Analog Devices AN-826 Application Note page 7

Finding the IQ Modulator Optimum Operating Point
Optimum output level for the modulator can be determined by
measuring the upconverted signal distortion in the frequency
domain. Input voltage at the modulator input is swept such that
the output power level varies from −6 dBm to −20 dBm.
The spectral masks are currently being defined but are typically
imposed by local regulations for out-of-band emissions. As of
today, the Korean WiBro standard provides a specific spectral
mask that applies in the 2.3 GHz to 2.4 GHz band, as shown in
Figure 10. Other masks have been defined for fixed and mobile
radio systems like in the U.S. for deployments in the 2.5 GHz to
2.69 GHz (FCC 04-258).
As an example, Figure 10 shows the characteristics of the WiBro
BTS mask, the modulated signal bandwidth being 8.75 MHz.
dBr
P1
P2
f
1
Figure 10. BTS WiBro Spectral Mask (RBW = 100 kHz)
The WiBro standard specifies different ACPR requirements
depending on the transmitted power at the antenna. Here the
requirements are derived for a power level at the antenna of
+33 dBm:
• P1 ≤ −34.5 dBr at f1 = 4.77 MHz (edge of the main
channel)
• P2 ≤ −52.4 dBr at f2 = 9.23 MHz (center of the adjacent
channel)
Figure 11 shows the modulator spectral performance with a
16 QAM, 256-OFDM signal according to this mask, with the
offsets scaled for a 10 MHz OFDM signal. An input drive level
has been varied such that the IQ modulator output power
ranges from −20 dBm to −6 dBm. An optimum operating point
is obtained at about −12 dBm output rms, where the mask is
met with more than 20 dB margin.
Figure 12 displays a more generic ACPR characteristic, again for
a 10 MHz OFDM signal, where adjacent and alternate powers
are compared to the main channel power level. All channel
power levels are integrated in a 9 MHz bandwidth and ACP1
and ACP2 are respectively calculated at 10 MHz and 20 MHz
offsets from the carrier.
f
2
FREQUENCY
OFFSET
Rev. B | Page 7 of 16
At the first frequency offset (ACP1), as the power level drops,
more backoff from the compression and third-order intercept
point helps reduce the signal distortion and improve ACPR
performance. As the output power drops further, degradation
of SNR (due to less signal energy compared to modulator noise
floor) is the reason for the dB-per-dB ACPR degradation. At the
second frequency offset given by ACP2, there is no spectral
regrowth and ACPR basically degrades with decreasing SNR.
–50
–55
–60
CHANNEL EDGE @ 5.45MHz
–65
–70
–75
ADJ CHANNEL @ 10.5MHz
–80
–22 –20 –18 –16
IQ MODULATOR OUTPUT POWER (dBm)
Figure 11. Modulator Performance According to WiBro Mask, with 16 QAM
OFDM Modulation, Function of I and Q Input Voltage (or IQ Modulator
Output Power), at 2400 MHz
–50
–55
–60
–65
–70
–75
–80
–22 –20 –18 –16
IQ MODULATOR OUTPUT POWER (dBm)
Figure 12. Modulator Output ACPR with 16 QAM OFDM Modulation,
Function of I and Q Input Voltage (or IQ Modulator Output Power), at
2400 MHz
The EVM performance with a WiMAX OFDM waveform,
for the IQ modulator itself, is also quite good at 0.6% rms. Most
of it is due to finite upper sideband cancellation as well as
second- and third-order intermodulation for each subcarrier
that falls within the main channel.
AN-826
–14 –12 –10 –8 –6 –4
ACP1
ACP2
–14 –12 –10 –8 –6 –4