AD6650ABCZ Analog Devices Inc, AD6650ABCZ Datasheet - Page 19

• Manufacturer Part Number: AD6650ABCZ
• Description: DIVERSITY IF-TO-BASEBAND GSM/EDGE NARROW-BAND RECEIVER
• Manufacturer: Analog Devices Inc

RCF Filter Length
The maximum number of taps this filter can calculate, N
given by Equation 10. The value N
channel register within the AD6650 at Address 0x1B.
where:
f
M
f
The RCF coefficients are located in Address 0x40 to Address 0x6F,
and are interpreted as 20-bit twos complement numbers. When
writing the coefficient RAM, the lower addresses are multiplied
by relatively older data from the IIR, and the higher coefficient
addresses are multiplied by relatively newer data from the IIR.
The coefficients need not be symmetric, and the coefficient
length, N
symmetric, both sides of the impulse response must be written
into the coefficient RAM.
The RCF stores the data from the IIR into a 46 × 48 RAM. A
RAM of 23 × 48 is assigned to I data, and a RAM of 23 × 48 is
assigned to Q data.
When the RCF is triggered to calculate a filter output, it starts
by multiplying the oldest value in the data RAM by the first
coefficient, which is pointed to by the RCF coefficient offset
register (Address 0x1A). This value is accumulated with the
products of newer data-words multiplied by the subsequent
locations in the coefficient RAM until the coefficient address
RCF
Table 11. Three-Tap Filter
Coefficient Address
0
1
2 = (N
The RCF coefficient offset register can be used for two purposes.
The main purpose is to allow multiple filters to be loaded into
memory and selected simply by changing the offset. The other
is to contribute to the symbol timing adjustment. If the desired
filter length is padded with 0s on the ends, the starting point
can be adjusted to form slight delays in the time the filter is
computed with reference to the high speed clock. This allows
for vernier adjustment of the symbol timing. Coarse adjustments
can be made with the RCF decimation phase.
CLK
SAMPIIR
RCF
is the external frequency oscillator.
OFF
N
is the RCF filter decimation rate.
taps
is the input rate to the RCF.
taps
+ N
− 1)
taps
≤
, can be even or odd. If the coefficients are
taps
min
− 1 is reached.
⎛
⎜
⎜
⎝
f
CLK
f
SAMPIIR
×
M
Impulse Response
h(0)
h(1)
h(2)
RCF
,
48
taps
⎞
⎟
⎟
⎠
− 1 is written to the
Data
N(0) oldest
N(1)
N(2) newest
taps
, is
(10)
Rev. A | Page 19 of 44
The output rate of this filter (f
rate of the IIR stage and M
where:
f
M
RCF Output Scale Factor and Control Register
Address 0x1C is used to configure the scale factor for the RCF
filter. This 2-bit register is used to scale the output data in 6 dB
increments. The possible output scales range from 0 dB to −18 dB.
The AD6650 RCF uses a recirculating multiply accumulator
(MAC) to compute the filter. This accumulator has three bits of
growth, allowing the output of the accumulator to be up to eight
times as large as the input signal. To achieve the best filter
performance, the coefficients should be as large as possible
without overflowing the accumulator. The gain of a filter is
merely the sum of the coefficients; therefore, for normal steady
state signals, the sum of the coefficients must be less than 8. If
the sum of the coefficients is 8 or slightly less, very rare
transient events can overflow the accumulator. To prevent this,
the sum of the absolute values of the coefficients should be less
than 8. It is then impossible for the RCF filter to overflow.
The RCF filter has a 4-position mux at the output of the
accumulator. This mux chooses which 24 bits are propagated to
the output and adjusts the rounding appropriately. This can be
viewed as a gain block that can be varied in 6 dB steps and is
controlled by the 2-bit RCF scale register.
The resulting gain of the RCF (RCFgain) is then represented by
the following equation:
where RCFScale is the value in the RCF scale register.
COMPOSITE FILTER
The total gain of the digital filters can be calculated with
Equation 13 and must be less than or equal to 1 (0 dB).
Typically, the RCF coefficient gain is scaled to compensate for
the gain of the CIC and IIR, and the RCF scale factor is set to 3.
where:
Gain is the gain of the digital filters.
M
S
RCFScale is the value in the RCF scale register.
The individual responses of the CIC4 and IIR filters, along with
the composite response of all the filters, are shown in Figure 26.
SAMPIIR
CIC4
RCF
CIC4
is the CIC4 scale factor.
Gain
is the RCF filter decimation rate.
RCFgain
f
is the CIC4 decimation ratio.
SAMPR
is the input rate to the RCF.
=
=
2
M
S
=
CIC
f
M
CIC
SAMPIIR
∑
4
+
RCF
4
Coef
12
4
×
1
2
×
×
2
RCF
3
⎛
⎜
⎝
−
.
∑
SAMPR
RCFScale
1
Coef
) is determined by the output
×
2
3
−
RCFScale
1
⎞
⎟
⎠
AD6650
(11)
(12)
(13)