ADRF6720-27ACPZ-R7 datasheet(27/44 Pages) AD

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ADRF6720-27

Data Sheet

Rev. B | Page 26 of 43

BASEBAND BANDWIDTH

The ADRF6720-27 can be used with a DAC generating a

complex IF (CIF), as well as a zero IF signal (ZIF). The 1 dB

bandwidth of the ADRF6720-27 is more than 1000 MHz.

Figure 48 shows the baseband frequency response of

ADRF6720-27, facilitating high CIF and providing sufficient

flat bandwidth for digital predistortion (DPD) algorithms. Any

flatness variations across frequency at the ADRF6720-27 RF

output have been calibrated out of this measurement.

Figure 48. ADRF6720-27 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 ADRF6720-27 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 ADRF6720-27 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 ADRF6720-27 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

–9

–8

–7

–6

–5

–4

–3

–2

–1

200

400

600

800

1000

BB FREQUENCY (MHz)

I_LO

Q_LO

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