AD6640

–21–

REV. A

NOISE SOURCE

(SEE FIGURE 30)

LPF

AIN

0.01 F

AIN

0.1 F

AD6640

COMBINER

BPF

FROM

RF/IF

IF AMP

Figure 31. Using the AD6640 with Dither

Receiver Example

To determine how the ADC performance relates to overall receiver

sensitivity, the simple receiver in Figure 32 will be examined.

This example assumes that the overall down conversion process

can be grouped into one set of specifications, instead of individu-

ally 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

ENCODE

61.44MHz

GAIN = 30dB

NF = 10dB

BW =12.5MHz

SINGLE CHANNEL

BW = 30kHz

Figure 32. Receiver Analysis

The first noise calculation to make is based on the signal bandwidth

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

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

k = 1.38

temperature, this gives an input noise power of 5.18

–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 required 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 equiva-

lent internal converter noise is 0.140 mV rms. Therefore, total

broadband noise is 0.21 mV rms. Before processing 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 increased 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 18 dB of

range is given away and the carrier-to-noise ratio (C/N) is reduced

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 domi-

nates the 0.16 mV rms, each dB reduction in noise figure translates

to an additional dB of sensitivity. Second, providing broadband

AGC can improve sensitivity by the range of the AGC. However,

the AGC would only provide useful improvements if all in-band

signals were kept to an absolute minimal power level so that

AGC could 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 (TPCs 13 and 16). In many cases, the addition of the

out-of-band dither can improve receiver sensitivity nearly to that

limited by thermal noise.

IF Sampling Using the AD6640 as a Mix-Down Stage

Since performance of the AD6640 extends beyond the baseband

region into the third Nyquist zone, the converter has many uses

as a mix-down converter in both narrow-band and wideband

applications. This application is called band-pass sampling. Doing

this has several positive implications in terms of the selection of

the IF drive amplifier. Not only is filtering a bit easier, the selection

of drive amplifiers is extended to classical IF gain blocks. In the

third Nyquist zone and above, the second and third harmonics

are easily filtered with a band-pass filter. Now only in-band

spurs that result from third order products are important.