AD9864BCPZRL Analog Devices Inc, AD9864BCPZRL Datasheet - Page 27

• Manufacturer Part Number: AD9864BCPZRL
• Description: IC,RF/Baseband Circuit,LLCC,48PIN,PLASTIC
• Manufacturer: Analog Devices Inc

Specifications of AD9864BCPZRL

• Rf Type: UHF, Cellular, GSM, EDGE, TETRA
• Frequency: 10MHz ~ 300MHz
• Features: General Purpose IF Subsystem
• Package / Case: 48-LFCSP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

For example, at f
250 pF is needed. However, in order to accommodate an induc-
tor tolerance of ±10%, the tank capacitance must be adjustable
from 227 pF to 278 pF. Selecting an external capacitor of 180 pF
ensures that even with a 10% tolerance and stray capacitances
as high as 30 pF, the total capacitance will be less than the
minimum value needed by the tank. Extra capacitance is sup-
plied by the AD9864’s on-chip programmable capacitor array.
Since the programming range of the capacitor array is at least
160 pF, the AD9864 has plenty of range to make up for the tol-
erances of low cost external components. Note that if f
increased by a factor of 1.44 MHz to 26 MHz so that f
becomes 3.25 MHz, reducing L and C by approximately the
same factor (i.e., L = 6.9 µH and C = 120 pF) still satisfies the
requirements stated above.
A 16 dB step attenuator is also included within the LNA/mixer
circuitry to prevent large signals (i.e., > –18 dBm) from over-
driving the Σ-∆ modulator. In such instances, the Σ-∆ modula-
tor will become unstable, thus severely desensitizing the
receiver. The 16 dB step attenuator can be invoked by setting
the ATTEN bit (Register 0x03, Bit 7), causing the mixer gain to
be reduced by 16 dB. The 16 dB step attenuator could be used
in applications in which a potential target or blocker signal
could exceed the IF input clip point. Although the LNA will be
driven into compression, it may still be possible to recover the
desired signal if it is FM. Refer to Table 14 to see the gain com-
pression characteristics of the LNA and mixer with the 16 dB
attenuator enabled.
Table 14. SPI Registers Associated with LNA/Mixer
Address
(Hex)
0x00
0x03
BAND-PASS ∑-∆ ADC
The ADC of the AD9864 is shown in Figure 46. The ADC con-
tains a sixth order multibit band-pass Σ-∆ modulator that
achieves very high instantaneous dynamic range over a narrow
frequency band. The loop filter of the band-pass Σ-∆ modulator
consists of two continuous-time resonators followed by a discrete
time resonator, with each resonator stage contributing a pair of
complex poles. The first resonator is an external LC tank, while
the second is an on-chip active RC filter. The output of the LC
resonator is ac-coupled to the second resonator input via 100 pF
capacitors. The center frequencies of these two continuous-time
resonators must be tuned to f
erly. The center frequency of the discrete-time resonator auto-
matically scales with f
f
CLK
8 /
=
Bit
Breakdown
(7:0)
(7)
1
2 /
[
CLK
×
= 18 MHz and L = 10 µH, a capacitance of
π
CLK
×
, thus no tuning is required.
(
2
L
8
Width
1
×
CLK
C
/8 for the ADC to function prop-
)
]
Default
Value
0xFF
0
CLK
Name
STBY
ATTEN
CLK
/8
is
Rev. 0 | Page 27 of 44
Figure 47 shows the measured power spectral density measured at
the output of the undecimated band-pass Σ-∆ modulator. Note
that the wide dynamic range achieved at the center frequency,
f
modulator have been successfully tuned. The out-of-band noise is
removed by the decimation filters following quadrature mixer.
The signal transfer function of the AD9864 possesses inherent
anti-alias filtering by virtue of the continuous-time portions of
the loop filter in the band-pass Σ-∆ modulator. Figure 48 illus-
trates this property by plotting the nominal signal transfer
function of the ADC for frequencies up to 2f
that naturally occur for all frequencies that alias to the f
pass band are clearly visible. Even at the widest bandwidth set-
ting, the notches are deep enough to provide greater than 80 dB
of alias protection. Thus, the wideband IF filtering require-
ments preceding the AD9864 will be determined mostly by the
mixer’s image band, which is offset from the desired IF input
frequency by f
associated with the ADC.
MXOP
CLK
/8, is achieved once the LC and RC resonators of the Σ-∆
Figure 46. Equivalent Circuit of Sixth Order Band-Pass Σ-∆ Modulator
Modulator ADC with f
–100
OUTPUT
MIXER
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
Figure 47. Measured Undecimated Spectral Output of Σ-∆
0
MXON
IF2P
IF2N
1
CLK
EXTERNAL
LC
/4 (i.e., 2 × f
CONTROL
DAC1
GAIN
2
CLK
= 18 MSPS and Noise Bandwidth of 3.3 kHz
NATOR
3
RESO-
FREQUENCY (MHz)
–2dBFS OUTPUT
RC
CLK
4
f
CLK
/8) rather than any aliasing
NATOR
= 13 MSPS TO 26 MSPS
RESO-
SC
5
6
LEVEL
FLASH
NINE-
ESL
f
NBW = 3.3kHz
CLK
CLK
7
. The notches
= 18MHz
8
AD9864
TO DIGITAL
FILTER
CLK
9
/8