EVAL-ADF7021DBZ5 Datasheet, PDF (36/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Data Sheet
DEMODULATOR CONSIDERATIONS
2FSK Preamble
The recommended preamble bit pattern for 2FSK is a dc-free
pattern (such as a 10101010… pattern). Preamble patterns with
longer run-length constraints (such as 11001100…) can also be
used but result in a longer synchronization time of the received
bit stream in the receiver. The preamble needs to allow enough
bits for AGC settling of the receiver and CDR acquisition. A
minimum of 16 preamble bits is recommended. When the receiver
is using the internal AFC, the minimum recommended number
of preamble bits is 48.
The remaining fields that follow the preamble header do not
have to use dc-free coding. For these fields, the ADF7021 can
accommodate coding schemes with a run length of up to
eight bits without any performance degradation. If longer run
lengths are required, an encoding scheme such as 8B/10B or
Manchester encoding is recommended.
4FSK Preamble and Data Coding
The recommended preamble bit pattern for 4FSK is a repeating
00100010… bit sequence. This 2-level sequence of repeating
−3, +3, −3, +3 symbols is dc-free and maximizes the symbol
timing performance and data recovery of the 4FSK preamble in
the receiver. The minimum recommended length of the
preamble is 32 bits (16 symbols).
The remainder of the 4FSK packet should be constructed so
that the transmitted symbols retain close to dc-free property by
using data scrambling and/or by inserting specific dc balancing
symbols in the transmitted bit stream at regular intervals, such
as after every 8 or 16 symbols.
2FSK Correlator Demodulator and Frequency Errors
The ADF7021 has a number of options to combat frequency
errors that exist due to mismatches between the transmit and
receive crystals/TCXOs.
The receiver sensitivity performance can be maximized at low
modulation index by increasing the discriminator bandwidth of
the correlator demodulator. For modulation indices of less than
0.4, it is recommended to double the correlator bandwidth by
calculating K as follows:
K
=
Round

100
2× f
e3
DEV

The DISCRIMINATOR_BW should be recalculated using the
new K value. Figure 26 highlights the improved sensitivity that
can be achieved for 2FSK modulation, at low modulation
indices, by doubling the correlator bandwidth.
AFC OPERATION
The ADF7021 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.
Two methods of AFC supported on the ADF7021 are external
and internal.
External AFC
Here, the user reads back the frequency information through
the ADF7021 serial port and applies a frequency correction
value to the fractional-N synthesizer-N divider.
The frequency information is obtained by reading the 16-bit
signed AFC readback, as described in the Readback Format
section, and by applying the following formula:
Frequency Readback [Hz] =
(AFC_READBACK × DEMOD CLK)/2¹⁸
With AFC disabled, the correlator demodulator is tolerant to
frequency errors over the ±0.4 × fDEV range, where fDEV is the
FSK frequency deviation. For larger frequency errors, the
frequency tolerance can be widened to ±0.8 × fDEV by adjusting
the value of K and thus doubling the correlator bandwidth.
K should then be calculated as
K
=
Round

100
2×
× 10 ³
f _DEV

The DISCRIMINATOR_BW setting should also be recalculated
using the new K value. Doubling the correlator bandwidth to
improve frequency error tolerance in this manner typically
results in a 1 dB to 2 dB loss in receiver sensitivity.
Correlator Demodulator and Low Modulation Indices
The modulation index in 2FSK is defined as
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.
Internal AFC
The ADF7021 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
R10_DB4 to 1. A scaling coefficient must also be entered, based
on the crystal frequency in use. This is set up in R10_DB[5:16]
and should be calculated using
Modulation Index = ^{2 × f DEV}
Data Rate
AFC
_
SCALING
_
FACTOR
=
Round


2
24 _{× 500}
XTAL


Rev. B | Page 36 of 64