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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 90 dB gain range from -48dB
power gain to +42dB power gain and is powered from
a single 3.6 V 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.

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

RF2609 Gain vs. Gain Control Voltage

(Vcc=3.6 V, 130 MHz)

-60

-50

-40

-30

-20

-10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

GC (volts)

Gain (dB)

+25癈

-30癈

+80癈

Figure 1. Gain response of the RF2609 as the gain
control voltage is swept

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RF2607, RF2609: Transmit and Receive AGC Amplifiers for CDMA Cellu-

lar/PCS Phones

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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.

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 1000

so for maximum power transfer, the system designer
need only provide a source impedance of 1000

. Typi-

cally, an intermediate frequency (IF) filter will precede
the RF2609 and provide a 1000

source impedance.

If a 1000

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 3.3 k

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 500

(1000

output impedance in

parallel with 1000

load) but other impedance levels

can be used if the change in power gain is taken into
account. Referring back to Figure 4, a 1000

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 1000

If the filter impedance is 1000

, 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.

RF2609 IIP3 vs. Gain

(Vcc=3.6 V, 130 MHz)

-60

-50

-40

-30

-20

-10

-60

-40

-20

Gain (dB)

IIP3 (dBm)

Figure 2. The RF2609 IIP3 vs. Gain

RF2609 Noise Figure vs. Gain

(Vcc=3.6 V, 130 MHz)

-60

-40

-20

Gain (dB)

Noise Figure (dB)

Figure 3. The RF2609 Noise Figure vs. Gain

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The RF2607 features a 96 dB gain range from -48dB
power gain to +48dB power gain and is powered from
a single 3.6 V supply. As the transmit AGC amplifier is
instructed to adjust its gain, the receive AGC amplifier
will track, so that they work in parallel. As the mobile
strays further from the base station, the RF2607 is
directed to increase its gain while the reverse occurs
as the mobile approaches the base station. The gain of
the IC, like the RF2609, is controlled by a single DC
voltage externally supplied by a digital-to-analog (D/A)
converter which is swept from 0 to 3VDC. Figure 5
illustrates the gain response of the RF2607 as the gain
control voltage is swept.

With this IC, the more demanding specification to meet
is noise figure. In a CDMA receiver, this component fol-
lows the downconverting mixer and must retain good
noise performance at the high end of the gain range. At
the same time the IIP3 must increase steadily as the
gain drops from the high end of the gain range. Using a
variable gain amplifier scheme similar to that used in
the RF2609, both requirements are achievable. At 45
dB gain the RF2607 typically produces a noise figure
of 4.5dB and an IIP3 of -41dBm. Figures 6 and 7 illus-
trate nominal performance of the IC over the entire
gain range.

Incorporating the RF2607 into a system is very simple.

The RF2607 has two input ports, one for CDMA opera-
tion and one for FM operation. This feature allows the
IC to be used in dual mode phones that operate in both
Advanced Mobile Phone Service (AMPS) systems and
IS-95 North American Digital Cellular (NADC) sys-
tems. In dual mode systems, two signal paths are
required at IF in order to filter properly. The CDMA path
requires a filter with a 1.26 MHz bandwidth while the

1 6

1 5

1 4

1 3

1 2

1 1

1 0

10 nF

R2:

1 k

10 nF

C D M A F i l t e r

OUT+

OUT-

IN+

IN-

Measurement

Reference Plane

=1 k

OUT

=1 k

LOAD

=1 k

R2 sets the balanced output impedance to 1 k

. L1 and C2

serve dual purposes. L1 serves as an output bias choke,
and C2 serves as a series DC block. In addition, the values
of L1 and C2 may be chosen to form an impedance
matching network if the load impedance is not 1k

Otherwise, the values of L1 and C1 are chosen to form a
parallel-resonant tank circuit at the IF when the IF filter's
input impedance is 1 k

LOAD,EFF

=500

Measurement

Reference Plane

10 nF

3.3 k

GAIN

CONTROL

Figure 4. RF2609 Application Circuit

RF2607 CDMA Gain vs. Gain Control Voltage

(Vcc=3.6 V, 85 MHz)

-60

-50

-40

-30

-20

-10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

GC (volts)

Gain (dB)

+25癈

-30癈

+80癈

Figure 5. RF2607 CDMA Gain vs. Gain Control Voltage

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FM path requires a filter with a 30 kHz bandwidth. The
RF2607 accommodates both paths by providing two
separate inputs that can be switched. Even though
there are two inputs, there is virtually no difference in
RF2607 performance between the two modes. The
SELECT pin (pin 7) is a digital switch which determines
in which mode the IC will be. Logical high corresponds
with CDMA mode while logical low corresponds with
FM mode.

