AD6640AST datasheet(21/24 Pages) AD

21 / 24 page

AD6640

REV. 0

–21–

The simplest method for generating dither is through the use of

a noise diode (Figure 48). In this circuit, the noise diode NC202

generates the reference noise that is gained up and driven by the

AD600 and OP27 amplifier chain. The level of noise may be

controlled by either presetting the control voltage when the

system is set up, or by using a digital-to-analog converter (DAC)

to adjust the noise level based on input signal conditions. Once

generated, the signal must be introduced to the receiver strip.

The easiest method is to inject the signal into the drive chain

after the last down conversion as shown in Figure 49.

NOISE SOURCE

(REF. FIGURE 48)

LPF

AIN

0.01 F

AIN

VREF

0.1 F

AD6640

COMBINER

BPF

FROM

RF/IF

IF AMP

Figure 49. Using the AD6640 with Dither

Receiver Example

To determine how the ADC performance relates to overall re-

ceiver sensitivity, the simple receiver in Figure 50 will be exam-

ined. This example assumes that the overall down conversion

process can be grouped into one set of specifications, instead of

individually examining all components within the system and

summing them together. Although a more detailed analysis

should be employed in a real design, this model will provide a

good approximation.

In examining a wideband digital receiver, several considerations

must be applied. Although other specifications are important,

receiver sensitivity determines the absolute limits of a radio

excluding the effects of other outside influences. Assuming that

receiver sensitivity is limited by noise and not adjacent signal

strength, several sources of noise can be identified and their

overall contribution to receiver sensitivity calculated.

RF/IF

AD6640

CHANNELIZER

REF IN

DSP

ENC

61.44MHz

GAIN = 30dB

NF = 10dB

BW =12.5MHz

SINGLE CHANNEL

BW = 30kHz

Figure 50. Receiver Analysis

The first noise calculation to make is based on the signal band-

width at the antenna. In a typical broadband cellular receiver,

the IF bandwidth is 12.5 MHz. Given that the power of noise in

a given bandwidth is defined by Pn = kTB, where B is band-

width, k = 1.38

× 10–23 is Boltzman’s constant and T = 300k

is absolute temperature, this gives an input noise power of

5.18

× 10–14 watts or –102.86 dBm. If our receiver front end has

a gain of 30 dB and a noise figure of 10 dB, then the total noise

presented to the ADC input becomes –62.86 dBm (–102.86 + 30

+ 10) or 0.16 mV rms. Comparing receiver noise to dither re-

quired for good SFDR, we see that in this example, our receiver

supplies about 3% of the dither required for good SFDR.

Based on a typical ADC SNR specification of 68 dB, the

equivalent internal converter noise is 0.140 mV rms. There-

fore total broadband noise is 0.21 mV rms. Before process-

ing gain, this is an equivalent SNR (with respect to full scale)

of 64.5 dB. Assuming a 30 kHz AMPS signal and a sample

rate of 61.44 MSPS, the SNR through processing gain is in-

creased by approximately 33 dB to 97.5 dB. However, if eight

strong and equal signals are present in the ADC bandwidth,

then each must be placed 18 dB below full scale to prevent

ADC overdrive. Therefore we give away 18 dB of range and

reduce the carrier-to-noise ratio (C/N) to 79.5 dB.

Assuming that the C/N ratio must be 10 dB or better for

accurate demodulation, one of the eight signals may be reduced by

66.5 dB before demodulation becomes unreliable. At this point,

the input signal power would be –90.5 dBm. Referenced to the

antenna, this is –120.5 dBm.

To improve sensitivity, several things can be done. First, the

noise figure of the receiver can be reduced. Since front end

noise dominates the 0.16 mV rms, each dB reduction in noise

figure translates to an additional dB of sensitivity. Second, pro-

viding broadband AGC can improve sensitivity by the range of

the AGC. However, the AGC would only provide useful im-

provements if all in-band signals are kept to an absolute minimal

power level so that AGC can be kept near the maximum gain.

This noise limited example does not adequately demonstrate the

true limitations in a wideband receiver. Other limitations such

as SFDR are more restrictive than SNR and noise. Assume that

the analog-to-digital converter has an SFDR specification of

–80 dBFS or –76 dBm (Full scale = +4 dBm). Also assume

that a tolerable carrier-to-interferer (C/I) (different from C/N)

ratio is 18 dB. This means that the minimum signal level is

–62 dBFS (–80 plus 18) or –58 dBm. At the antenna, this is

–88 dBm. Therefore, as can be seen, SFDR (single or multi-

tone) would limit receiver performance in this example. How-

ever, as shown previously, SFDR can be greatly improved

through the use of dither (Figures 19, 22). In many cases, the

addition of the out-of-band dither can improve receiver sensitiv-

ity nearly to that limited by thermal noise.

AD6640AST 数据表(PDF) 21 Page - Analog Devices

AD6640AST 数据表(HTML) 21 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

数据表下载

链接网址

AD6640AST 数据表(PDF) 21 Page - Analog Devices

AD6640AST 数据表(HTML) 21 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

数据表 下载

链接网址

数据表下载