UG-570
AD9361 Reference Manual
| Page 38 of 128
SPLIT TABLE MODE
In situations where high power out-of-band interfering signals are
often present, it can be advantageous to split the gain table to
optimize noise figure in the presence of these interferers. In this
case, separate pointers control the LMT gain and the LPF gain
independently (and digital gain if it is enabled). This allows the
gain to be changed in the area of the receive path that is
overloading. Recall that for the full gain table, gain changes could
affect any or all of the gain blocks in the receive path regardless of
where the overload occurs.
The architecture of the LMT table depends on which gain control
mode is used so this aspect is covered in the next sections, which
cover the various gain control modes in detail. An LMT table
must be written to the
AD9361
using the ad9361_load_gt
function. A LPF table per se does not exist. Instead, the LPF index
directly translates to LPF gain in dB. The same is true of digital
gain (if it is enabled). LPF gain ranges from 0 to 24(d) while
digital ranges as mentioned previously from 0 to 31(d).
The total gain in dB of the
AD9361
is not necessarily equal to the
LMT and LPF indices added together. The actual gain of the LMT
stages vary with LO frequency and in addition some of the LMT
steps are larger than one dB. Thus, changing the LMT index by
one may not change the gain by 1 dB. This concept is easier to
understand when looking at the tables.
The maximum index when using the split table mode is 40(d).
To read back the split table gain indices in any gain control mode,
use the ad9361_get_rx_rf_gain function.
Table 16. Reading Split Table Gain Indices
Register(hex)
Function
2B0[D6:D0]
Rx1 LMT gain index
2B1[D4:D0]
Rx1 LPF gain index
2B2D4:D0][
Rx1 digital gain index (if enabled)
2B5[D6:D0]
Rx2 LMT gain index
2B6[D4:D0]
Rx2 LPF gain index
2B7[D4:D0][
Rx2 digital gain index (if enabled)
DIGITAL GAIN
All modes (MGC, AGC) and both gain table modes allow for the
addition of digital gain. The maximum allowable index for a full
gain table is 90(d). The maximum digital index is 31(d). A
standard full gain table with only analog gain has a maximum
index of 76(d). For the gain tables provided by Analog Devices,
this leaves 24(d) indices left over for digital gain. Alternative gain
tables that reach their maximum analog gain at an index lower
than 76(d) can accommodate more digital gain steps (up to 31).
It is important to point out that digital gain does not increase
signal-to-noise (SNR) ratio as it is merely multiplies the digital
word by a factor. In this way, both noise and signal are increased
and thus SNR remains the same. For many applications, digital
gain is not needed. Further, modifying the gain table and
sacrificing analog gain to add more digital gain will decrease
the performance of the system.
In some cases, however, it is desired that the signal power
received by the BBP, be equal to some nominal value. For very
low signal levels in which the maximum analog gain is still not
high enough to achieve this goal, variable digital gain can be used.
MGC OVERVIEW
In MGC mode, the BBP controls the gain index pointer(s). This
mode is setup with the ad9361_set_rx_gain_control_mode
function. In its simplest form, in MGC the BBP evaluates the
digital signal level at the I/O port and then adjusts the gain
appropriately. In this scenario, the BBP needs no other
information other than the digital signal level that it receives. For
the full (single) gain table, this is all that is needed—an overload
requires that the gain be decreased.
However, the
AD9361
has programmable thresholds that indicate
the condition of the signal in each receiver. Routing these signals
to the control output pins and then connecting them to the BBP
inputs allows the BBP to determine the status of the received
signals in more detail. For a split gain table, this information
allows the BBP to adjust the gain in the area, which is overloading
because it indicates where the overload is occurring (LMT, LPF,
digital).
Rev. A
