ADRF6720ACPZ-R7 datasheet(26/44 Pages) AD

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ADRF6720

Data Sheet

baseband frequency response of ADRF6720, facilitating high

CIF and providing sufficient flat bandwidth for digital

predistortion (DPD) algorithms. Any flatness variations across

frequency at the ADRF6720 RF output have been calibrated out

of this measurement.

Figure 48. ADRF6720 Baseband Frequency Response

CARRIER FEEDTHROUGH NULLING

Carrier feedthrough results from minute dc offsets that occur

on the differential baseband inputs. In an I/Q modulator,

nonzero differential offsets mix with the LO and result in

carrier feedthrough to the RF output. In addition to this effect,

some of the signal power at the LO input couples directly to the

RF output (this may be as a result of bond wire to bond wire

coupling or coupling through the silicon substrate). The net

carrier feedthrough at the RF output is the vector combination

of the signals that appear at the output as a result of these two

effects.

The ADRF6720 has a feature to add dc current, positive or

negative, to both the I and Q channels for carrier feedthrough

nulling. Figure 49 shows carrier feedthrough vs. DCOFF_I

(Register 0x33[15:8]) and DCOFF_Q (Register 0x33[7:0]).

The carrier feedthrough nulling can also be accomplished

externally by a TxDAC.

Figure 49. Carrier Feedthrough Optimization Through DCOFF_I and

DCOFF_Q Adjustment

SIDEBAND SUPPRESSION OPTIMIZATION

Sideband suppression results from gain and phase imperfection

between the I and Q channels. Sideband suppression also results

from the quadrature error in generating quadrature LO signals.

The net unwanted sideband signal at the RF output is the vector

combination of the signals as a result of these effects.

The ADRF6720 offers quadrature phase adjustment through the

I_LO (Register 0x32[3:0]) and Q_LO (Register 0x32[7:4])

parameters to reject unwanted sideband signal.

Figure 50 shows the level of unwanted sideband signal

achievable from the ADRF6720 across the I_LO and Q_LO

parameters

If further optimization is required, the amplitude and phase

adjustments can be made externally by a TxDAC. The result of

this type of adjustment is shown in Figure 51.

Figure 50. Sideband Suppression Optimization Through I_LO and Q_LO

Adjustment ; LO = 2140 MHz

Figure 51. Sideband Suppression Before and After Nulling Using I_LO and

Q_LO Through External Adjustment; LO = 2140 MHz

–1

–2

–3

–4

–5

–6

200

400

600

BB FREQUENCY (MHz)

800

1000

–20

–30

–40

–50

–60

–70

–30

–40

–50

–60

–70

DCO

FF_Q

300

200

100

150

200

250

300

–30

–35

–40

–50

–55

–65

–45

–60

–70

–30

–35

–40

–50

–55

–65

–45

–60

–70

Q_LO

–90

–80

–70

–60

–50

–40

–30

–20

–10

700

1200

1700

2200

2700

LO FREQUENCY (MHz)

BEFORE NULLING

AFTER NULLING BY I_LO, Q_LO IN ADRF6720

AFTER NULLING EXTERNALLY

Rev. 0 | Page 26 of 44

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