Single-Chip Low Power RF Transceiver for Narrowband Systems ...

• Frequency register setting
is switched
between A and B
This feature is useful to avoid AGC
operation
during start-up transients and to
ensure minimum dwell time using
frequency hopping. This means that bit
synchronization can be maintained from
hop to hop.
VGA_WAIT determines the time to hold
the present bit synchronization and RSSI
levels after changing VGA gain. This
feature is useful to avoid AGC operation
during the settling of transients after a
VGA
gain
change. Some transients are
expected due to DC offsets in the VGA.
At the sensitivity limit, the VGA gain is set
by VGA_SETTING. In order to optimize
selectivity,
this gain should not be set
®
higher than necessary. The SmartRF
Studio software gives the settings for
VGA1 – VGA4 registers. For reference,
the following method can be used to find
the AGC settings:
Note that the AGC works with "raw" filter output signal
strength, while the RSSI readout value is compensated for
VGA gain changes by the AGC.
The AGC keeps the signal strength in this range.
Minimize
VGA_DOWN for best selectivity, but leave some margin to
avoid frequent VGA gain changes during reception.
The AGC keeps the signal strength above carrier
sense level
+ VGA_UP. Minimize VGA_UP for best selectivity,
but
increase if first VGA gain reduction occurs too close
to the
noise floor.
To set CS_LEVEL, subtract 8 from RSSI readout with
RF
input signal at desired carrier sense level.
Zero level depends on front-end settings and VGA_SETTING
value.
CC1021
1. Disable AGC and use maximum LNA2 gain by
writing BFh to the VGA2 register. Set minimum
VGA gain by writing to the VGA3 register with
VGA_SETTING = 0.
2 . Apply no RF input signal, and measure ADC noise
floor by reading the RSSI register.
3.
Apply no RF input signal, and write VGA3 register
with increasing VGA_SETTING value until the
RSSI register value is approximately 4 larger than
the value read in step 2. This places the front-end
noise floor around 6 dB above the ADC noise floor.
4. Apply an RF signal
with strength equal the desired
carrier sense threshold. The RF signal should
preferably be modulated with correct Baud rate and
deviation. Read the RSSI register value, subtract 8,
and write to CS_LEVEL in the VGA4 register. Vary
the RF signal level slightly and check that carrier
sense indication (bit 3 in STATUS register)
switches at the desired input level.
5.
If desired, adjust the VGA_UP and VGA_DOWN
settings according to the explanation in Figure 20.
6. Enable
AGC and select LNA2 gain change level.
Write 55h to VGA2 register if the resulting
VGA_SETTING>10. Otherwise, write 45h to VGA2.
Modify AGC_AVG in the above VGA2 value if
faster carrier sense and AGC settling is desired.
RSSI Level
(signal strength, 1.5dB/step)
AGC decreases gain if above
this level (unless at minimum).
VGA_DOWN+3
AGC increases gain if below this
level (unless at maximum).
VGA_UP
Carrier sense is turned on here.
CS_LEVEL+8
Figure 20. Relationship between RSSI, carrier
sense
level, and AGC settings CS_LEVEL,
VGA_UP and
VGA_DOWN
0
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