Analog Devices AN-826 Application Note page 6

AN-826
0
–10
–20
–30
–40
–50
–60
10k 100k
FREQUENCY (Hz)
Figure 7. Baseband Filter Gain Response in Decibels
40
30
20
10
0
10k 100k
FREQUENCY (Hz)
Figure 8. Baseband Filter Group Delay Response in Seconds
It is important to consider passive components with the lowest
tolerance for this filter, to minimize mismatches between I and
Q signal paths.
LO Feedthrough and Sideband Nulling
LO leakage at the output of the modulator comes from different
sources:
•
DC offsets between I and Q
•
DC offsets causing imbalance between the differential
signals I and I or Q and Q
•
Imperfect LO-to-RF isolation
Usually the most important sources of LO leakage are the
unwanted cumulated dc offsets on the baseband signals,
between the signal generation and the modulator mixers input.
On the other hand, amplitude and phase mismatches between
the I and Q signals and an inaccurate 90° LO phase shifter
result in an unwanted upper sideband image. When the Tx
DACs are configured for complex outputs, good image rejection
at the modulator output is critical because this spur falls inside
the channel and cannot be filtered. Phase mismatches cannot be
compensated for in this design, but amplitude matching may be
achieved through independent gain correction at the DAC level.
1M 10M 100M
1M 10M 100M
Similarly, the AD9862
to achieve LO leakage suppression.
I and Q amplitude mismatch correction is achieved by current
scaling. There is both a fine and coarse gain control (Register 14
and Register 15) to adjust the full-scale output current of either Tx
channel independently. The coarse gain control can be bypassed
where no current scaling is done or it can be scaled by 1/2 or 1/11
of the full-scale current. This translates to a −6 dB or −20 dB
change in the current. For finer resolution, the fine gain control
scales the full-scale currents individually on each leg by ±4%.
For LO suppression, a positive or negative offset can be applied
on either the I channel or the Q channel. With a 10-bit accuracy
(Register 10 to Register 13), an offset current of up to ±12% or
±2.4 mA for 20 mA full scale can be applied to either differential
channel. This is far more than what is usually required for LO
nulling.
The gain and offset mismatches are corrected after the analog
conversion, therefore maintaining the signal resolution. LO
leakage can be suppressed down to 75 dBc at the modulator
output and unwanted sideband can be reduced to −60 dBc at
room temperature.
Although the WiMAX OFDM signal has no subcarrier at dc, it
is important to achieve good dc offset correction to help the
demodulator distinguish between the on and off times of the
WiMAX burst and to make sure it does not saturate the receiver
ADC at low transmitted power.
Figure 9 shows the single sideband spectral characteristic at
the IQ modulator output once dc offset and gain calibration
have been applied. The unwanted sideband is at −60 dBc and
LO leakage at −70 dBc.
REF 10dBm ATT 15dBm
10
10 OFFSET 3.5dB
0
1 SA
–10
AVG
–20
–30
–40
–50
–60
–70
–80
–90
CENTER 2.4GHz
Figure 9. Single Sideband Spectrum at 2.4 GHz,
After DC/Gain Calibration (DAC+IQ Modulator Output)
Rev. B | Page 6 of 16
DACs allow dc offset correction voltages
*
RBW 50kHz MARKER 1 [T1]
VBW 200kHz
*
SWT 5ms
2.399000000GHz
DELTA 2 [T1]
1
2.000000000MHz
MARKER 3 [T1]
2.400000000GHz
2
3
650kHz/ SPAN 6.5MHz
–1.59dBm
–59.81dB
A
–69.84dBm