AD9042

–22–

REV. A

AD9042

NOISE SOURCE

(REF. FIGURE 53)

LPF

FROM

RF/IF

AIN

Figure 54. Using the AD9042 with Dither

Receiver Example

To determine how the ADC performance relates to overall

receiver sensitivity, the simple receiver in Figure 55 will be

examined. 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

AD9042

CHANNELIZER

REF IN

DSP

ENC

40.96MHz

GAIN = 30dB

NF = 20dB

BW =12.5MHz

SINGLE CHANNEL

BW = 30kHz

Figure 55. 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

bandwidth, k = 1.38

T = 300k is absolute temperature, this gives an input noise

power of 5.18

front end has a gain of 30 dB and a noise figure of 20 dB, then

the total noise presented to the ADC input becomes –52.86 dBm

(–102.86 + 30 + 20) or 0.51 mV rms. Comparing receiver

noise to dither required for good SFDR, we see that in this

example, our receiver supplies about 10% 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. Therefore

total broadband noise is 0.529 mV rms. Before processing gain,

this is an equivalent SNR (with respect to full scale) of 56.5 dB.

Assuming a 30 kHz AMPS signal and a sample rate of

40.96 MSPS, the SNR through processing gain is increased by

28.3 dB to 84.8 dB. However, if 8 strong and equal signals are

present in the ADC bandwidth, then each must be placed 18 dB

below full scale to prevent ADC overdrive. In addition, 3 dB to

15 dB should be used for ADC headroom should another signal

come in-band unexpectedly. For this example, 12 dB of

headroom will be allocated. Therefore we give away 30 dB of

range and reduce the carrier-to-noise ratio (C/N)* to 54.8 dB.

Assuming that the C/N ratio must be 6 dB or better for accurate

demodulation, one of the eight signals may be reduced by 48.8 dB

before demodulation becomes unreliable. At this point, the

input signal power would be 40.6

µV rms on the ADC input or

–74.8 dBm. Referenced to the antenna, this is –104.8 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 0.529 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.

However, as shown previously, SFDR can be greatly improved

through the use of dither (Figures 22, 25). In many cases, the

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

sensitivity nearly to that limited by thermal noise.

Multitone Performance

The plot below shows the AD9042 in a worst case scenario of

four strong tones spaced fairly close together. In this plot no

dither was used, and the converter still maintained 85 dBFS of

spurious-free range. As illustrated previously, a modest amount

of dither introduced out-of-band could be used to lower the

nonlinear components.

FREQUENCY – MHz

0

–80

–120

–40

–100

–20

–60

dc

20.5

4.1

8.2

12.3

16.4

ENCODE = 41 MSPS

3

6

9

7

4

258

Figure 56. Multitone Performance

**C/I is the ratio of signal to inband interferer.