AD6640STSLASHPCB datasheet(23/25 Pages) AD

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AD6640

–22–

REV. A

In narrow-band applications, harmonics of the ADC can be

placed out-of-band. One example is the digitization of a

201 MHz IF signal using a 17.333 MHz clock. As shown in

Figure 33, the spurious performance has diminished due to

internal slew rate limitations of the ADC. However, the SNR of

the converter is still quite good. Subsequent digital filtering with

a channelizer chip such as the AD6620 will yield even better SNR.

For multicarrier applications, third order intercept of the drive

amplifier is important. If the input network is matched to the

internal 900

Ω input impedance, the required full-scale drive

level is –3 dBm. If spurious products delivered to the ADC are

required to be below –90 dBFS, the typical performance of the

ADC with dither applied, then the required third order intercept

point for the drive amplifier can be calculated.

For multicarrier applications, the AD6640 is useful up to about

80 MHz analog in. For single channel applications, the AD6640

is useful to 200 MHz as shown in the bandwidth charts. In

either case, many common IF frequencies exist in this range of

frequencies. If the ADC is used to sample these signals, they will

be aliased down to baseband during the sampling process in

much the same manner that a mixer will down-convert a signal.

For signals in various Nyquist zones, the following equations

may be used to determine the final frequency after aliasing.

1NYQUISTS = f SAMPLE − f SIGNAL

2NYQUISTS = abs ( f SAMPLE − f SIGNAL )

3NYQUISTS = 2 × f SAMPLE − f SIGNAL

4NYQUISTS = abs (2 × f SAMPLE − f SIGNAL )

Using the converter to alias down these narrow-band or wide-

band signals has many potential benefits. First and foremost is

the elimination of a complete mixer stage along with amplifiers,

filters, and other devices, reducing cost and power dissipation. In

some cases, the elimination of two IF stages is possible.

TPCs 15 and 18 illustrate a multicarrier, IF sampling system. By

using dither, all spurious components are forced below 90 dBFS

(TPC 18). The dashed line illustrates how a 5 MHz band-pass

filter could be centered at 67.5 MHz. As discussed earlier, this

approach greatly reduces the size and complexity of the receiver’s

RF/IF section.

FREQUENCY – MHz

198

207

199.8

201.6

203.4

205.2

100

ALIASED

SECOND HARMONIC

ALIASED

THIRD HARMONIC

ANALOG IF

FILTER MASK

ALIASED

SIGNALS

Figure 33. IF Sampling a 201 MHz Input

RECEIVE CHAIN FOR A PHASED-ARRAY CELLULAR

BASE STATION

The AD6640 is an excellent digitizer for beam forming in phased-

array antenna systems. The price performance of the AD6640

and AD6620 channelizers allows for a very competitive solution.

Phased-array base stations allow better coverage by focusing the

receivers’ sensitivity in the direction needed. Phased-array systems

allow for the electronic beam to form on the receive antennas.

A typical phased-array system may have eight antennas, as shown

in Figure 34. Since a typical base station will handle 32 calls,

each antenna would have to be connected to 32 receivers. If done

with analog or traditional radios, the system grows quite rapidly.

With a multicarrier receiver, however, the design is quite compact.

Each antenna would have a wideband down-converter with one

AD6640 per receiver. The output of each AD6640 would drive

32 AD6620 channelizers, which are phase locked in groups of

eight—one per antenna. This allows each group of eight AD6620s

to tune and lock onto a different user. When the incoming signal

direction is determined, the relative phase of each AD6620 in

the group can be adjusted so that the output signals sum together

in a constructive manner, giving high gain and directivity in the

direction of the caller. This application would not be possible

with traditional receiver designs.

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