LT5518 데이터 시트보기 (PDF) - Linear Technology

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LT5518

1.5GHz–2.4GHz High Linearity Direct Quadrature Modulator

Linear Technology 

LT5518

APPLICATIO S I FOR ATIO

Baseband Interface

The baseband inputs (BBPI, BBMI), (BBPQ, BBMQ) pres-

ent a differential input impedance of about 2.9kΩ. At each

of the four baseband inputs, a lowpass filter using 200Ω

and 1.8pF to ground is incorporated (see Figure 1), which

limits the baseband bandwidth to approximately 250MHz

(–1dB point). The common mode voltage is about 2.06V

and is slightly temperature dependent. At TA = – 40oC, the

common mode voltage is about 2.19V and at TA = 85oC

it is about 1.92V.

If the I/Q signals are DC-coupled to the LT5518, it is

important that the applied common mode voltage level

of the I and Q inputs is about 2.06V in order to properly

bias the LT5518. Some I/Q test generators allow setting

the common mode voltage independently. In this case, the

common mode voltage of those generators must be set

to 1.03V to match the LT5518 internal bias, because for

DC signals, there is no – 6dB source-load voltage division

(see Figure 2).

50Ω

–+ 2.06VDC

GENERATOR

1.03VDC

50Ω

50Ω

1.5k

–+ 2.06VDC

GENERATOR

2.06VDC

2.06VDC –+

LT5518

5518 F02

Figure 2. DC Voltage Levels for a Generator Programmed at

1.03VDC for a 50Ω Load and for the LT5518 as a Load

The LT5518 should be driven differentially; otherwise, the

even-order distortion products will degrade the overall

linearity severely. Typically, a DAC will be the signal source

for the LT5518. A reconstruction filter should be placed

between the DAC output and the LT5518’s baseband

inputs. DC coupling between the DAC outputs and the

LT5518 baseband inputs is recommended. Active level

shifters may be required to adapt the common mode level

of the DAC outputs to the common mode input voltage

of the LT5518. It is also possible to achieve a DC level

shift with passive components, depending on the appli-

cation. For example, if flat frequency response to DC is

not required, then the interface circuit of Figure 3 may be

used. This figure shows a commonly used 0mA – 20mA

DAC output followed by a passive 5th order lowpass

filter. The DC-coupled interface allows adjustment of the

8

DAC’s differential output current to minimize the LO to RF

feedthrough. Resistors R3A, R3B, R4A and R4B translate

the DAC’s output common mode level from about 0.5VDC

to the LT5518’s input at about 2.06VDC. For these resis-

tors, 1% accuracy is recommended. For different ambi-

ent temperatures, the LT5518 input common mode level

varies with a temperature coefficient of about –2.7mV/°C.

The internal common mode feedback loop will correct

these level changes in order to bias the LT5518 at the

correct operating point. Resistors R3 and R4 are chosen

high enough that the LT5518 common mode compliance

current value will not be exceeded at the inputs of the

LT5518 as a result of temperature shifts. Capacitors C4A

and C4B minimize the input signal attenuation caused by

the network R3A, R3B, R4A and R4B. This results in a

gain difference between low frequency and high frequency

baseband signals. The high frequency baseband –3dB

corner point is approximately given by:

f–3dB = 1/[2π • C4A • (R3A||R4A||(RIN, DIFF/2)]

In this example, f–3dB = 58kHz.

This corner point should be set significantly lower than the

minimum baseband signal frequency by choosing large

enough capacitors C4A and C4B. For signal frequencies

significantly lower than f–3dB, the gain is reduced by ap-

proximately

GDC = 20 • log [R3A||(RIN, DIFF/2)]/[R3A||(RIN, DIFF/2)

+ R4A]

In this example, GDC = –11dB.

Inserting the network of R3A, R3B, R4A, R4B, C4A and

C4B has the following consequences:

• Reduced LO feedthrough adjustment range. LO to RF

feedthrough can be reduced by adjusting the differential

DC offset voltage applied to the I and/or Q inputs. Be-

cause of the DC gain reduction, the range of adjustment

is reduced. The resolution of the offset adjustment is

improved by the same gain reduction factor.

• DC notch for uneven number of channels. The interface

drawn in Figure 3 might not be practical for an uneven

number of channels, since the gain at DC is lower and

will appear in the center of (one of) the channel(s). In

that case, a DC-coupled level shifting circuit is required,

or the LT5528 might be a better solution.

5518f

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