GC4114 Datasheet, PDF (33/45 Pages) Texas Instruments – QUAD TRANSMIT CHIP

GC4114 QUAD TRANSMIT CHIP
DATA SHEET REV 1.0
7.5.1 Modulating a 25K-Baud QPSK signal in the 1X Mode
This section illustrates how to modulate signals in the 1X mode. The signal parameters used in this example
are shown in Table 8 below.
Table 8: Example QPSK Signal Parameters
Parameter
Baud Rate
Excess Bandwidth
Digital to Analog Converter (DAC) Size
Desired Crest Factor for the DAC
Number of Channels
Symbol
B
A
CF
M
Value
25
0.35
14
0.2
16
Units
Kbaud
Bits
(-14dB)
It is assumed that the QPSK symbols have been mapped into I/Q pairs as shown in Table 9. The value of
Table 9: QPSK Symbol Map
Symbol
0
1
2
3
I
16384
-16384
16384
-16384
Q
16384
16384
-16384
-16384
16384 is arbitrary, but should be greater than 8192 to allow reasonable gain settings.
The information in Tables 8 and 9 can be used to derive the CIC interpolation ratio (N) and the gain settings
(G, SCALE, BIG_SHIFT and SUM_SCALE). The filter coefficients for the 1X mode root-raised cosine filter with an
excess bandwidth of 0.35 can be derived using the formula shown above. These coefficients are listed in Section
3.4.1. The sum of these coefficients (PFIR_SUM) is used in the gain calculation and is equal to 59835.
In the 1X mode the clock rate is equal to 4BN, so N is 600 for F_CK equal to 60MHz and B equal to 25KHz.
The optimal gain setting for the chip is described in Section 3.7. The first equation to be optimized is
CF
=
. ^


--R----M-----S---
32768



--^G------
128
 ^{P----F---I6--R-5---_5---S3---6U-----M----}



The
crest
factor
is
0.2,
the
RMS
input
level¹
is
16384,
and
the
PFIR_SUM
is
59835.
Solving
this equation for G gives a value of G=56. The second equation to satisfy is
CF
=
. ^


--R----M-----S---
32768



--^G------
128



P----F---I--R----_---S---U-----M----
65536
^{

N³
2^–(
SCALE
+
12
×
BIG_SHIFT
+
3
)
}
The
term
N³
is
slightly
less
than
2²⁸,
so
a
value
of
BIG_SHIFT=1
and SCALE=13 is appropriate. Solving this equation using the complex data’s RMS level of 23170 gives G=49.
The sum tree gain (SUM_SCALE) should be set to keep the final RMS output level, after adding M signals
together, at the desired crest factor. The RMS output level increases by M , which for M=16 is a gain of 4. Setting
SUM_SCALE=2 will cancel the gain of 4 in the sum tree and leave the crest factor at 0.25 (-12dB).
The suggested control register settings for the chip with these parameters are shown in Table 10.
1. The RMS level of the I data is 16384, the RMS level of the Q data is 16384, but the RMS level of the combined
complex data is 2 times16384 = 23170. The RMS level of the combined data must be considered in the second
equation in order to prevent overflow in the mixer.
Texas Instruments Inc.
- 29 -
MAY 22, 2000
This document contains information which may be changed at any time without notice