EVAL-ADF7025DB1 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-ADF7025DB1' PDF : 44 Pages View PDF
ADF7025
TRANSMITTER
RF OUTPUT STAGE
The PA of the ADF7025 is based on a single-ended, controlled
current, open-drain amplifier that has been designed to deliver
up to 13 dBm into a 50 Ω load at a maximum frequency of
928 MHz.
The PA output current and, consequently, the output power are
programmable over a wide range. The PA configuration is
shown in Figure 25. The output power is independent of the
state of the DATA I/O pin. The output power is set using Bits
R2_DB [9:14].
R2_DB(30:31)
2
IDAC
6
R2_DB(9:14)
RFOUT
+
RFGND
FROM VCO
Figure 25. PA Configuration
R2_DB4
R2_DB5
DIGITAL
LOCK DETECT
The PA is equipped with overvoltage protection, which makes it
robust in severe mismatch conditions. Depending on the
application, one can design a matching network for the PA to
exhibit optimum efficiency at the desired radiated output power
level for a wide range of different antennas, such as loop or
monopole antennas. See the LNA/PA Matching section for
details.
PA Bias Currents
Control Bits R2_DB [30:31] facilitate an adjustment of the PA
bias current to further extend the output power control range, if
necessary. If this feature is not required, the default value of
7 µA is recommended. The output stage is powered down by
resetting Bit R2_DB4.
MODULATION SCHEME
Frequency Shift Keying (FSK)
Frequency shift keying is implemented by setting the N value for
the center frequency and then toggling this with the TxData
line. The deviation from the center frequency is set using
Bits R2_DB [15:23]. The deviation from the center frequency in
Hz is
PFD × Modulation Number
FSKDEVIATION [Hz] =
214
where Modulation Number is a number from 1 to 511
(R2_DB(15:23)).
Select FSK using Bits R2_DB [6:8].
4R
FSK DEVIATION
FREQUENCY
–FDEV
+FDEV
TxDATA
PFD/
CHARGE
PUMP
PA STAGE
VCO
÷N
THIRD-ORDER
Σ-∆ MODULATOR
FRACTIONAL-N
INTEGER-N
Figure 26. FSK Implementation
Modulation Index
The choice of deviation frequency for a given data rate is critical
to get optimum sensitivity performance from the ADF7025.
The modulation index (MI) of an FSK modulated signal is
defined as
MI = 2 × Frequency Deviation[Hz]
Data Rate [bps]
It is recommended to use a MI > 1 for the ADF7025. The
variation of receiver sensitivity with modulation index, for
various data rates, can be observed in Figure 16, Figure 17,
and Figure 18.
Rev. A | Page 18 of 44
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