adl5385 Analog Devices, Inc., adl5385 Datasheet - Page 17

• Manufacturer Part Number: adl5385
• Description: 50 Mhz To 2200 Mhz Quadrature Modulator
• Manufacturer: Analog Devices, Inc.

SPECTRAL PRODUCTS FROM HARMONIC MIXING
For broadband applications such as cable TV head-end
modulators, special attention must be paid to harmonics of the
LO. Figure 39 shows the level of these harmonics (out to 3 GHz)
as a function of the output frequency from 50 MHz to 1000 MHz,
in a single-sideband (SSB) test configuration, with a baseband
signal of 1 MHz and a SSB level of approximately −5 dBm. To
read this plot correctly, first pick the output frequency of
interest on the trace called P
be read off the harmonic traces at multiples of this frequency.
For example, at an output frequency of 500 MHz, the
fundamental power is −5 dBm. The power of the second
(P
and −16 dBm (at 1500 MHz), respectively. Of particular
importance are the products from odd-harmonics of the LO,
generated from the switching operation in the mixers.
For cable TV operation at frequencies above approximately
500 MHz, these harmonics fall out of the band and can be
filtered by a fixed filter. However, as the frequency drops below
500 MHz, these harmonics start to fall close to or inside the
cable band. This calls for either limitation of the frequency
range to above 500 MHz or the use of a switchable filter bank to
block in-band harmonics at low frequencies.
Figure 38. ACP1, ACP2, ACP3, and Modulation Error Ratio (MER) vs. Output
2fc − BB
–50
–55
–60
–65
–70
–75
–80
–85
–90
) and third (P
–24
–22
ACPR2 (5.25MHz)
ACPR3 (6.00MHz)
Power for 256 QAM Transmitter
3fc + BB
–20
CARRIER POWER (dBm)
) harmonics is −63 dBm (at 1000 MHz)
–18
OUT
ACPR1 (750kHz)
. The associated harmonics can
MER
–16
–14
–12
–10
48
47
46
45
44
43
42
41
40
Rev. 0 | Page 17 of 24
RF SECOND-ORDER PRODUCTS
A two-tone RF output signal produces second-order spectral
components at sum and difference frequencies. In broadband
systems, these intermodulation products fall inside the carrier
or in the adjacent channels. Output second-order RF
intermodulation intercept is defined as
where P
F
as a function of carrier frequency in Figure 40, where the
baseband tones are 3.5 MHz and 4.5 MHz at −5 dBm each.
OUT1
Figure 40. Output Second-Order Intermodulation vs. Carrier Frequency
OIP2_RF = P
+ F
–10
–20
–30
–40
–50
–60
–70
–80
–90
70
60
50
40
30
20
10
IM(RF)
0
Figure 39. Spectral Components for Output Frequencies
0
OUT2
0
0
. OIP2_RF levels from a two-tone test are plotted
is the level of the intermodulation product at
100
250
P
7LO + BB
OUT
200
500
from 50 MHz to 1000 MHz
+ (P
300
OUTPUT FREQUENCY (MHz)
OUTPUT FREQUENCY (MHz)
750
OUT
400
P
6LO – BB
− P
1000
500
IM(RF)
1250
P
5LO – BB
)
600
1500
P
700
4LO + BB
P
3LO + BB
1750
P
800
OUT
P
2LO – BB
ADL5385
2000
900
1000
2250