AD6620
RCF Output Scale Factor
Table VI.
The scale factor associated with the RCF, SOUT, behaves differ-
ently than the scale factors in the CIC stages. This scalar, at the
RCF output, controls the weight of the 16-bit output data going
to the parallel port or to the serial port when using 16-bit words.
SOUT determines which of the 23 RCF output bits are used
based on the equation below. OLRCF is the 23-bit RCF output
data; POL represents the output port data. POL is rounded to
the 16 bits desired. The weight of the rounding is adjusted by
SOUT. When the serial port is used with 24-bit or 32-bit words,
RCF Gain
RCF Scale Factor (Address 309h)
1/8
1/4
1/2
1
2
4
7
6
5
4
3
2
1
0
8
16
S
OUT is ignored.
)
POL = round16bits (OLRCF × 2(4–S
)
OUT
Gain through the RCF of the AD6620 is thus:
Another way to consider the effects of the RCF Output Scale
factor is discussed here. If both CIC scalars follow the previous
recommendations, the following chart can be used to determine
what value to use for the RCF scale factor. In order to determine
this, the “gain” of the impulse response must first be determined.
This can be done by integrating the coefficients used for the
RCF filter remembering to normalize the values against the full-
scale input range of 219.
Gaincoefficients × GainRCF
Unique B Operation
Unique B works in conjunction with dual channel mode. In this
mode, both the A and B channels can have different FIR coeffi-
cients. This can prove useful in many applications where each
signal path has known differences. Another option is that FIR
gain for one path could be different than the other. During
diversity selection, one path could be tailored for weak signals
and the other for strong signals, providing extra dynamic range.
There are several possibilities when setting the “gain” of the
RCF coefficients. Following these guidelines will preserve at
least three bits in the sum of products registers.
To use the Unique B mode, set Bit 3 high in register 309h. This
will cause the internal state machine to use a different set of
coefficients for the B channel than the A. With Bit 3 set low for
normal operation, the FIR coefficient index is incremented only
after both the A and B channels are computed. However when
this bit is set high, the index is incremented after each A channel
and B channel computation. Therefore, filters are computed nor-
mally. When downloaded to the AD6620, they should be inter-
leaved with the A channel terms occupying the even RCF
Coefficient locations and the B channel terms occupying the
odd locations. Both filters must be the same length and fit in the
allocated memory space.
1.
h n = 1; 0 dB dc gain in RCF filter. Numeric wraparound
(
)
∑
very unlikely. The RCF Scale factor should be nominally
set to 4.
h n ≤ 1
2.
3.
; slight loss in RCF filter. Numeric wraparound
(
)
∑
is impossible. The RCF Scale factor should be nominally
set to 4.
h n = m; where the absolute value of m is a number less
(
)
∑
With Unique B set to ‘0,’ the following table illustrates how the
coefficients are distributed.
than 1 and is scaled to account for losses elsewhere in the
system, such as conversion gain errors, attenuator losses or
CIC scaling errors. The gain should be scaled down to avoid
wraparound in the RCF process, however, then the RCF
Scale Factor can be adjusted up to increase the signal level.
The value of m can also be negative to account for an inver-
sion through an amplifier. The RCF Scale factor should be
set where needed to produce the desired full-scale results
with a fully loaded receiver input signal.
Table VII.
Coefficient
Address
W(0)
W(1)
W(2)
W(3)
…
0
1
2
3
The RCF Scale factor has the effect as shown in the following
table. Each successive gain step doubles or halves the overall
gain of the stage. Overall gain through the RCF stage is the
cascaded gain of the RCF Scale factor shown below and the
RCF coefficient gain discussed previously.
…
With Unique B set to ‘1,’ the following table illustrates how the
coefficients are distributed.
Table VIII.
Coefficient
Address
Wa(0)
Wb(0)
Wa(1)
Wb(1)
…
0
1
2
3
…
REV. A
–26–
ADI [ ADI ]
