AD8340 Просмотр технического описания (PDF) - Analog Devices

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AD8340

700 MHz to 1000 MHz RF Vector Modulator

Analog Devices 

AD8340

INTERFACING TO HIGH SPEED DACs

The AD977x family of dual DACs is well suited for driving the

I and Q vector controls of the AD8340. While these inputs can

in general be driven by any DAC, the differential outputs and

bias level of the Analog Devices TxDAC® family allow for a

direct connection between DAC and modulator.

The AD977x family of dual DACs has differential current

outputs. The full-scale current is user programmable and is

usually set to 20 mA, that is, each output swings from 0 mA

to 20 mA.

The basic interface between the AD9777 DAC outputs and the

AD8340 I and Q inputs is shown in Figure 33. The resistors R1

and R2 set the dc bias level according to the equation:

Bias Level = Average Output Current × R1

For example, if the full-scale current from each output is 20 mA,

each output will have an average current of 10 mA. Therefore,

to set the bias level to the recommended 0.5 V, R1 and R2

should be set to 50 Ω each. R1 and R2 should always be equal.

If R3 is omitted, this results in an available swing from the DAC

of 2 V p-p differential, which is twice the maximum voltage

range required by the AD8340. DAC resolution can be

maximized by adding R3, which scales down this voltage

according to the following equation:

Full − Scale Swing =

( ) 2× I MAX

R1

(R2 + R3)

×

1



−

R2

R2 + R3





AD9777

IOUTA1

R1

R2

IOUTB1

OPTIONAL

LOW-PASS

FILTER

AD8340

IBBP

R3

IBBM

IOUTA2

R1

R2

IOUTB2

OPTIONAL

LOW-PASS

FILTER

QBBP

R3

QBBM

Figure 33. Basic AD9777 to AD8340 Interface

Data Sheet

1.15

1.13

1.10

1.08

1.05

1.02

1.00

0.97

0.95

0.92

0.90

0.88

0.85

0.82

0.80

0.77

0.75

0.72

0.70

50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130

R3

Figure 34. Peak-to-Peak DAC Output Swing vs.

Swing Scaling Resistor R3 (R1 = R2 = 50 Ω)

Figure 34 shows the relationship between the value of R3 and

the peak baseband voltage with R1 and R2 equal to 50 Ω. Figure 34

shows that a value of 100 Ω for R3 provides a peak-to-peak

swing of 1 V p-p differential into the AD8340 I and Q inputs.

When using a DAC, low-pass image reject filters are typically

used to eliminate the Nyquist images produced by the DAC.

They also provide the added benefit of eliminating broadband

noise that might feed into the modulator from the DAC.

CDMA2000 APPLICATION

To test the compliance to the CDMA2000 base station standard,

a single-carrier CDMA2000 test model signal (forward pilot, sync,

paging, and six traffic as per 3GPP2 C.S0010-B, Table 6.5.2.1)

was applied to the AD8340. A cavity-tuned filter was used to

reduce noise from the signal source being applied to the device.

The 4.6 MHz pass band of this filter is apparent in the

subsequent spectral plots (see Figure 35 to Figure 38).

Figure 35 shows a plot of the spectrum of the output signal

under nominal conditions. POUT is equal to −5 dBm and VI = VQ

= 0.353 V, that is, VIBBP − VIBBM = VQBBP − VQBBM =

0.353 V. Adjacent channel power is measured in 30 kHz

resolution bandwidth at 750 kHz and 1.98 MHz carrier offset.

Noise floor is measured at ±4 MHz carrier offset.

Rev. C | Page 14 of 20

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