ADRF6780ACPZN-R7 datasheet(23/35 Pages) AD

23 / 35 page

Data Sheet

ADRF6780

Rev. D | Page 23 of 35

APPLICATIONS INFORMATION

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 because of the 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 because of these two effects. A

TxDAC can externally accomplish carrier feedthrough nulling.

SIDEBAND SUPPRESSION OPTIMIZATION

Sideband results from gain and phase imperfections between

the I and Q channels. Sideband also results from the quadrature

error in generating the quadrature LO signals. Quadrature I and Q

signals are constructed in the LO path, and the vector combination

of these signals at the RF output results in suppression of the

unwanted sideband. Deviation from perfect quadrature on these

signals limits the amount of achievable sideband suppression.

The ADRF6780 offers quadrature phase adjustment in the LO

path quadrature signals. Make these adjustments through the

I_PATH_PHASE_ACCURACY bits (Register 0x05, Bits[3:0])

and Q_PATH_PHASE_ACCURACY (Register 0x05, Bits[7:4])

bits to reject the unwanted sideband signal.

Figure 14 shows the level of unwanted sideband signal achievable

from the ADRF6780 across the I_PATH_PHASE_ACCURACY

bits (Register 0x05, Bits[3:0]) and Q_PATH_PHASE_ACCURACY

(Register 0x05, Bits[7:4]) bits.

If further optimization is needed, adjust the amplitude and

phase externally through a TxDAC.

LINEARITY

The linearity in the ADRF6780 can be optimized through the

distortion cancellation circuit that is set up by the RDAC_

LINEARIZE bits (Register 0x04, Bits[7:0]) SPI settings. The

distortion cancellation circuit connects in parallel with the

baseband signal path in such a way that the fundamental is

minimally affected whereas the third-order portions cancel to

some degree. Adjusting the value of the RDAC_LINEARIZE

bits (Register 0x04, Bits[7:0]) changes the resistance value in

the cancellation path by fine tuning the amount of third-order

destructively added to the main signal path. It also serves as a

form or predistortion for third-order impedance generated

further down in the signal path.

Figure 16 and Figure 44 show the level of linearity improvement

achievable across the ADRF6780 RDAC_LINEARIZE bits.

ADC

The ADRF6780 includes an ADC that connects to a detector.

The user has an option to read the detector output from the

detector output pin (VDET, Pin 1) or using the ADC from the

SPI. Figure 71 shows normal operation at I/Q mode, an RF output

of 6.7 GHz, an I/Q input of 1 MHz, and a maximum gain. Figure 72

shows normal operation at IF mode and an RF output of 6.7 GHz,

an IF input of 800 MHz, and a maximum gain.

0.05

0.25

0.45

0.65

0.85

1.05

1.25

100

150

200

250

300

RF OUTPUT POWER (dBm)

ADC VALUE

DETECTOR VOLTAGE(V)

–35

–31

–27

–23

–19

–15

–11

–7

–3

Figure 71. ADC Detector Output and Detector Output Power, I/Q Mode

0.05

0.25

0.45

0.65

0.85

1.05

1.25

100

150

200

250

300

–35

–31

–27

–23

–19

–15

–11

–7

–3

RF OUTPUT POWER (dBm)

ADC VALUE

DETECTOR VOLTAGE (V)

Figure 72. ADC Detector Output and Detector Output Power, IF Mode

The following procedure is to read back from the detector for

the first time. For each subsequent readback, repeat Step 4 to

Step 8. To read back from the detector using the ADC, take the

following steps:

1. Set Bit 7 of Register 0x03 to 1 (DETECTOR_ENABLE).

2. Set Bit 1 of Register 0x06 to 1 (ADC_ENABLE). Write

0x02 to Register 0x06.

3. Set Bit 0 of Register 0x06 to 1 (ADC_CLOCK_ENABLE).

Write 0x03 to Register 0x06.

4. Set Bit 2 of Register 0x06 to 1 (ADC_START). Write 0x07

to Register 0x06.

5. Wait approximately 200 µs for the ADC to be ready.

6. Read back Bit 8 of Register 0x0C to 1 (ADC_STATUS).

When the bit is high, proceed to Step 7.

7. Set Bit 2 of Register 0x06 to 0 (ADC_START).

8. Read back Bits[7:0] of Register 0x0C for the ADC

value (ADC_VALUE).

To disable the ADC, disable the ADC_CLOCK_ENABLE,

ADC_ENABLE, and ADC_START bits.

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