AD6640

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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 narrowband and wideband

applications. This application is called bandpass sampling. Do-

ing 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 bandpass filter. Now

only in-band spurs that result from third order products are

important.

In narrowband 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 51, 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 ohm 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 from 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.

f

f

f

f

Using the converter to alias down these narrowband or wideband

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.

Figures 21 and 24 in Typical Performance Characteristics illus-

trate a multicarrier, IF Sampling System. By using dither, all

spurious components are forced below 90 dBFS (Figure 24).

The dashed line illustrates how a 5 MHz bandpass 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

0

60

198

207

199.8

201.6

203.4

205.2

40

80

20

100

ALIASED

2ND HARMONIC

ALIASED

3RD HARMONIC

ANALOG IF

FILTER MASK

ALIASED

SIGNALS

Figure 51. 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 followed by AD6620 channelizers allows for a very

competitive solution. Phase 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 52. 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 will 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 AD6620’s 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 such the

output signals sum together in a constructive manner, giving

high gain and directivity in the direction of the caller. This ap-

plication would not be possible with traditional receiver designs.