Pentek 6210 Operating Manual page 132

AD6640
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
1NYQUISTS SAMPLE
= abs ( f
f
2NYQUISTS SAMPLE
= 2 × f
f
3NYQUISTS SAMPLE
= abs (2 × f
f
4NYQUISTS
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.
− f
SIGNAL
− f
)
SIGNAL
− f
SIGNAL
− f
)
SAMPLE SIGNAL
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.
0
20
40
ANALOG IF
FILTER MASK
60
80
100
198 199.8
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.
–22–
ALIASED
SIGNALS
ALIASED
3RD HARMONIC
ALIASED
2ND HARMONIC
201.6 203.4 205.2 207
FREQUENCY – MHz
REV. 0