same booster.
To minimize power consumption.
Using the minimum power necessary for reliable communication with the
selected base station, based on distance.
3. Theory of the Automatic Gain Control system
Many attempts have been made to fully describe an AGC system in terms of control
system theory, from pseudo linear approximations to multivariable systems. Each
model has its advantages and disadvantages, first order models are easy to analyze
and understand but sometimes the final results show a high degree of inaccuracy
when they are compared with practical results. On the other hand, non-linear and
multivariable systems show a relative high degree of accuracy but the theory and
physical implementation of the system can become really tedious
From a practical point of view, the most general description of an AGC system is
presented in figure 1.
Figure 1. Typical PAC Loop with Proportional Control
Figure 1 shows a typical PAC loop that uses proportional control. The control signal
is proportional to the difference between the Detector/Coupler feedback and the DSP
signal.
Since an AGC is essentially a negative feedback system, the system can be described
in terms of its transfer function which can be done by noting the following
relationships.
Feedback=KdKc (Vo)
(1)
Error=Ks (DSP-Feedback)
(2)
Vo= Ka (Error)
(3)
Vo= Ka (Ks (DSP- KdKc (Vo)))
(4)
Simplifying:
(5)
(6)
This expression, T is commonly known as the closed loop gain. The product of gains,
Ka Ks KdKc is known as the loop gain.
This control scheme is usually called proportional control, because the output
voltage, Vo, is directly proportional to the error signal.
The closed loop gain is divided into a forward path gain (A) and a feedback path gain
(B) Figure 2.
Substituting into the original equation produces the following more familiar output
voltage expression:
(7)
(8)
where A =
Ka Ks, and B = KdKc
Figure 2. Closed Loop Gain with Forward Path Gain A and Feedback Path Gain B
3.1 Benefits of Closed Loop Control
In this section, the study compares between open loop control( B = 0) and closed
loop control. The advantages of closed loop control become apparent.
3.1.1 Temperature, Battery Insensitivity, and Linearization
If the loop gain remains >> 1 over temperature and battery ranges, then the output
voltage, Vo, becomes insensitive to temperature and battery effects.
If AB >> 1, then the closed loop transfer function becomes:
(9)
Since KdKc is part of the feedback network and is usually a stable, temperature
compensated block, there is a gain independence from battery supply and
temperature. There is also a gain independence from KsKa variations when the radio
environment differs from the one in which it was calibrated.
3.1.2 Sensitivity to Power Amplifier Gain Changes
In general, the power amplifier (PA) gain, Ka is sensitive to both temperature and
supply level. This can be stated mathematically as:
Since the design scheme is proportional control, there is a concern as to just how
much the output power varies with a change in the PA gain.
Assume Ks = 1 for the following calculations, that is A = Ka, and starting with
equation (8).
(10)
Writing this expression in incremental form yields the following:
(11)
To make this expression more useful, it can be written in terms of a fractional change
of output voltage by dividing equation (11) by (8), which results in the following:
(12)
Now the relative change in output voltage vs the change in PA gain is obvious.
If the loop gain is >>1, then the gain change has very little effect on the output
voltage change.
But, as the loop gain changes from the calibration point, the change in output voltage
becomes more and more sensitive to PA changes.
3.2 Adding an Integrator to the Forward Path
Most PAC loops employ an integrator in the control loop forward path, Figure 3. This
changes the topology from proportional control to integral control. As the previous
section pointed out, the output power is a function of both the feedback gain, which is
relatively stable, and the PA gain, which is sensitive to both temperature and supply,.
This is a problem when the loop gain is relatively low. Unfortunately, this low gain
condition usually occurs at the highest system output power.
Figure 3. PAC Loop with an Integrator in the Forward Path
From Figure 3, it can be seen that the control voltage driving the PA is the integral of
the error.
In theory, integral control has the advantage that the loop settles to a point where the
error is zero. Yet the integrator output provides the correct control voltage to the PA
by integrating past errors.
Re-writing the closed loop expression taking into account the integrator in the forward
path provides insight as to what effect the integrator has on the steady operation of the
loop.
(13)
(14)
Now differentiating to time yields,
(15)
For steady state, when
we have
(16)
(17)
This is a design benefit. By using an integrator, the PA gain is essentially removed
from loop steady state operations. In more practical terms, output voltage is expected
to be the same even if the PA gain changes. This is intuitively obvious as the
integrator drives the PA to the calibrated power level, so long as the feedback gain,
KdKc, remains the same.
Although the integrator seems like an attractive solution, it does have a few
drawbacks that the designer should be aware of.
● The loop now pushes the PA to get what it wants. If there are any conditions in
which the PA cannot meet the calibrated output power, the loop drives the PA into
saturation. This, in turn, can cause problems for switching transients.
● The integrator adds a phase shift/delay of 90° to the loop, so stability needs careful
attention over all areas of operation.
4. PAC Loop Components
This section covers the following basic PAC loop components; see Figure 7. to
include how they are commonly implemented:
4.1 RF Detector
The RF detector converts an output voltage scaled value into a DC voltage used for
comparison with the DSP value.
it is important that the detector is temperature compensated and Detector gain should
also be stable over all operating conditions. detector gain is bounded by –7 dB and –3
dB.
