AD6649 Analog Devices, AD6649 Datasheet - Page 22

• Manufacturer Part Number: AD6649
• Description: IF Diversity Receiver
• Manufacturer: Analog Devices

AD6649
DIGITAL PROCESSING
The AD6649 includes a digital processing section that provides
filtering. This digital processing section includes a numerically
controlled oscillator (NCO), a selectable FIR filter (high perfor-
mance or low latency), and a second coarse NCO (f
value) for output frequency translation (complex to real). These
blocks can be configured in several modes to implement a
signal processing function. Refer to Figure 1 for the functional
block diagram of the AD6649.
NUMERICALLY CONTROLLED OSCILLATOR (NCO)
Frequency translation is accomplished with an NCO shared
between the two channels. Amplitude and phase dither can be
enabled on chip to improve the noise and spurious performance of
the NCO.
Because the filtering prevents usage of part of the Nyquist
spectrum, a means is needed to translate the sampled input
spectrum into the usable range of the decimation filter. To
achieve this, a 32-bit, tuning, complex NCO is provided. This
NCO/mixer allows the input spectrum to be tuned to dc, where
it can be effectively filtered by the subsequent filter blocks to
prevent aliasing.
When using the low latency FIR, the NCO must be tuned to f
(0x40000000). This prevents unwanted aliases from falling back
into the band of interest.
NCO AND FIR FILTER MODES
The NCO and FIR blocks can be used in two modes depending on
the bandwidth and latency requirement of the application. The two
modes of operation of these blocks are summarized in Table 11.
Table 11. Signal Path Modes
Mode
Fixed-Frequency NCO,
Tunable-Frequency NCO,
95 MHz FIR Filter
100 MHz FIR Filter
FIR
Low latency
(default)
High performance
Output
Bandwidth at
245.76 MSPS
95 MHz
99.5 MHz
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Two fixed-coefficient FIR filters provide filtering capability. A
low latency FIR or a high performance FIR can be selected. It
removes the negative frequency images to avoid aliasing negative
frequencies for real outputs. Figure 38, Figure 39, and Figure 40
show the progression of a 95 MHz bandwidth signal through the
filter stages when using the fixed-frequency NCO and 95 MHz FIR
filter with a sample rate of 245.76 MSPS. The tunable-frequency
NCO can be used instead and operates in a similar fashion. In
these modes, the output is centered at 61.44 MHz, assuming a
245.76 MSPS sample rate.
f
A fixed-frequency f
decimated signal from dc to f
NCO is required in all operation modes because complex
output from the part is not supported.
Figure 38. Example AD6649 Real 95 MHz Bandwidth Input Signal Centered at
Figure 40. Example AD6649 95 MHz Bandwidth Output Signal Tuned to f
S
–122.88
Figure 39. Example AD6649 95 MHz Bandwidth Input Signal Tuned to DC
/4 FIXED-FREQUENCY NCO
–108.94
–75.38
Using the NCO (NCO Frequency = 61.44 MHz)
–61.44
0
COMPLEX ADC OUTPUT/NCO OUTPUT
S
/4 NCO is provided to translate the filtered,
61.44 MHz (f
13.94
(NCO Frequency = 61.44 MHz)
–47.5
–13.94
TUNED NCO OUTPUT
REAL ADC INPUT
61.44
S
0
/4 to allow a real output. The f
ADC
0
= 245.76 MHz)
13.94
108.94
47.5
122.88
61.44
Data Sheet
75.38
108.94
122.88
122.88
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/4
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/4