EVAL-ADF7021-VDB2Z Analog Devices Inc, EVAL-ADF7021-VDB2Z Datasheet - Page 34

• Manufacturer Part Number: EVAL-ADF7021-VDB2Z
• Description: 868 - 870MHz - EVALUATION BOARD
• Manufacturer: Analog Devices Inc
• Type: Transceiverr

Specifications of EVAL-ADF7021-VDB2Z

• Frequency: 868MHz ~ 870MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: ADF7021
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ADF7021-V
Correlator Demodulator and Low Modulation Indexes
The modulation index in 2FSK is defined as
The receiver sensitivity performance and receiver frequency
tolerance can be maximized at low modulation indexes by
increasing the discriminator bandwidth of the correlator
demodulator. For modulation indexes of less than 0.4, it is
recommended that the correlator bandwidth be doubled by
calculating K as follows:
The DISCRIMINATOR_BW value in Register 4 should be
recalculated using the new K value. Figure 29 illustrates the
improved sensitivity that can be achieved for 2FSK modulation,
at low modulation indexes, by doubling the correlator bandwidth.
AFC OPERATION
The ADF7021-V also supports a real-time AFC loop that is
used to remove frequency errors due to mismatches between
the transmit and receive crystals/TCXOs. The AFC loop uses
the linear frequency discriminator block to estimate frequency
errors. The linear FSK discriminator output is filtered and
averaged to remove the FSK frequency modulation using a
combined averaging filter and envelope detector. In receive
mode, the output of the envelope detector provides an estimate
of the average IF frequency.
The two methods of AFC supported on the ADF7021-V are
external AFC and internal AFC.
External AFC
With external AFC, the user reads back the frequency infor-
mation through the ADF7021-V serial port and applies a
frequency correction value to the synthesizer-N divider.
The frequency information is obtained by reading the
signed, 16-bit AFC readback value, as described in the
Readback Format section, and by applying the following
formula:
Although the AFC readback value is a signed number, under
normal operating conditions, it is positive. In the absence of
frequency errors, the frequency readback value is equal to the
IF frequency of 100 kHz.
Frequency Readback (Hz) = (AFC READBACK ×
DEMOD CLK)/2
Modulation
K
=
Round
⎛
⎜ ⎜
⎝
Index
2
100
×
18
f
DEV
=
3
Data
2
⎞
⎟ ⎟
⎠
×
f
DEV
Rate
Rev. 0 | Page 34 of 60
Internal AFC
The ADF7021-V supports a real-time, internal, automatic
frequency control loop. In this mode, an internal control loop
automatically monitors the frequency error and adjusts the
synthesizer-N divider using an internal proportional integral
(PI) control loop.
The internal AFC control loop parameters are controlled in
Register 10. The internal AFC loop is activated by setting
Bit DB4 in Register 10 to 1. A scaling coefficient must also be
entered, based on the crystal frequency in use. This is set up
using Bits[DB16:DB5] in Register 10 and should be calculated
as follows:
Maximum AFC Range
The maximum frequency correction range of the AFC loop
is programmable using Register 10, Bits[DB31:DB24]. The
maximum AFC correction range is the difference in frequency
between the upper and lower limits of the AFC tuning range.
For example, if the maximum AFC correction range is set to
10 kHz, the AFC can adjust the receiver LO within the f
5 kHz range.
However, when RF_DIVIDE_BY_2 (Register 1, Bit DB18) is
enabled, the programmed range is halved. The user should
account for this halving by doubling the programmed maxi-
mum AFC range.
The recommended maximum AFC correction range should be
≤1.5 × IF filter bandwidth. If the maximum frequency correction
range is set to be >1.5 × IF filter bandwidth, the attenuation of
the IF filter can degrade the AFC loop sensitivity.
The adjacent channel rejection (ACR) performance of the receiver
can be degraded when AFC is enabled and the AFC correction
range is close to or greater than the IF filter bandwidth. However,
because the AFC correction range is programmable, the user
can trade off AFC correction range and ACR performance of
the receiver.
When AFC errors are removed using either the internal or
external AFC, further improvement in receiver sensitivity can
be obtained by reducing the IF filter bandwidth using the
IF_FILTER_BW bits (Register 4, Bits[DB31:DB30]).
AFC
_
SCALING
_
FACTOR
=
Round
⎛
⎜
⎜
⎝
2
24
XTAL
×
500
⎞
⎟
⎟
⎠
LO
±