13-154
TA0030
13
Copyright 1997-2000 RF Micro Devices, Inc.
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
Ω. To
guarantee a good match to a 500
Ω IF filter (see Figure
8), a 1000
Ω 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
Ω 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
Ω. 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
Ω.
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
Ω 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
Ω. If the filter impedance is
500
Ω, 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
0
-60
-40
-20
0
20
40
60
Gain (dB)
Figure 6. RF2607 CDMA IIP3 vs. Gain
RF2607 CDMA Noise Figure vs. Gain
(Vcc=3.6 V, 85 MHz)
0
10
20
30
40
50
60
70
80
-60
-40
-20
0
20
40
60
Gain (dB)
Figure 7. RF2607 CDMA Noise Figure vs. Gain
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