4.2 Coupler
The coupler is a passive temperature stable device ideal for a feedback block. The
coupler is important beyond the obvious connection to the feedback path. The amount
of coupling defines not only dynamic range, but it also limits the minimum insertion
loss in the output power path. Generally, couplers are found in the range of 10 dB to
20 dB, with the least amount of coupling being preferred for insertion loss.
Coupler gain
(18)
The amount of coupling also defines the amount of feedback gain, and ultimately
limits the maximum amount of loop gain.
4.3 Difference amplifier
Error signal computation can be done using a difference amplifier shown in Figure 4.
Figure 4. Difference Amplifier Circuitry
Where
Vdet is the Rf detector o/p
Vdsp is the desired power level
4.4. Differential Integrator
The differential integrator is usually used in a PAC loop instead of a simple
differential amplifier It is used to take advantage of an integrator in the forward path,
discussed in Section 3.2. Figure 5 shows implementation using an operational
amplifier.
Figure 5. Implementation Using an Operational Amplifier
We can combine amplifier with the integrator into one operational amplifier, the
Differential Integrator block.as we can see in figure 6.
Figure6. Differential Integrator Block Diagram
we can combine all the previous components in one circuit as in figure7.
Figure7. Automatic gain control
Where
TX_EN=2.8 v
VPAC =2.8 v
4.5. Power amplifier (PA)
An RF power amplifier is a type of amplifier used to convert a low power radio
frequency signal into a larger signal of significant power. It's usually optimized to
have high efficiency, high output power, good return loss on the input and output,
good gain and optimum heat dissipation.
ALM-31122 power amplifier
1. General description:
ALM-31122 is a high linearity 1 Watt PA with good OIP3( third order intercept
point) performance and exceptionally good PAE at 1dB gain compression point,
achieved through the use of Avago Technologies’ proprietary 0.25um GaAs Enhancement-mode pHEMT process.
All matching components are fully integrated within the module and the 50Ω RF
input and output pins are already internally AC-coupled. This makes the ALM-31122
extremely easy to use as the only external parts are DC supply bypass capacitors.
The adjustable temperature-compensated internal bias circuit allows the device to be
operated at either class A or class AB operation.
The ALM-31122 is housed inside a miniature 5.0 x 6.0 x 1.1 mm3 22-lead multiplechips-on-board (MCOB) module package.
2. Features
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Fully matched, input and output
High linearity at P1dB
Unconditionally stable across load condition
Built-in adjustable temperature-compensated internal bias circuitry
GaAs E-pHEMT Technology[1]
5V supply
Excellent uniformity in product specifications
Tape-and-Reel packaging option available
MSL-3 and Lead-free
High MTTF for base station application
3. Specifications
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900 MHz; 5V, 394mA (typical)
15.6 dB Gain
47.6 dBm Output IP3
31.6 dBm Output Power at 1dB gain compression
52.5% PAE at P1dB
2 dB Noise Figure
4. Applications
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Class A driver amplifier for GSM/CDMA Base Stations.
General purpose gain block.
5. Component Image
Notes:
Package marking provides orientation and identification
“31122” = Device Part Number
“WWYY” = Work week and Year of manufacture
“XXXX” = Last 4 digit of Lot number
6. Electrical Specifications
TA = 25 °C, Vdd =5V, Vctrl=5V, RF performance at 900MHz, measured on demo
board (see Figure 8) unless otherwise specified.
Symbol
Parameter and Test Condition
Ids
Quiescent current
Ictrl
Vctrl current
Gain
Gain
Min.
Typ.
Max
Units
340
394
440
mA
-
10.4
-
mA
13.7
15.6
17.3
dB
OIP3 [8]
Output Third Order Intercept Point
45
47.6
-
dBm
OP1dB
Output Power at 1dB Gain Compression
30
31.6
-
dBm
Power Added Efficiency
-
52
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%
NF
Noise Figure
-
2.0
-
dB
S11
Input Return Loss, 50Ω source
-
-14
-
dB
S22
Output Return Loss, 50Ω load
-
-11
-
dB
S12
Reverse Isolation
-
-21
-
dB
PAE
Table 1.
7. Demo Board Schematic
Figure 8.
1.
2.
3.
4.
5.
6.
References
J.R smith ''Modern Communication Circuits'', McGraw Hill
Electrical and Computer Engineering Series.2nd edition, New
York, 1988.
U.L. Rhode, T.T.N. bucher ''Communication Receivers:
principle and design'', McGraw hill, New York, 1988.
Automatic gain control in receivers by Iulian Rosu, VA3IUL.
A discussion on the Automatic Gain Control (AGC) – Phil
Harman, VK6APH.
Analysis and Design of Analog Integrated Circuits
Grey, Paul R.; Robert G. Meyer.
GSM Handset Power Amplifier Control Loop Design An
Analog Approachby Jason Millard & Darioush Agahi