AD8349AREZ Analog Devices Inc, AD8349AREZ Datasheet - Page 19

• Manufacturer Part Number: AD8349AREZ
• Description: IC QUADRATURE MOD 700MHZ 16TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD8349AREZ

• Function: Modulator
• Lo Frequency: 700MHz ~ 2.7GHz
• Rf Frequency: 700MHz ~ 2.7GHz
• P1db: 5.6dBm
• Noise Floor: -156dBm/Hz
• Output Power: 5.1dBm
• Current - Supply: 150mA
• Voltage - Supply: 4.75 V ~ 5.5 V
• Test Frequency: 2.14GHz
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Frequency Range: 700MHz To 2700MHz
• Rf Type: Quadrature
• Supply Voltage Range: 4.75V To 5.5V
• Rf Ic Case Style: TSSOP
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +85°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

REDUCING UNDESIRED SIDEBAND LEAKAGE
Undesired sideband leakage is the result of phase and amplitude
imbalances between the I and Q channel baseband signals.
Therefore, to reduce the undesired sideband leakage, the
amplitude and phase of the baseband signals have to be
matched at the mixer cores. Because of mismatches in the
baseband input paths leading to the mixers, perfectly matched
baseband signals at the pins of the device may not be perfectly
matched when they reach the mixers. Therefore, slight
adjustments have to be made to the phase and amplitudes of the
baseband signals to compensate for these mismatches.
Begin by making one of the inputs, say the I channel, the
reference signal. Then adjust the amplitude and phase of the
Q channel’s signal until the unwanted sideband power reaches a
trough. The AD9777 has built-in gain adjust registers that allow
this to be performed easily. If an iterative adjustment is
performed between the amplitude and the phase, the undesired
sideband leakage can be minimized significantly.
Note that the compensated sideband rejection performance
degrades as the operating baseband frequency is moved away
from the frequency at which the compensation was performed.
As a result, the frequency of the I and Q sine waves should be
approximately half the baseband bandwidth of the modulated
carrier. For example, if the modulator is being used to transmit
a single WCDMA carrier whose baseband spectrum spans from
dc to 3.84/2 MHz, the calibration could be effectively performed
with 1 MHz I and Q sine waves.
REDUCTION OF LO FEEDTHROUGH
Because the I and Q signals are being multiplied with the LO,
any internal offset voltages on these inputs will result in leakage
of the LO to the output. Additionally, any imbalance in the LO
to RF in the mixers will also cause the LO signal to leak through
the mixer to the RF output. The LO feedthrough is clearly
visible in the single sideband spectrum. The nominal LO
feedthrough of –42 dBm can be reduced further by applying
offset compensation voltages on the I and Q inputs. Note that
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
Figure 53. AD8349 Single Sideband Spectrum at 2140 MHz
THIRD HARMONIC = –36.8dBc
USB = –52dBc
SSB = 1.7dBm
LO = –44.5dBm
SPAN 10MHz
Rev. A | Page 19 of 28
the LO feedthrough is reduced by varying the differential offset
voltages on the I and Q inputs (xBBP – xBBN), not by varying
the nominal bias level of 400 mV. This is easily accomplished by
programming and then storing the appropriate DAC offset code
required to minimize the LO feedthrough. This, however,
requires a dc-coupled path from the DAC to the I and Q inputs.
The procedure for reducing the LO feedthrough is simple. A
differential offset voltage is applied from the I DAC until the LO
feedthrough reaches a trough. With this offset level held, a
differential offset voltage is applied to the Q DAC until a lower
trough is reached (This is an iterative process).
Figure 54 shows a plot of LO feedthrough vs. I channel offset (in
mV) after the Q channel offset has been nulled. This suggests
that the compensating offset voltage should have a resolution of
at least 100 µV to reduce the LO feedthrough to be less than –
65 dBm. Figure 55 shows the single sideband spectrum at 2140
MHz after the nulling of the LO. The reduced LO feedthrough
can clearly be seen when compared with the performance
shown in Figure 53.
Compensated LO feedthrough degrades somewhat as the LO
frequency is moved away from the frequency at which the
compensation was performed. This variation is very small
across a 30 MHz or 60 MHz cellular band, however. This small
variation is due to the effects of LO-to-RF output leakage
around the package and on the board.
–52
–54
–56
–58
–60
–62
–64
–66
–68
–70
Figure 54. Plot of LO Feedthrough vs. I Channel Baseband Offset
3.0
3.5
(Q Channel Offset Nulled)
IOPP-IOPN (mV)
4.0
4.5
5.0
AD8349
5.5