In CDMA mode, pins 1 and 2 are the balanced input
port and the differential input impedance is 1000

. To

guarantee a good match to a 500

IF filter (see Figure

8), a 1000

1% resistor (R1) is placed across the input

port. The IC can operate with a higher or lower imped-
ance filter by merely adjusting the external 1% resistor.
The noise figure, gain, and IIP3 specifications, how-
ever, are all based upon the configuration in Figure 8.
Once the IF signal is fed into the IC, it travels through
four variable gain amplifier stages. Like the RF2609
each of these variable gain amplifiers is controlled by
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 4.7 k

resistor, which, together with a capac-

itor to ground, forms a lowpass filter to clean up the
signal from the D/A converter. Like the RF2609 the
gain control voltage range of 0 to 3VDC is referenced
to the GAIN label on Figure 8, not at pin 16.

In FM mode, pins 4 and 5 are the balanced input port
and the differential input impedance is 1000

. In FM

mode, however, most system designers prefer to cou-
ple into the part single-endedly. In this case pin 4 or pin
5 can be used as the input port with the other pin AC
coupled to ground. The single-ended input impedance
is then 850

The output port of the RF2607 consists of pins 9 and
10 and is open collector. Since the output is open col-
lector, the power supply must be fed through inductors.
As with the RF2609, the output impedance and load
can be varied. Typically, a 500

resistor is placed

across pins 9 and 10 to set the differential output
impedance of the IC. Referring to Figure 8, 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 fil-
ter does not look like 500

. If the filter impedance is

500

, then the values of L1 and C1 are chosen to form

a parallel-resonant tank circuit at the signal frequency.
In this case, C2 merely acts as a DC blocking capaci-
tor.

An alternative CDMA phone architecture integrates
additional components with both AGC amplifiers. In the
transmit chain, a quadrature modulator is added in
front of the AGC amplifier so that the system designer
interfaces with a baseband input port. In this case, I
and Q data drives the IC at baseband frequencies and
is upconverted to IF where the AGC amplifier takes
over. The RF9958 CDMA Transmit IC includes both the
modulator and AGC amplifier as well as an RF upcon-
verter. The upconverter can be powered down if the

RF2607 CDMA IIP3 vs. Gain

(Vcc=3.6 V, 85 MHz)

-60

-50

-40

-30

-20

-10

-60

-40

-20

Gain (dB)

IIP3 (dBm)

Figure 6. RF2607 CDMA IIP3 vs. Gain

RF2607 CDMA Noise Figure vs. Gain

(Vcc=3.6 V, 85 MHz)

-60

-40

-20

Gain (dB)

i
se

gure

(dB)

Figure 7. RF2607 CDMA Noise Figure vs. Gain

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system designer only wishes to use the modulator/
AGC section. In the receive chain, a demodulator is
added to the receive AGC amplifier. In this case, the
output of the part is I/Q data at baseband frequencies.

The RF9957 CDMA Receive IC includes both of these
functions. Both the RF9957 and RF9958 operate from
a single 3V power supply.

The RF2607 and the RF2609 are two critical compo

nents of the RF section of a CDMA phone (see Figure
9). Both of these IC's were designed to meet all of IS-
95's worst-case requirements and are available in high
volume at low cost. Please call RF Micro Devices' Mar-
keting Department for additional information and
details on these two products, or any others, at 910-
664-1233. Also, please check our web page at
www.rfmd.com.

1 6

1 5

1 4

1 3

1 2

1 1

1 0

GAIN

CONTROL

SEL.

10 nF

R1:

1 k

CDMA-

FM IN

CDMA+

IF IN
SELECT

4.7 k

10 nF

OUT+

OUT-

R2:

500

Measurement

Reference Plane

LOAD

=500

FM IF Filter

CDMA IF Filter

=500

IN, EFF

=500

=1 k

=850

S, EFF

=333

=850

Measurement

Reference Plane

LOAD,EFF

=250

OUT

=500

R1 sets the CDMA balanced input impedance. The effective input
impedance is then 500

R2 sets the balanced output impedance to 500

. L1 and C2 serve dual

purposes. L1 serves as an output bias choke, and C2 serves as a series DC
block. In addition, the values of L1 and C2 may be chosen to form an
impedance matching network of the load impedance is not 500

Otherwise, the values of L1 and C1 are chosen to form a parallel-resonant
tank circuit at the IF when the load impedance is 500

10 nF

Figure 8. RF2607 Application Circuit

协谢械泻褌褉芯薪薪褘泄 泻芯屑锌芯薪械薪褌: TA0030

协谢械泻褌褉芯薪薪褘泄泻芯屑锌芯薪械薪褌: TA0030