Pentek 6210 Operating Manual page 129

PRESELECT
LNA
FILTER
LO
DRIVE
1900MHz
SYNTHESIZER
65.00MHz
REFERENCE
System Requirements
Figure 46 shows a typical wideband receiver subsystem based
around the AD6640. This strip consists of a wideband IF filter,
amplifier, ADC, latches, channelizer and interface to a digital
signal processor. This design shows a typical clocking scheme
used in many receiver designs. All timing within the system is
referenced back to a single clock. While this is not necessary, it
does facilitate PLL design, ease of manufacturing, system test,
and calibration. Keeping in mind that the overall performance
goal is to maintain the best possible dynamic range, many con-
siderations must be made.
One of the biggest challenges is selecting the amplifier used to
drive the AD6640. Since this is a communications application,
it is common to directly sample an intermediate frequency (IF)
signal. As such, IF gain blocks can be implemented instead of
baseband op amps. For these gain block amplifiers, the critical
specifications are third order intercept point and noise figure. A
bandpass 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 47 shows a typical multitone test.
0
–20
ENCODE = 65MSPS
–40
–60
–80
–100
–120
dc 6.5 13.0
FREQUENCY – MHz
Figure 47. Multitone Performance
Two other key considerations for the digital wideband receiver
are converter sample rate and IF frequency range. Since per-
formance of the AD6640 converter is largely independent of
both sample rate and analog input frequency (Figures 10, 11
and 16), the designer has greater flexibility in the selection of
these parameters. Also, since the AD6640 is a bipolar device,
REV. 0
+5V (A)
5–15MHz
PASSBAND
AIN
AIN
ENCODE
M/N PLL
REF
ENCODE
IN
CLOCK
Figure 46. Simplified Wideband PCS Receiver
19.5 26.0 32.5
+3.3V (D) CMOS
BUFFER
348
D11
12
AD6640
D0
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 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 below.
Encode Rate
Fundamental
Second Harmonic
Third Harmonic
Another option can be found through bandpass sampling. If the
analog input signal range is from dc to FS/2, then the amplifier
and filter combination must perform to the specification re-
quired. However, if the signal is placed in the third Nyquist
zone (FS to 3 FS/2), the amplifier is no longer required to meet
the harmonic performance required by the system specifications
since all harmonics would fall outside the passband filter. For
example, the passband filter would range from FS to 3 FS/2.
The second harmonic would span from 2 FS to 3 FS, well out-
side the passband 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 worthwhile tradeoff since many complex filters can
easily be realized using SAW and LCR techniques alike at these
relatively high IF frequencies. Although harmonic performance
of the drive amplifier is relaxed by this technique, intermodula-
tion 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 sig-
nal, it is the bandwidth of the signal that is key. Bandpass 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. Oversampling leads to processing gains because the
–19–
AD6620
ADSP-2181
(REF. FIG 45)
I & Q
NETWORK
DATA
CONTROLLER
INTERFACE
CLK
Table II.
60 MSPS
7.5 MHz–15 MHz
15 MHz–30 MHz
22.5 MHz–30 MHz, 30 MHz–15 MHz
AD6640