AD8366ACPZ-R7 datasheet(18/28 Pages) AD

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AD8366

Rev. A | Page 18 of 28

MATCHING

NETWORK

PAD

FILTER

BALUN

LC LOW-

PASS

FILTER

LC LOW-

PASS

FILTER

LC LOW-

PASS

FILTER

LC LOW-

PASS

FILTER

ADL5523

ADL5380

AD8366

ADL5523

ADF4350

ADC

Figure 51. Direct Conversion Receiver Block Diagram

DIRECT CONVERSION RECEIVER DESIGN

A direct conversion receiver directly demodulates an RF modulated

carrier to baseband frequencies, where the signals can be detected

and the conveyed information recovered. Eliminating the IF

stages and directly converting the signal to effectively zero IF

results in reduced component count. The image problems

associated with the traditional superheterodyne architectures

can be ignored as well. However, there are different challenges

associated with direct conversion that include LO leakage, dc

offsets, quadrature imperfections, and image rejection. LO

leakage causes self mixing that results in squaring of the LO

waveform which generates a dc offset that falls in band for the

direct conversion receiver. Residual dc offsets create a similar

interfering signal that falls in band. I/Q amplitude and phase

mismatch lead to degraded SNR performance and poor image

rejection in the direct conversion system. Figure 51 shows the

block diagram for a direct conversion receiver system.

QUADRATURE ERRORS AND IMAGE REJECTION

An overall RF-to-baseband EVM performance was measured

with the ADL5380 IQ demodulator preceding the AD8366, as

shown in Figure 56. In this setup, no LC low-pass filters were used

between the ADL5380 and AD8366. A 1900 MHz W-CDMA RF

signal with a 3.84 MHz symbol rate was used. The local oscillator

(LO) is set at 1900 MHz to obtain a zero IF baseband signal.

The gain of the AD8366 is set to maximum gain (~20.25 dB).

Figure 52 shows the SNR vs. the input power of the cascaded

system for a 5 MHz analysis bandwidth. The broad input power

range over which the system exhibits strong SNR performance

reflects the superior dynamic range of the AD8366.

–75

–65

–55

–45

–35

–25

–15

–5

INPUT POWER (dBm)

Figure 52. SNR vs. RF Input Power Level

The image rejection ratio is the ratio of the intermediate frequency

(IF) signal level produced by the desired input frequency to that

produced by the image frequency. The image rejection ratio is

expressed in decibels (dB). Appropriate image rejection is critical

because the image power can be much higher than that of the

desired signal, thereby plaguing the downconversion process.

Amplitude and phase balance between the I/Q channels are

critical for high levels of image rejection. Image rejection of

greater than 47 dB was measured for the combined ADL5380

and the AD8366 for a 5 MHz baseband frequency, as seen in

Figure 53. This level of image rejection corresponds to a ±0.5°

phase mismatch and a ±0.05 dB of amplitude mismatch for the

combined ADL5380 and AD8366. Looking back to Figure 7 and

Figure 10, the AD8366 exhibits only ±0.05 dB of amplitude mismatch

and ±0.05o of phase mismatch, thus implying that the AD8366

does not introduce additional amplitude and phase imbalance.

900

1500

1300

1100

1700 1900 2100 2300 2500 2700 2900

RF FREQUENCY (MHz)

Figure 53. Image Rejection vs. RF Frequency

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