ADA2200-EVALZ User Guide
UG-702
SYNCHRONOUS DEMODULATION USING THE
INPUT SIGNAL SYNCHRONIZATION
By default,
filters and demodulates signals located
exactly at 1/64 of its clock frequency. For example, when using
the 400 kHz on-board oscillator, the demodulated signal
frequency must be 6.25 kHz. Because it is difficult to achieve
coherent, independently generated signals, the clock and the
signal source must be synchronized. This synchronization is
generally accomplished by deriving one signal from the other,
or by employing a phase-locked loop.
For this purpose, the reference clock signal (RCK) is derived
from the
clock, and the default RCK frequency is
f
CLK
/64. Therefore, if the input signal is synchronized to RCK,
the band-pass filter is automatically centered on the input
signal, and the signal is properly demodulated. In addition,
this configuration allows the system to track any changes in
frequency, because everything is derived from a single master.
Because of these properties, it is possible to use RCK to drive a
signal excitation source, to trigger a pattern generation, or as
the frequency reference in a phase-locked loop generating the
input signal.
OUTPUT SIGNAL SYNCHRONIZATION
An output synchronization pulse is available on the SYNCO
connector. The
generates this pulse every time the
output is updated and ready to be sampled. The frequency of
this pulse is 1/8 the clock frequency. By default, the pulse
polarity is positive, and it is generated 6.5 clock cycles after the
last output update.
When the
is clocked by the on-board oscillator circuit,
the frequency of the SYNCO pulse is 50 kHz (20 µs period); the
pulse duration is one clock cycle or 2.5 µs (12.5% duty cycle);
and the pulse occurs 16.25 µs after the last output update. The
polarity and its occurrence relative to the output update event
are programmable features.
Table 3. Default Clock Frequencies Relative to f
CLK
Signal
Ratio
Description
FCLK
1
Master clock
FS
1
Input sampling rate
FSN
1/2
Input sampling Nyquist rate
FD
1/8
Output sampling rate
FDN
1/16
Output sampling Nyquist rate
FSYNCO
1/8
Synchronization pulse frequency
FRCK
1/64
Reference clock frequency
FC
1/64
Band-pass filter center frequency
FC
1/64
Mixer frequency
PROGRAMMING THE
has many programmable features, such as
multiple filter configurations, quadrature demodulation, and
adjustable clock ratios. These features can be programmed via
the serial port interface (SPI) or at power-up when booting
from an I
2
C EEPROM. For a complete list of features and the
internal register map, refer to the
On the
board, the SPI configuration option
is not directly available. The device can be reprogrammed
through the EEPROM option only. A valid configuration must
be loaded on the EEPROM, and the toggle switch must be on
the EEPROM_BOOT position at power up.
The internal configuration memory is volatile, and the device
returns to its default value if it does not boot from the EEPROM.
Therefore, it is possible to toggle between the default and the
EEPROM configurations by selecting the appropriate position
of the toggle switch and by then pressing the
RESET
button.
SELECTING BETWEEN I AND Q DEMODULATION
COMPONENTS
By default, the
performs synchronous demodulation
of the in-phase signal component (I). To demodulate the
quadrature component (Q), set the toggle switch to the
EEPROM_BOOT position and press the
RESET
button. The
EEPROM contains the same default configuration as the
, but sets the quadrature demodulation bit.
The EEPROM must be configured to the factory contents.
Changing the EEPROM contents changes the behavior of the
device after switching the toggle switch.
DEFAULT FILTER CONFIGURATION
internal filter is configured by default as a
second-order, band-pass filter, as shown in Figure 3.
NORMALIZED FREQUENCY (Hz/Nyquist)
G
AI
N (
d
B)
10
0.25
–50
–40
–30
–20
–10
0
0.5
0.75
12359-
003
Figure 3.
Filter Transfer Function,
Normalized to Half the Output Sampling Rate
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