ADRF6821 datasheet(31/61 Pages) AD

31 / 61 page

Data Sheet

ADRF6821

Rev. A | Page 31 of 61

IMAGE REJECTION

For direct conversion systems, maximizing image rejection is

key to achieving performance and optimizing bandwidth. The

amplitude and phase mismatch of the baseband I and Q paths

directly translates to degradation in image rejection, as is shown

in the following equation. The equation translates the gain and

quadrature phase mismatch to the image rejection ratio (IRR)

performance.

)

cos(

)

cos(

10log

(dB)

IRR

where:

Ae is the amplitude error and is shown in Figure 58 for various

LO frequencies.

φe is the phase error and is shown in Figure 59 for various LO

frequencies.

The image rejection calculated with the given phase and gain

mismatches is shown in Figure 60.

0.50

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

100

200

300

400

500

150

250

350

450

IF FREQUENCY (MHz)

LO = 1000MHz

LO = 2000MHz

LO = 2800MHz

Figure 58. Gain Mismatch (Error) Between I and Q Outputs vs. IF Frequency

4.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

100

200

300

400

500

150

250

350

450

IF FREQUENCY (MHz)

LO = 1000MHz

LO = 2000MHz

LO = 2800MHz

Figure 59. Phase Mismatch (Error) Between I and Q Outputs vs. IF Frequency

One of the dominant sources of phase error in a system originates

from the demodulator where the quadrature phase split of the

LO signal occurs. The ADRF6821 offers phase and gain adjustment

of the I and Q paths independently to allow quadrature correction.

Adjusting the phase with the TRM_LODRV_CAPI bits

(Register 0x0092, Bits[3:0]) for the I path correction and the

TRM_LODRV_CAPQ bits (Register 0x0092, Bits[7:4]) for the

Q path correction accesses the quadrature correction. Adjust

the I_MIXER_GAIN_ADJ bits (Register 0x003A, Bits[3:2]) and

the Q_MIXER_GAIN_ADJ bits (Register 0x003A, Bits[5:4]) for

the I and Q outputs, respectively, to achieve gain correction.

Figure 60 shows uncalibrated and calibrated image rejection for

an LO frequency of 2800 MHz and across temperature.

100

200

300

400

500

150

250

350

450

IF FREQUENCY (MHz)

–40°C

+27°C

+105°C

CORRECTED

UNCORRECTED

Figure 60. Corrected and Uncorrected Image Rejection vs. IF Frequency for

Various Temperatures, fLO = 2800 MHz

For any correction circuit, it is important to observe the effect

of temperature on the correction level and settings. Figure 61

shows how the correction with a given phase and gain setting

holds across temperature.

4.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

100

200

300

400

500

150

250

350

450

IF FREQUENCY (MHz)

TRM_LODRV_CAPI = 0

TRM_LODRV_CAPI = 8

TRM_LODRV_CAPI = 15

Figure 61. Phase Mismatch vs. IF Frequency (fLO = 2800 MHz) for Various

Phase Setting Values

ADRF6821 датащи(PDF) 31 Page - Analog Devices

ADRF6821 датащи(HTML) 31 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

датащи скачать

ссылки URL