AD9853 Analog Devices, AD9853 Datasheet - Page 20

• Manufacturer Part Number: AD9853
• Description: Programmable Digital OPSK/16-QAM Modulator
• Manufacturer: Analog Devices

AD9853
COMPUTE SRRC FILTER COEFFICIENTS:
h(t) :=
COMPUTE SRRC FILTER COEFFICIENTS MODIFIED
FOR CORRECTION OF CIC RESPONSE:
tap := 0..TAPS – 1
DISPLAY FREQUENCY RESPONSE PLOTS:
h tap := h(t tap )
h1(t) :=
COMP(f)
f:= 0,0.001.. 0.5
FIR(f) := dB(SCALEsrrc | gain(h,f) |)
SCALEsrrc := (| gain(h,0) |)
CIC(f) := dB(SCALEcic | H(f,R) |)
COMP(f) := dB(SCALEcompsrrc | gain(h1,f) |)
SYSuncomp(f) := FIR(f) + CIC(f)
SYScomp(f) := COMP(f) + CIC(f)
SCALEsrrc := 5.559 10
R := 6
0
CIC(f)
BW
h1 tap := h1(t tap )
FIR(f)
h1 T =
h1
h
h T =
tap
tap
0
SRRC(f) cos(2
BW
–20
–40
–60
500
0
0
0
0
...CIC INTERPOLATION RATIO (USER PROGRAMMABLE)
MODIFICATION OF SQUARE-ROOT RAISED COSINE (SRRC) FIR FILTER RESPONSE
h := INT
0
0
TO COMPENSATE FOR CASCADED INTEGRATOR-COMB (CIC) FILTER RESPONSE
t
tap
SRRC (f) if
.
0
1
0
0
:=
.
FreqScale
h1 := INT
3
4
1
1
.
h PROC_GAINSCALE
.
.
–4
Figure 34. Mathcad Simulation of 41-Tap SRRC Filter with CIC Compensation
.
.
1
. .
–1
0.2
2
2
f t)df
f
2
2
5
max(h)
.
...NORMALIZED FREQUENCY RANGE [A REQUIREMENT OF MATHCAD'S GAIN() FUNCTION]
–2
–3
.
.
3
3
tap –
BW,
h1 PROC_GAINSCALE
–5
–6
SCALEcompsrrc := (| gain(h1,0) |)
.
SCALEcompsrrc := 5.79 10
4
4
TAPS – 1
0.4
H
max(h1)
–2
–1
2
5
5
FreqScale
H(0,R)
...FUNCTION TO COMPUTE NORMALIZED, UNCOMPENSATED FIR RESPONSE (SRRC) IN dB
...FUNCTION TO COMPUTE NORMALIZED CIC RESPONSE IN dB
...FUNCTION TO COMPUTE NORMALIZED, COMPENSATED FIR RESPONSE (SRRC + CIC
...FUNCTION TO COMPUTE OVERALL SYSTEM RESPONSE OF SRRC AND CIC TOGETHER IN dB
...FUNCTION TO COMPUTE OVERALL SYSTEM RESPONSE OF COMPENSATED SRRC AND CIC TOGETHER
10
5
7
6
6
f
...INVERSE FOURIER INTEGRAL COMPUTES SRRC IMPULSE RESPONSE
...MAP THE FILTER TAP INDEX TO TIME DOMAIN (CENTERED AT t = 0)
...SRRC FILTER COEFFICIENTS INTEGERIZED AND SCALED
7
9
7
7
(TIME DOMAIN) FROM THE SRRC FREQUENCY RESPONSE (FREQUENCY DOMAIN).
THE COS() FUNCTION REPLACES THE NORMAL COMPLEX EXPONENTIAL
BECAUSE WE ARE RESTICTED TO REAL FILTER COEFFICIENTS.
0.6
,R
SRRC AND MODIFIED SRRC IMPULSE RESPONSE
8
1
8
1
.
,1
...MODIFIED SRRC FILTER COEFFICIENTS INTEGERIZED AND SCALED TO 10-BIT RANGE
SRRC, CIC, AND CORRECTED SRRC RESPONSE
–7
–10 –9
–4
9
9
15
.
cos(2
–7
0.8
10
10
–1
FREQUENCY SCALE – f n
.
11
11
7
8
. .
f t)df
SCALEcic := (| H(0,R) |)
–20–
SCALEcic := 7.716 10
12
19
12
24
TAP
20
1.0
12
13
13
7
...INVERSE FOURIER INTERGRAL MODIFIES THE SRRC RESPONSE
BY THE RECIPROCAL OF THE NORMALIZED CIC FREQUENCY
RESPONSE. THE MODIFICATION IS ONLY PERFORMED OVER THE
FRACTION OF THE SRRC BANDWIDTH AS SPECIFIED BY .
–34 –71 –48
–34 –78 –61
14
14
15
15
1.2
.
25
16
16
–4
–1
17
71 260 438 511
17
56 251 435 511
...SCALE FACTORS TO ADJUST SRRC,
18
18
COMPENSATED SRRC, AND CIC
FREQUENCY RESPONSES TO UNITY AT f = 0
1.4
19
19
30
20
20
1.6
...FIR FILTER COEFFICIENTS
...FIR FILTER COEFFICIENTS
FOR SRRC RESPONSE
FOR SRRC RESPONSE
WITH CIC COMPENSATION
–1
35
) IN dB
1.8
40
2.0
REV. C