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TRANSMIT AND RECEIVE AGC AMPLIFIERS FOR CDMA CELLULAR /PCS PHONES
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TA0030
TA0030
RF2607, RF2609: Transmit and Receive AGC Amplifiers for CDMA Cellu-
lar/PCS Phones
Cellular and Personal Communication Services (PCS)
phones that are based on Code Division Multiple
Access (CDMA) need careful regulation of signal levels
on both the forward and reverse channels. In the
reverse channel (mobile phone to base station) a
transmit automatic gain control (AGC) amplifier must
carefully adjust the output power of the mobile so that it
does not dominate the input spectrum at the base sta-
tion.
Each mobile's signal should arrive at the base station
at the same power level; this helps ensure that capac-
ity is maximized. In the forward channel (base station
to mobile) a receive AGC amplifier adjusts to accom-
modate widely varying signal levels coming in from the
base station. At a CDMA mobile phone antenna
numerous signals sent from the cell base station are
layered on a single frequency band and within this
group of signals lays the desired data.
All of these signals (desired and undesired) pass
through a low noise amplifier front end and then
through a downconverting mixer. Immediately following
the mixer the waveforms pass through a CDMA inter-
mediate frequency (IF) bandpass filter. All of the afore-
mentioned signals are in-band and are not filtered.
Because of this condition the receive AGC amplifier
cannot simply limit. The strongest signal would be lim-
ited and compress all other signals that were received,
so if the desired data was not the strongest, it would be
lost. It becomes clear that the receive AGC amplifier
must provide linear amplification and attenuation to
prevent limiting of undesired signals. RF Micro Devices
has developed two integrated circuits (ICs) that per-
form both of these important functions. The RF2607
CDMA/FM Receive AGC Amplifier and the RF2609
CDMA/FM Transmit AGC Amplifier are both monolithic
ICs that are fabricated in an advanced bipolar Silicon
process. Both of these low cost, high performance ICs
pack variable gain differential amplifier stages, gain
control operational amplifiers, and temperature com-
pensation circuitry within small QSOP16 plastic pack-
ages.
The RF2609 features a 90dB gain range from -48dB
power gain to +42dB power gain and is powered from
a single 3.6V supply. In a cellular system, the cellular
base station sends control signals to the mobile phone
directing the RF2609 to increment or decrement its
gain in 1dB steps. As the mobile strays further from the
base station, the RF2609 is directed to increase its
gain and hence, its output power, while the reverse
occurs as the mobile approaches the base station. The
gain of the IC is controlled by a single DC voltage
externally supplied by a digital-to-analog (D/A) con-
verter which is swept from 0VDC to 3VDC. Figure 1
illustrates the gain response of the RF2609 as the gain
control voltage is swept.
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
0.0
RF2609 Gain vs. Gain Control Voltage
(Vcc=3.6 V, 130 MHz)
+25°C
-30°C
+80°C
0.5 1.0 1.5 2.0 2.5
GC (volts)
3.0
Figure 1. Gain response of the RF2609 as the gain
control voltage is swept
CDMA system specifications dictate that as the gain is
swept over its entire range, the transmit AGC amplifier
must maintain a minimum input third order intercept
point (IIP3). Another way to state this requirement is
that the adjacent channel power rejection of the ampli-
fier must remain constant regardless of the output
level. This specification ensures that IS-95 transmitted
spectrum requirements are met under the entire gain
range of the AGC amplifier. To clarify how this affects
the design, in a CDMA phone the input signal provided
to the RF2609 will remain constant even while the gain
is increased or decreased. Thus, the output signal will
vary proportionally with the gain of the IC. Under these
conditions, the third order (IM3) products must remain
constant and not rise as the gain moves upward. The
RF2609 uses a proprietary variable gain amplifier
scheme that achieves excellent IM3 performance (see
13
Copyright 1997-2000 RF Micro Devices, Inc.
13-151
TA0030
Figure 2) while keeping down the noise figure of the
device (see Figure 3). Although the noise requirements
for the transmit AGC amplifier are not as stringent as
that of the receive AGC amplifier, the challenging IIP3
requirements for the RF2609 make the noise figure
more difficult to achieve. The final design, however,
was able to meet both IS-95 specifications under nomi-
nal and worst-case conditions.
RF2609 IIP3 vs. Gain
(Vcc=3.6 V, 130 MHz)
0
-10
-20
-30
-40
-50
-60
-60 -40 -20 0 20
Gain (dB)
Figure 2. The RF2609 IIP3 vs. Gain
40
60
13
RF2609 Noise Figure vs. Gain
(Vcc=3.6 V, 130 MHz)
80
70
60
50
40
30
20
10
0
-60 -40 -20 0 20
Gain (dB)
40
Figure 3. The RF2609 Noise Figure vs. Gain
60
Understanding how to incorporate the RF2609 into a
transmit chain is straightforward (see Figure 4). Pins 1
and 2 are the input port for the IC.
The differential impedance of the input port is 1000Ω,
so for maximum power transfer, the system designer
need only provide a source impedance of 1000 Ω. Typi-
cally, an intermediate frequency (IF) filter will precede
the RF2609 and provide a 1000Ω source impedance.
If a 1000Ω filter cannot be used, a simple L-C network
can be designed to perform an impedance transforma-
tion. Since there is DC present on pins 1 and 2, the
source should be AC coupled through capacitors as
shown in Figure 4.
Once the IF signal is fed into the IC, it travels through
four variable gain amplifier stages. Each of these
amplifiers is controlled by gain control circuitry, which
primarily consist of operational amplifiers. External to
the part, a DC gain control voltage is fed from a D/A
converter and enters the IC through pin 16. In order to
achieve the correct gain curve, the DC gain control
voltage must pass through a 3.3kΩ resistor. A capaci-
tor is placed from pin 16 to ground in order to lowpass
filter the signal from the D/A converter. The earlier
mentioned gain control voltage range of 0VDC to
3VDC is referenced to the GAIN label on Figure 4, not
at pin 16.
The output port of the RF2609 consists of pins 9 and
10. The output of the IC is open collector, which means
that it looks like a high impedance. Open collector also
means that the output pins must be supplied DC volt-
age externally for the internal output circuitry to oper-
ate.
The output is left high impedance for greater flexibility
and greater precision. A system designer can choose
whatever output impedance they desire and use 1%
resistors to guarantee good matching. The IC was
designed to drive 500Ω (1000Ω output impedance in
parallel with 1000Ω load) but other impedance levels
can be used if the change in power gain is taken into
account. Referring back to Figure 4, a 1000Ω resistor
is placed across pins 9 and 10 to set the differential
output impedance of the IC.
Inductors (L1) connect the power supply to the output
pins. The inductors can be used with series capacitors
(C2) to form an impedance transformation network if
the IF filter does not look like 1000Ω.
If the filter impedance is 1000Ω, then the values of L1
and C1 are chosen to form a parallel-resonant tank cir-
cuit at the signal frequency. In this case, C2 merely
acts as a DC blocking capacitor.
13-152
Copyright 1997-2000 RF Micro Devices, Inc.
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