ADRF6820ACPZ-R7 datasheet(27/45 Pages) AD

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Data Sheet

ADRF6820

Rev. C | Page 27 of 45

IMAGE REJECTION

The amplitude and phase mismatch of the baseband I and Q

paths directly translates to degradations in image rejection, and

for direct conversion systems, maximizing image rejection is

key to achieving performance and optimizing bandwidth. The

ADRF6820 offers phase adjustment of the I and Q paths

independently to allow quadrature correction. The quadrature

correction can be accessed by writing to Register 0x32, Bits[3:0]

for the I path correction and Register 0x32, Bits[7:4] for the Q

path correction. Figure 49 shows the available correction range

for various LO frequencies.

12 13 14 15

ILO OR QLO SETTING

3.0

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

ILO ADJUST

LO = 740MHz

LO = 940MHz

LO = 1940MHz

LO = 2540MHz

QLO ADJUST

2.9°

2.5°

1.1°

0.9°

Figure 49. Quadrature Correction Range

Use the following equation to translate the gain and quadrature

phase mismatch to image rejection ratio (IRR) performance.

( )

IRR

−

cos

log

where:

Ae is the amplitude error.

φe is the phase error.

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. Figure 50 to Figure 52 show the

level of image rejection achievable from the ADRF6820 across

different sweep parameters with no correction applied.

700

900

1100 1300 1500 1700 1900 2100 2300 2500 2700

RF FREQUENCY (MHz)

HIGH-SIDE LO: INT 2× LO

HIGH-SIDE LO: EXT. 1× LO, POLYPHASE

LOW-SIDE LO: INT 2× LO

LOW-SIDE LO: EXT. 1× LO, POLYPHASE

Figure 50. Image Rejection vs. RF Frequency, fIF = 200 MHz

–10

–8

–6

–4

–2

RF SIGNAL LEVEL (dBm)

LOW-SIDE LO: INT 2× LO

HIGH-SIDE LO: INT 2× LO

LOW-SIDE LO: EXT 1× LO, POLYPHASE

HIGH-SIDE LO: EXT 1× LO, POLYPHASE

Figure 51. Image Rejection vs. RF Signal Level, IF = 200 MHz, for High-Side LO

Injection fLO = 2000 MHz and fRF = 1800 MHz and Vice Versa for Low-Side Injection

100

200

300

400

500

600

EXTERNAL LO: POLYPHASE

INTERNAL 2× LO

IF FREQUENCY (MHz)

Figure 52. Image Rejection vs. IF Frequency, fLO = 1800 MHz

ADRF6820ACPZ-R7 数据表(PDF) 27 Page - Analog Devices

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