SI2107-X-FM SILABS [Silicon Laboratories], SI2107-X-FM Datasheet - Page 29

• Manufacturer Part Number: SI2107-X-FM
• Description: SATELLITE RECEIVER FOR DVB-S/DSS WITH QUICKLOCK AND QUICKSCAN
• Manufacturer: SILABS [Silicon Laboratories]

6.6.2. Digital AGC
Downstream of the analog VGAs, after A/D conversion
of the signal, there are two points at which the digital
gain can be programmed. Digital AGC1 is used to
change signal power after removal of adjacent channels
by the (digital) anti-aliasing filter.
By default, DAGC1 is enabled and periodically adjusts
the gain of the I & Q data streams based on a
comparison of the measured complex RMS level and a
target value. The target value can be selected with the
DAGC1T register. Two levels are provided to allow
operation with additional headroom for signal peaks
during signal acquisition. The gain function of DAGC1
can be disabled using DAGC1_EN; then, no gain is
applied to I & Q data streams. The signal measurement
and gain adjustment normally operate continuously,
allowing the gain to track the input level. The
measurement window can be adjusted by register
DAGC1W. The automatic updating of the gain can be
frozen by register bit DAGC1HOLD. This holds the gain
to the last setting. The value of the gain can be read
from the DAGC1 register. It is possible to override the
internal AGC algorithm and provide host-based control
of AGC1 by appropriately programming register bit
DAGC1HOST.
Digital AGC2 (DAGC2) is intended to optimally scale the
soft decision outputs of the demodulator prior to Viterbi
decoding. This allows it to compensate for signal level
variations after matched filtering and equalization.
Normally, operation is continuous, but tracking can be
disabled using register bit DAGC2_TDIS. This holds the
gain to the last setting.
During AGC operation, the average power of the signal
is compared to a threshold set by register DAGC2T. The
signal power is measured over a finite window specified
by DAGC2W. The gain applied to the signal to make the
input match the programmed threshold can be read
from register DAGC2GA.
6.7. LNB Signaling Controller
All device versions provide LNB signaling capability.
The device supports several LNB signaling methods
including dc voltage selection, continuous tone, tone
burst,
messaging. A description of each method follows.
6.7.1. DC Voltage Selection
A constant dc voltage of 18 or 13 V is typically used to
switch the LNB between horizontal and vertical polarity
or clockwise and counterclockwise polarization. The
LNBV bit is used to select the desired voltage.
DiSEqC
1.x-
and
DiSEqC
2.x-compliant
Preliminary Rev. 0.81
When an external LNB supply regulator is used, the
DCS pin is driven high or low depending on the
selection of high or low voltage.
6.7.2. Tone Generation
Tone-related information is communicated to external
devices via the TGEN pin. The tone format select bit,
TFS, specifies whether the output of TGEN is an
internally-generated tone or a tone envelope. The
frequency of the internal tone generator is governed by
the following equation:
Frequencies between 20 and 24 kHz are supported.
The default value of TFQ results in a nominal tone
frequency of 22 kHz. When tone envelope output is
selected, a high signal on TGEN corresponds to "tone
on" while a low signal corresponds to "tone off." When
operating in the “Manual LNB messaging mode”, the TT
bit directly controls the output of the tone or tone
envelope.
6.7.2.1. Continuous Tone
A continuous tone is typically used to select between
the high and low band of an incoming satellite signal.
The LNBCT bit can be set to one to generate a
continuous tone.
6.7.2.2. Tone Burst
The tone burst signaling method can be used to
facilitate the control of a simple two-way switch. Two
types of tone burst are available, as shown in Figure 17.
An unmodulated tone burst persists for 12.5 ms. A
modulated tone burst lasts for the same duration but
consists of a sequence of nine 0.5 ms pulses and 1 ms
gaps. Tone burst selection is controlled via the LNBB
bit. The tone burst command can optionally be disabled
to support systems that do not use tone burst signaling
by setting the burst disable bit, BRST_DS, to one. This
disables the tone/burst generation as part of the
DiSEqC signaling sequence when the device uses
“Automatic LNB messaging mode” as described below.
Envelope
Envelope
Tone
Tone
Figure 17. Tone Burst Modulation
f
tone
=
--------------------------------------------------------- - MHz
[
32
Si2107/08/09/10
×
'0' Tone Burst (Satellite A)
'1' Tone Burst (Satellite B)
(
TFQ 7:0
100
12.5 ms
[
]
+
1
)
]
29