Modulation - Texas Instruments TMS570LC4357 Technical Reference Manual

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14.5.1 Modulation

Optionally, the frequency can be modulated, that is, a controlled jitter is introduced onto the baseline
frequency of the PLL. This modulation mechanism is not shown in
the modulating mode, the programmable modulation block varies the PLL frequency from the baseline
frequency (f = (f
baseline
modulation waveform is triangular and should be enabled after lock.
f
0
f - n%
0
f -2n%
0
The modulation is digital and the spreading profile is triangular, down-spread which implies:
• the modulation waveform is composed of a series of frequency steps.
• the modulation frequency and modulation depth are both well controlled due to their digital character.
• the average frequency during modulation is lower than the average frequency prior to enabling
modulation. The depth of modulation, however, sets the new average frequency.
• the modulation frequency must be selected slower than the loop bandwidth. From a practical
perspective, NS should be near 20.
The modulation fields have a simple geometric meaning:
• the modulation step size is:
N V
f
´
O u t p u t C L K
N F
• the number of steps per modulation period is 2 × NS
• the modulation depth is given by:
N S
D f =
2
D e p t h [ % ]
• the modulation frequency is:
f
OSC
T
=
mod
2 NR
´
• MULMOD minimizes frequency offset when programmed as:
( S P R _ A M O U N T
NOTE: Modulation should be enabled after Lock
Enable modulation after the lock is completed.
SPNU563A – March 2018
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/NR) × NF/(OD × R)) to f
CLKIN
Modulation Period (1/f
N V
´ ´ f
O u t p u t C L K
N F
N S N V
= ´
2 N F
NS
´
8 [ ... ] 0 1 )(
+
1 6
Copyright © 2018, Texas Instruments Incorporated
Figure
× (1 - 2 × Depth) in a period defined by 1/f
baseline
)
s
Time (Ps)
S P R R AT E 8 [ ... ] 0
+
14-4. When the PLL is used in
) 1
Oscillator and PLL
PLL
; the
s
(8)
(9)
(10)
(11)
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