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GC4016-PB Datasheet(PDF) 10 Page - Texas Instruments |
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GC4016-PB Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 83 page  © GRAYCHIP,INC. - 5 - August 27, 2001 GC4016 MULTI-STANDARD QUAD DDC CHIP DATA SHEET REV 1.0 This document contains information which may be changed at any time without notice compensating for the droop in the CIC filter’s passband. The second stage is a 63 tap decimate by 2 filter (PFIR) with user programmable tap weights. The user can customize the channel’s spectral response using the PFIR filter. A typical use of the PFIR is to perform matched (root-raised cosine) filtering. The 63 tap symmetrical filter is downloaded into the chip as 32 words, 16 bits each. Both filters must be programmed as there are no default filter coefficient sets. The CIC, CFIR, and PFIR filters can all be configured in a SPLITIQ mode where the channel only processes the real portion of the mixer output. In this case the minimum CIC decimation is 4. By using two channels in parallel one can process twice the output bandwidth by processing the real portion of the mixer output in one channel and the imaginary portion in a second channel (hence the name “split I/Q”). One can also offset the decimation timing between channels so that two or even four channels can be used to generate oversampled data. Using this capability each GC4016 chip can downconvert wideband signals such as 4X oversampled 5.12 Msymbol/sec MCNS data, or 4X oversampled 5 MHz WB-CDMA (UMTS) data, or two 8X oversampled NB-CDMA IS95 signals. The PFIR will also, if desired, convert the complex output data to real. The complex to real conversion also doubles the output sample rate so that the PFIR does not decimate in this mode. This mode is useful when one wants to output real data. The PFIR filter is followed by a resampler. The resampler filters the PFIR output to generate new sample points in between the PFIR output samples. The resampler is programmed to generate new output samples spaced in time by “R” times the PFIR output’s sample period. The value R is called the resampling ratio. If R is less than one, the resampler will interpolate. If R is greater than one, the resampler will decimate. The value of R is specified as a 32 bit word (6 integer bits and 26 fractional bits). The time resolution of the new sampling points is user programmable down to 1/64th of the PFIR output’s sample period. If R is a multiple of the selected time resolution, then the resampling process will have no resampling time jitter. If R is not a multiple of the time resolution, then the output will be generated at the time instant closest to the desired output time. The resampler is typically used to either generate oversampled output data (for instance 4x oversampled QPSK or GMSK data), or to alter the sampler rate to match a multiple of the baud rate of a QPSK or GMSK signal. The resampling operation allows the user to design a system where the ADC sample rate does not need to be equal to an integer multiple of the baud rate of the desired output signal. The resampler can also be used as a final filtering stage. A final shift circuit allows the user to shift the data by up to 15 bits to properly place it in the output word. The output word may be rounded to 12,16, 20, or 24 bits. The data may be output from the chip via a microprocessor interface, serial pins, or using a parallel port. The parallel port is particularly valuable for wideband output data. 3.3.1 Zero Padding The input samples are normally clocked into the chip at the clock rate, i.e., the input sample rate is equal to the clock rate. Input rates lower than the clock rate can be accepted by using the zero pad mode. When enabled by setting the ZPAD_EN bit in address 19 of the channel control pages, the zero pad mode will insert “NZERO” zeroes between each input sample, where NZERO ranges from 0 to 15, allowing input data rates down to 1/16th the clock rate. NZERO is set in address 19 NCO TUNING FREQUENCY PHASE OFFSET IN TO OUTPUT CIRCUIT Q I 20 24 20 20 24 12,16, 20 or 24 bits 20 Figure 4. The Down Converter Channel |
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