lt5571 Linear Technology Corporation, lt5571 Datasheet - Page 13

• Manufacturer Part Number: lt5571
• Description: 620mhz - 1100mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Application Measurements
The LT5571 is recommended for base-station applications
using various modulation formats. Figure 10 shows a
typical application.
Figure 11 shows the ACPR performance for CDMA2000
using one and three channel modulation. Figures 12 and 13
illustrate the 1- and 3-channel CDMA2000 measurement.
To calculate ACPR, a correction is made for the spectrum
analyzer’s noise fl oor (Application Note 99).
If the output power is high, the ACPR will be limited by the
linearity performance of the part. If the output power is
low, the ACPR will be limited by the noise performance of
the part. In the middle, an optimum ACPR is obtained.
Because of the LT5571’s very high dynamic-range, the test
equipment can limit the accuracy of the ACPR measure-
ment. Consult Design Note 375 or the factory for advice
on ACPR measurement if needed.
Figure 12. 1-Channel CDMA2000 Spectrum
Figure 10. 620MHz to 1.1GHz Direct Conversion Transmitter Application
–100
–110
–120
–130
GENERATOR
BASEBAND
–30
–40
–50
–60
–70
–80
–90
Q-DAC
896.25
I-DAC
DOWNLINK TEST
MODEL 64 DPCH
UNCORRECTED
SPECTRUM
EN
SPECTRUM ANALYSER NOISE FLOOR
897.75
2, 4, 6, 9, 10, 12, 15, 17
RF FREQUENCY (MHz)
899.25
14
16
1
7
5
900.75
V-I
Q-CH
V-I
I-CH
CORRECTED
SPECTRUM
902.25 903.75
90°
5571 F12
VCO/SYNTHESIZER
0°
V
CC
8, 13
3
BALUN
Figure 13. 3-Channel CDMA2000 Spectrum
LT5571
–100
–110
–120
–130
–30
–40
–50
–60
–70
–80
–90
894
11
5571 F10
DOWNLINK
TEST MODEL
64 DPCH
CORRECTED
SPECTRUM
UN-
896
100nF
×2
CORRECTED SPECTRUM
PA
RF FREQUENCY (MHz)
898
TO 1100MvHz
RF = 620MHz
5V
SPECTRUM
ANALYSER
FLOOR
NOISE
900
The ACPR performance is sensitive to the amplitude
mismatch of the BBIP and BBIM (or BBQP and BBQM)
input voltage. This is because a difference in AC voltage
amplitude will give rise to a difference in amplitude between
the even-order harmonic products generated in the internal
V-I converter. As a result, they will not cancel out entirely.
Therefore, it is important to keep the amplitudes at the BBIP
and BBIM (or BBQP and BBQM) as equal as possible.
LO feedthrough and image rejection performance may
be improved by means of a calibration procedure. LO
feedthrough is minimized by adjusting the differential DC
offsets at the I and the Q baseband inputs. Image rejection
can be improved by adjusting the amplitude and phase
difference between the I and the Q baseband inputs. The
LO feedthrough and Image Rejection can also change
as a function of the baseband drive level, as depicted in
Figure 14.
902
904
5571 F13
Figure 11. CDMA2000 ACPR, ALTCPR and Noise vs
RF Output Power at 900MHz for 1 and 3 Carriers
906
–40
–50
–60
–70
–80
–90
–30
DOWNLINK TEST
MODEL 64 DPCH
Figure 14. Image Rejection and LO Feed-
Through vs Baseband Drive Voltage After
Calibration at 25°C
RF OUTPUT POWER PER CARRIER (dBm)
3-CH NOISE
–25
3-CH AltCPR
1-CH AltCPR
–10
–20
–30
–40
–50
–60
–70
–80
–90
20
10
0
–20
0
3-CH ACPR
I AND Q BASEBAND VOLTAGE (V
–15
1
P
RF
–10
25°C
85°C
–40°C
2
f
V
EN = HIGH, f
f
1-CH NOISE
BBI
LO
CC
= 900MHz, P
–5
= 2MHz, 0°
= 5V, f
ACPR
1-CH
5571 F11
3
BBQ
LT5571
RF
0
IR
–110
–120
–130
–140
–150
–160
= 2MHz, 90°
= f
LO
P-P,DIFF
BB
4
LO FT
= 0dBm
+ f
13
5571 F14
LO
)
5
5571f