GC4114 Datasheet, PDF (10/45 Pages) Texas Instruments – QUAD TRANSMIT CHIP

GC4114 QUAD TRANSMIT CHIP
DATA SHEET REV 1.0
The PFIR will also, if desired, convert real input data to single-sideband complex data. In this mode the PFIR
does not interpolate by a factor of 2. Instead it down-converts the input data by F_S/4, where F_S is the input sample
rate, and low pass filters the result.
The second interpolate by 2 filter is a 31 tap compensating filter (CFIR) which pre-compensates for the
droop associated with the CIC filter that follows it.
The CIC filter interpolates by another factor of N=8 to 16,384 to give an overall interpolation factor of 32 to
65,536 (16 to 32,768 in the real input mode).
The interpolated signal is up-converted by a sine/cosine sequence generated by the NCO. The real part
(I-half) of the complex result is saved as the channel output.
3.4.1 The Programmable Input Filter (PFIR)
The programmable input filter uses either internal ROM based coefficients, or externally downloaded filter
coefficients. The 63 filter coefficients are quantized to 16 bit words. The internal 80% bandwidth filter is flat over the
range of -0.4F_S to +0.4F_S with 0.03 dB of passband ripple, and with 80db of out of band image rejection. The 32
unique coefficients in the 63 tap symmetric filter are:
-14, -20, 19, 73, 43, -70, -82, 84, 171, -49, -269, -34, 374, 192, -449,
-430, 460, 751, -357, -1144, 81, 1581, 443, -2026, -1337, 2437, 2886,
-2770, -6127, 2987, 20544, 29647
The internal filter is typically used for signals that are digitized analog signals such as voice, AM, FM or SSB
signals. The internal filter is also used for digital signals (FSK, PSK, QAM or GMSK signals for example) that have
already been modulated and filtered and are ready to be up-converted to their desired carrier frequency.
The external coefficients are typically used to “pulse shape” and then up-convert digital data. The PFIR will
accept QPSK, O-QPSK, PSK, PAM, OOK, π/4-QPSK, or QAM symbols and then filter them by the desired pulse
shaping filter. A common pulse shaping filter is the root-raised-cosine (RRC) filter. The symbols can be entered
directly into the chip at the desired symbol (baud) rate, or entered at twice the baud rate by alternately entering
symbol data and entering zeroes.¹ The “twice the baud rate” or “2X” method results in better out of band image
rejection. See the application note in Section 7.5 for more details.
(a) Internal Coefficients
(b) RRC (Alpha=0.35) Filter (c) RRC (Alpha=0.35) At 2X
Figure 5. PFIR Spectral Response
Figure 5a shows the spectral response of the internal filter. Figure 5b shows the root-raised-cosine pulse
shaping filter with 35% excess bandwidth (alpha=0.35). Figure 5c shows the RRC shape for the 2X mode.
1. Called zero padding.
Texas Instruments Inc.
-6-
MAY 22, 2000
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