LT5528 Ver la hoja de datos (PDF) - Linear Technology

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LT5528

1.5GHz to 2.4GHz High Linearity Direct Quadrature Modulator

Linear Technology 

LT5528

APPLICATIO S I FOR ATIO

internal circuit of the LT5528 is drawn. For best results,

a high ohmic source is recommended; for example, the

interface circuit drawn in Figure 3, modified by pulling

resistors R5 and R6 to a –5V supply and adjusting their

values to 550Ω, with T1 omitted.

Another method to reduce current mismatch between

the currents flowing in the BBIP and BBIM pins (or the

BBQP and BBQM pins) is to use a 1:1 transformer with

the two windings in the DC path (T1 in Figure 3). For DC,

the transformer forms a short, and for AC, the transformer

will reduce the common-mode current component, which

forces the two currents to be better matched. Alternatively,

a transformer with 1:2 impedance ratio can be used, which

gives a convenient DC separation between primary and

secondary in combination with the required impedance

match. The secondary center tap should not be connected,

which allows some voltage swing if there is a single-

ended input impedance difference at the baseband pins.

As a result, both currents will be equal. The disadvantage

is that there is no DC coupling, so the LO feed-through

calibration cannot be performed via the BB connections.

After calibration when the temperature changes, the LO

feed-through and the Image Rejection performance will

change. This is illustrated in Figure 17. The LO feed-through

and Image Rejection can also change as a function of the

baseband drive level, as is depicted in Figure 18. The RF

output power, IM2 and IM3 vs a two-tone baseband drive

are given in Figure 19.

–50

–55

LO FEED-THROUGH

–60

–65

–70 IMAGE REJECTION

–75

–80

CALIBRATED WITH PRF = –10dBm

–85

–40 –20 0 20 40 60 80

TEMPERATURE (°C)

EN = HIGH

VCC = 5V

fBBI = 2MHz, 0°

fBBQ = 2MHz, 90°

fLO = 2.14GHz

fRF = fBB + fLO

PLO = 0dBm

5528 F18

Figure 17: LO Feed-Through and Image Rejection vs Temperature

after Calibration at 25°C

–20

10

PRF

–30

0

LOFT

–40

–10

–50

–20

–60

IR –30

–70

–40

–80

–40°C –50

25°C

85°C

–90

–60

0

1

2

3

4

5

I AND Q BASEBAND VOLTAGE (VP-P, DIFF)

EN = HIGH

VCC = 5V

fBBI = 2MHz, 0°

fBBQ = 2MHz, 90°

fLO = 2.14GHz

fRF = fBB + fLO

PLO = 0dBm

5528 F18

Figure 18: LO Feed-Through and Image Rejection vs Baseband

Drive Voltage after Calibration at 25°C

14

10

PRF

0

–10

IM3

–20

–30

–40

IM2

–50

–60

–70

–80

–90

0.1

–40°C

25°C

85°C

1

10

I AND Q BASEBAND VOLTAGE (VP-P, DIFF EACH TONE)

EN = HIGH

fLO = 2.14GHz

VCC = 5V

fRF = fBB + fLO

fBBI = 2MHz, 2.1MHz, 0°

PLO = 0dBm

fBBQ = 2MHz, 2.1MHz, 90°

IM2 = POWER AT fLO + 4.1MHz

IM3 = MAX POWER AT fLO + 1.9MHz OR

fLO + 2.2MHz

5528 F19

Figure 19: RF Two-Tone Power, IM2 and IM3 at 2140MHz vs Baseband Voltage

5528f

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