STV0196 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'STV0196' PDF : 23 Pages View PDF
STV0196B
FUNCTIONAL DESCRIPTION (continued)
V - CARRIER RECOVERY ; DEROTATOR
The input of the circuit is a pair of demodulated
signals ; however, there may subsist some phase
error not corrected by the front end loop.
Furthermore, the demodulation may be done at
constant frequency; the tuner is trimmed in order
to make the useful signal bandwidth centered on
this demodulation frequency ; in that case, a carrier
offset frequencymay subsist; it is fixed by the mean
of the on-chip derotator which acts as a fine tuning
carrier loop.
The derotator frequency range is limited to an
interval corresponding to ±Fs/16.
V.1 - Loop Parameters
Like the timing loop, the carrier loop is a second
order system where two parameters α and β may
be programmed respectively with alpha_car and
beta_car.
Carrier Loop Parameter Registers
Internal Address : Hex0E
1
0
1
0
0
0
1
1
alpha_carrier
beta_carrier
Derotator Frequency Register
Internal Address : Hex0F
Signed number
The shaded area correspond to the reset values.
beta_car
(reg. value)
0
1
2 34567
Tn = 2π/ωn
(symb per)
NA
907
642 454 321 227 160 113
fn (kHz) for
F = 20Mbd
22 31 44 62 88 125 177
alpha_car
(reg. value)
Damping Factor
0 NA NA NA NA NA NA NA NA
1 NA 0.89 0.63 0.44 0.31 0.22 0.16 0.11
2 NA 1.77 1.25 0.89 0.63 0.44 0.31 0.22
3 NA 3.54 2.51 1.77 1.25 0.89 0.63 0.44
4 NA 7.09 5.01 3.54 2.51 1.77 1.25 0.89
5 NA 14.18 10.03 7.09 5.01 3.54 2.51 1.77
VI - CARRIER OFFSET EVALUATOR
An 8 bit register may be read at any time; it gives
a signed value proportionnal to the carrier fre-
quency offset according to the expression :
∆f = 1.8 . 10-6 . m2 . N . Fs (in mode A)
where Fs is the symbol frequency, m the symbol
module (AGC reference), N the read value.
The maximum value for N is reached in nominal
conditions for a carrier offset of 16% of Fs ; if
greater, N remains saturated, giving a reliable sign
indication over more than ±50% Fs range.
Carrier Offset Register
Internal Address : Hex10
Signed number
This 8 bit R/W register may be written at any time
to force the central frequency of the derotator to
start the carrier research, or read, when the loop is
locked,in order to know the current carrier offset
(one LSB correspond to Fs/2048).
V.2 - Loop Equations
The natural pulsation is :
ωn = 10−3 ⋅ fs ⋅ √m⋅ 2beta_car
and the damping factor is :
√ ξ = 0.128 ⋅ 2alpha_car ⋅
m
2beta_car
.
where m is the reference value (see AGC registers).
The next table gives for the nominal amplitude
m = 24 the natural period (in symbols), and the
damping factor for the possible values of alpha_car.
As an example, the corresponding natural frequency
is given assuming a symbol frequency of 20MBauds.
VI.1 - Lock Indicator
This 1 bit Carrier Found flag may be read (see
Viterbi Status register) at any time ; it indicates that
a QPSK signal is found, and that the carrier loop is
closed ; This flag allows to detect false lock that can
happen if the loop bandwidth is small regarding the
frequency offset.
VII - CARRIER TO NOISE INDICATOR
Internal Address : Hex14. Read only register.
b7 b6 b5 b4 b3 b2 b1 b0
This register can be used to estimate the carrier to
noise level (Eb/No) in a range from 4 to 16dB.
The register value dependson both the AGC refer-
ence level ”m” (see paragraph VIII) and the control
bits ”SN[1..0]” (see paragraph IX). For more details
about how to use this register, please refer to the
Annexe 1.
10/23
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