ADL5385

Rev. 0 | Page 17 of 24

–50

–90

–85

–80

–75

–70

–65

–60

–55

48

40

41

42

43

44

45

46

47

–24

–10

–12

–14

–16

–18

–20

–22

CARRIER POWER (dBm)

ACPR1 (750kHz)

ACPR2 (5.25MHz)

ACPR3 (6.00MHz)

MER

Figure 38. ACP1, ACP2, ACP3, and Modulation Error Ratio (MER) vs. Output

Power for 256 QAM Transmitter

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

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

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.

0

–90

0

1000

OUTPUT FREQUENCY (MHz)

–80

–70

–60

–50

–40

–30

–20

–10

100

200

300

400

500

600

700

800

900

Figure 39. Spectral Components for Output Frequencies

from 50 MHz to 1000 MHz

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

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.

70

0

0

2250

OUTPUT FREQUENCY (MHz)

60

50

40

30

20

10

2000

1750

1500

1250

1000

750

500

250

Figure 40. Output Second-Order Intermodulation vs. Carrier Frequency