Datasheet

MAX5884
3.3V, 14-Bit, 200Msps High Dynamic
Performance DAC with CMOS Inputs
______________________________________________________________________________________ 13
Multitone Testing for GSM/EDGE
Applications
The transmitter sections of multicarrier base station
transceiver systems for GSM/EDGE usually present
communication DAC manufacturers with the difficult
task of providing devices with higher resolution, while
simultaneously reducing noise and spurious emissions
over a desired bandwidth.
To specify noise and spurious emissions from base sta-
tions, a GSM/EDGE Tx mask is used to identify the DAC
requirements for these parameters. This mask shows
that the allowable levels for noise and spurious emis-
sions are dependent on the offset frequency from the
transmitted carrier frequency. The GSM/EDGE mask
and its specifications are based on a single active car-
rier with any other carriers in the transmitter being dis-
abled. Specifications displayed in Figure 10 support
per-carrier output power levels of 20W or greater.
Lower output power levels yield less stringent emission
requirements.
For GSM/EDGE applications, the DAC demands spuri-
ous emission levels of less than -80dBc for offset fre-
quencies ≥6MHz. Spurious products from the DAC can
combine with both random noise and spurious prod-
ucts from other circuit elements. The spurious products
from the DAC should therefore be backed off by 6dB or
more to allow for these other sources and still avoid sig-
nal clipping.
The number of carriers and their signal levels with
respect to the full scale of the DAC are important as
well. Unlike a full-scale sine wave, the inherent nature
of a multitone signal contains higher peak-to-RMS
ratios, raising the prospect for potential clipping, if the
signal level is not backed off appropriately. If a trans-
mitter operates with four/eight in-band carriers, each
individual carrier must be operated at less than
-12dB FS/-18dB FS to avoid waveform clipping.
The noise density requirements (Table 2) for a
GSM/EDGE-based system can again be derived from
the system’s Tx mask. With a worst-case noise level of
-80dBc at frequency offsets of ≥6MHz and a measure-
ment bandwidth of 100kHz, the minimum noise density
per hertz is calculated as follows:
SNR
MIN
= -80dBc - 10 ✕ log
10
(100 ✕ 10
3
Hz)
SNR
MIN
= -130dBc/Hz
Since random DAC noise adds to both the spurious tones
and to random noise from other circuit elements, it is rec-
ommended reducing the specification limits by about
10dB to allow for these additional noise contributions
while maintaining compliance with the Tx mask values.
-120
-90
-110
-100
-80
-70
-60
-50
-40
-30
ANALOG OUTPUT POWER (dBm)
-20
4MHz/div
f
CLK
= 184.32MHz
f
CENTER
= 30.72MHz
ACLR = 72dB
Figure 8. ACLR for W-CDMA Modulation, Single Carrier
-120
-110
-80
-90
-100
-70
-60
-50
-40
-30
-20
ANALOG OUTPUT POWER (dBm)
4MHz/div
f
CLK
= 184.32MHz, f
CENTER
= 30.72MHz
ACLR = 68dB
Figure 9. ACLR for W-CDMA Modulation, Four Carriers
O
-30
-60
-70
-73
-75
-80
-90
0.2 0.4 0.6 1.2 1.8 6.0
IMD REQUIREMENT: < -70dBc
30kHz 100kHz
MEASUREMENT BANDWIDTH
TRANSMITTER EDGE
INBAND OUTBAND
WORST-CASE
NOISE LEVEL
AMPLITUDE (dBc)
FREQUENCY OFFSET FROM CARRIER (MHz)
Figure 10. GSM/EDGE Tx Mask Requirements