AD6640

–19–

REV. A

band-pass filter will remove harmonics generated within the

amplifier, but intermods should be better than the performance

of the A/D converter. In the case of the AD6640, amplifier

intermods must be better than –80 dBFS when driving full-

scale power. As mentioned earlier, there are several amplifiers

to choose from and the specifications depend on the end

application. Figure 29 shows a typical multitone test.

FREQUENCY – MHz

0

–80

–120

–40

–100

–20

–60

dc

32.5

6.5

13.0

19.5

26.0

ENCODE = 65MSPS

Figure 29. Multitone Performance

Two other key considerations for the digital wideband receiver

are converter sample rate and IF frequency range. Since perfor-

mance of the AD6640 converter is largely independent of both

sample rate and analog input frequency (TPCs 4, 5, and 10), the

designer has greater flexibility in the selection of these parameters.

Also, since the AD6640 is a bipolar device, power dissipation is

not a function of sample rate. Thus there is no penalty paid in

power by operating at faster sample rates. All of this is good

because, by carefully selecting the input frequency range and

sample rate, some of the drive amplifier and ADC harmonics

can actually be placed out-of-band.

For example, if the system has second and third harmonics that

are unacceptably high, by carefully selecting the ENCODE rate

and signal bandwidth, these second and third harmonics can be

placed out-of-band. For the case of an ENCODE rate equal to

60 MSPS and a signal bandwidth of 7.5 MHz, placing the fun-

damental at 7.5 MHz places the second and third harmonics out

of band as shown in the Table II.

Table II.

ENCODE Rate

60 MSPS

Fundamental

7.5 MHz–15 MHz

Second Harmonic

15 MHz–30 MHz

Third Harmonic

22.5 MHz–30 MHz, 30 MHz–15 MHz

Another option can be found through band-pass sampling. If the

and filter combination must perform to the specification required.

performance required by the system specifications since all

harmonics would fall outside the pass-band filter. For example,

band filter’s range. The burden then has been passed off to the

filter design, provided that the ADC meets the basic specifications

at the frequency of interest. In many applications, this is a worth-

while trade-off since many complex filters can easily be realized

using SAW and LCR techniques at these relatively high IF fre-

quencies. Although harmonic performance of the drive amplifier

is relaxed by this technique, intermodulation performance cannot

be sacrificed since intermods must be assumed to fall in-band for

both amplifiers and converters.

Noise Floor and SNR

Oversampling is sampling at a rate that is greater than twice the

bandwidth of the signal desired. Oversampling does not have

anything to do with the actual frequency of the sampled signal;

it is the bandwidth of the signal that is key. Band-pass or IF

sampling refers to sampling a frequency that is higher than Nyquist

and often provides additional benefits such as down conversion

using the ADC and replacing a mixer with a track-and-hold. Over-

sampling leads to processing gains because the faster the signal is

digitized, the wider the distribution of noise. Since the integrated

noise must remain constant, the actual noise floor is lowered by

3 dB each time the sample rate is doubled. The effective noise

density for an ADC may be calculated by the equation

V

Hz

=

SNR /20

4 FS

For a typical SNR of 68 dB and a sample rate of 65 MSPS, this

is equivalent to 25 nV/

√Hz. This equation shows the relationship

equation may be used for computational purposes to determine

overall receiver noise.

PRESELECT

FILTER

LNA

5MHz–15MHz

PASS BAND

348

CMOS

BUFFER

D11

D0

+3.3V (D)

+5V (A)

AD6640

AIN

ENCODE

ENCODE

M/N PLL

SYNTHESIZER

LO

DRIVE

REF

IN

1900MHz

REFERENCE

CLOCK

65MHz

12

AD6620

(REF. FIG 27)

I & Q

DATA

CLK

ADSP-2181

NETWORK

CONTROLLER

INTERFACE

AIN

Figure 28. Simplified Wideband PCS Receiver