W3020 Datasheet, PDF (12/44 Pages) Agere Systems – GSM Multiband RF Transceiver

W3020 GSM Multiband RF Transceiver
Advance Data Sheet
December 1999
IF/Baseband Amplifier
The IF amplifier is a balanced-input/balanced-output
type and is connected to a balanced SAW filter. It
consists of three gain stages: an IF amplifier and two
sections of baseband amplifiers. The gain can be
changed in steps of 32, 16, 8, and 4 dB. The base-
band also contains a level-shifter stage to drive an
A/D converter directly. The level-shifter stage has 21
dB of gain that can be switched off. The specifications
below are for the two modes of operation.
The gain of the IF section is programmed via the
three-wire serial bus.
The IF amplifier contains the 32 dB amplifier stage
and has a gain of either 0 or 32 dB. The IF amplifier is
followed by a quadrature mixer with a fixed gain of
4 dB. The first baseband amplifier (G3, G2, G6) after
the low-pass filter and demodulator has gains
selectable between 0, 4, 8, 12, 16, 20, 24, and 28 dB.
Using the other gain steps, the IF and baseband gain
can be varied by 64 dB in 4 dB steps. The second
baseband amplifier (G5, G1) has gains selectable
between 0, 4, 21, and 25 dB. The 21 dB gain step in
the second baseband amplifier section is not tested
and should therefore not be used. Figure 3 is a
diagram of the gain steps.
G4
LPF1 G3 G2 G6* LPF2 G5* G1
32/0 6
*Not tested.
16/0 8/0 4/0
21/0 4/0
Figure 3. IF Amplifier Gain Steps
The baseband amplifier section contains dc correction
circuitry that minimizes dc offsets at the I/Q outputs.
The low-pass filters in the baseband contain a self-
calibrating circuit for tuning of filter cut-off frequency.
The selectable gain settings are programmed via the
TR register as described in the Programming
Information section. Filter tuning and dc calibration
are also explained in that section.
To achieve the specified absolute gain accuracy, the
total gain should be calibrated at room temperature.
This would normally be part of the overall phone
calibration. Absolute gain accuracy measures the gain
change over a specified temperature range relative to
the room temperature measurement. In the GSM
system, this specification is dependent on all the RX
functional blocks and not solely on the IF strip. The
relative gain accuracy is a measure of the gain stage
accuracy over a 20 dB range (see Figure 4). Relative
12
gain accuracy is determined after calibration of the
32 dB amplifier.
ACTUAL
GAIN (dB)
ACTUAL = REQUESTED
ACTUAL GAIN
X
Y
20 dB
REQUESTED GAIN (dB)
Note: X , Y = relative gain accuracy.
Figure 4. Actual Gain vs. Requested Gain
The input impedance of the IF strip will vary slightly
when the 32 dB amplifier is switched between the ON
and OFF states. We recommend that the IF strip be
matched with the 32 dB amplifier in the ON state to
provide the best match to the SAW filter when the
input level is at a minimum. The input matching
network can match the IF input directly to the SAW
filter or to 50 Ω.
A matching network to 50 Ω was chosen for the
evaluation board to allow for convenient laboratory
measurements. To keep the input impedance low and
minimize impedance variation between gain settings
of the IF stage, a resistor is shunt-connected between
the input terminals. The input network can then be
matched to the desired input impedance. (The
specified gain includes a resistor value of 500 Ω.) For
testing purposes, the input has been matched to 50 Ω,
and the gains of the IF/baseband amplifier are all
referred to a 50 Ω matched input impedance. The I/Q
outputs are terminated in high-impedance loads. The
gains are voltage gains and include the voltage gain in
the impedance transformation of the input matching
network. The network is illustrated in Figure 5.
W3020 G4 bit
IFIP PIN 35
INPUT
IMPEDANCE
50 Ω
1:1
R1
500 Ω
32/0
IFIN PIN 34
Note: Balun is shown for testing purposes only.
Figure 5. IF Strip Balanced Input Matching
Network
Lucent Technologies Inc.