[ ]
CHP _ CURRENT / 4
CHP
I
16
2
A
=
´
m
ref
2
f
PLL _ BW
174
16 log
7.126
æ
ö
=
+
ç
÷
è
ø
SWRS046H – NOVEMBER 2006 – REVISED MARCH 2015
After calibration the PLL bandwidth is set by the PLL_BW register in combination with the external loop
filter components calculated above. The PLL_BW can be found from
.
(31)
Where:
f
ref
is the reference frequency (in MHz).
The PLL loop filter bandwidth increases with increasing PLL_BW setting. Note that in SmartRF Studio
PLL_BW is fixed to 9E hex when the channel spacing is set up for 12.5 kHz, optimized selectivity.
After calibration the applied charge pump current (CHP_CURRENT[3:0]) can be read in the STATUS1
register. The charge pump current is approximately given by
(32)
The combined charge pump and phase detector gain (in A/rad) is given by the charge pump current
divided by 2
π
.
The PLL bandwidth will limit the maximum modulation frequency and hence, data rate.
5.12.2 VCO and PLL Self-Calibration
To compensate for supply voltage, temperature and process variations, the VCO and PLL must be
calibrated. The calibration is performed automatically and sets the maximum VCO tuning range and
optimum charge pump current for PLL stability. After setting up the device at the operating frequency, the
self-calibration can be initiated by setting the CAL_START bit in the CALIBRATE register. The calibration
result is stored internally in the chip, and is valid as long as power is not turned off. If large supply voltage
drops (typically more than 0.25 V) or temperature variations (typically more than 40°C) occur after
calibration, a new calibration should be performed.
The nominal VCO control voltage is set by the CAL_ITERATE[2:0] bits in the CALIBRATE register.
The CAL_COMPLETE bit in the STATUS register indicates that calibration has finished. The calibration
wait time (CAL_WAIT) is programmable and is inverse proportional to the internal PLL reference
frequency. The highest possible reference frequency should be used to get the minimum calibration time.
It is recommended to use CAL_WAIT[1:0] = 11 in order to get the most accurate loop bandwidth.
Table 5-11. Typical Calibration Times
CALIBRATION
REFERENCE FREQUENCY [MHz]
TIME [ms]
CAL_WAIT
1.8432
7.3728
9.8304
00
49 ms
12 ms
10 ms
01
60 ms
15 ms
11 ms
10
71 ms
18 ms
13 ms
11
109 ms
27 ms
20 ms
The CAL_COMPLETE bit can also be monitored at the LOCK pin, configured by LOCK_SELECT[3:0] =
0101, and used as an interrupt input to the microcontroller.
To check that the PLL is in lock the user should monitor the LOCK_CONTINUOUS bit in the STATUS
register. The LOCK_CONTINUOUS bit can also be monitored at the LOCK pin, configured by
LOCK_SELECT[3:0] = 0010.
Copyright © 2006–2015, Texas Instruments Incorporated
Detailed Description
47
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