TA0030 Datasheet, PDF (3/6 Pages) RF Micro Devices – TRANSMIT AND RECEIVE AGC AMPLIFIERS FOR CDMA CELLULAR /PCS PHONES

TA0030
  
The RF2607 features a 96dB gain range from -48dB
power gain to +48dB power gain and is powered from
a single 3.6V 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.
RF2607 CDMA Gain vs. Gain Control Voltage
(Vcc=3.6 V, 85 MHz)
60
50
+25°C
40
-30°C
30
+80°C
20
10
0
-10
-20
-30
-40
-50
-60
0.0
0.5
1.0
1.5
2.0
2.5
3.0
GC (volts)
Figure 5. RF2607 CDMA Gain vs. Gain Control Voltage
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 1.26MHz bandwidth while the
Measurement
Reference Plane
Z_S=1 kΩ
IN+
IN-
10 nF
1
10 nF
2
Z_IN=1 kΩ
3
4
5
GAIN
CONTROL
3.3 kΩ
16
GAIN
15
10 nF
14
10 nF
13
12
V_CC
6
11
7
10
8
9
Z_LOAD,EFF=500 Ω
R2 sets the balanced output impedance to 1 k Ω. 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 Ω.
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 Ω.
Figure 4. RF2609 Application Circuit
C1
R2:
1 kΩ
L1
C2
C2
C1
L1
Z_OUT=1 kΩ
V_CC
10 nF
Z_LOAD=1 kΩ
CDMA Filter
OUT+
OUT-
Measurement
Reference Plane
13
Copyright 1997-2000 RF Micro Devices, Inc.
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