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ISL5217 Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL5217 Datasheet(HTML) 9 Page - Intersil Corporation |
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9 / 43 page  9 FN6004.3 July 8, 2005 Modulation Mode 00 - QASK This modulation mode configures the QPUC as a BPSK, QPSK, OQPSK, MSK or m-QAM modulator. The block diagram is shown in Figure 7. The data FIFO outputs are routed to the shaping filters. Here the samples are interpolated by 4, 8, or 16 and shaped using a FIR filter with up to a 256 taps. The filter impulse response can span 4-16 input samples. A half (input) sample delay can be inserted in the I/Q path after the FIR and is enabled through Main Control (0xc, bit 13). The 20-bit output of the shaping filter is routed through a gain adjust multiplier controlled by 0x0a, bits 11:0 and into the interpolation filter. The interpolation filter interpolates by a factor set in the resampling NCO with the Interpolation Phases controlled by 0xd, bits 1:0. The output of the interpolation filter is at the master clock frequency, CLK. The samples are then mixed with the carrier L.O. for quadrature upconversion. The output is then summed with the cascade input signal, saturated (in the case of overflow), and formatted for output. Modulation Mode 01 - FM with Bandlimiting Filter This mode configures the QPUC as an FM modulator with post-modulation filtering. The block diagram is shown in Figure 8. This mode provides for FSK and FM modulation schemes. In this mode, the I input samples drive the frequency control section of a quadrature NCO to produce a zero IF FM signal. The 16-bit FM quadrature signals are then routed to the shaping FIR filter and into the interpolation filter for bandlimiting and interpolation up to the master clock rate. The quadrature filtered FM signals are then upconverted to the carrier frequency by the carrier NCO and mixers. The output is then summed with the cascade input signal, saturated (in the case of overflow), and formatted for output. Note that pulse shaping in this mode must be provided prior to the QPUC. Modulation Mode 10 - FM with Pulse Shaping This mode configures the QPUC as a FM modulator with pre-modulation baseband pulse shaping. The block diagram is shown in Figure 9. The data from the FIFO (I channel only) is routed to the FIR shaping filter. The FIR shaping filter output drives the frequency control section of a quadrature NCO to produce a zero IF FM signal. These 18-bit FM modulated quadrature samples are then up sampled in the interpolation filter to the output sample rate. The baseband modulated signal is then upconverted to the carrier frequency by the carrier NCO and mixers. The output is then summed with the cascade input signal, saturated, and formatted for output. In Mode 10, the amplitude out of the shaping filter needs to be limited in order to prevent frequency excursions that cannot be filtered out in the interpolation filter. NOTE: THE QUALITY OF THE FM SIGNAL IS AFFECTED BY THE AMPLITUDE SLEW RATE OUT OF THE SHAPING FILTER. AS A RULE OF THUMB, LIMITING THIS SLEW RATE TO LESS THAN 1/8 THE SAMPLE RATE WILL MINIMIZE THIS DISTORTION. FM Modulator The FM modulator provides for frequency modulation of the carrier center frequency by the QPUC input data. The FM modulator is driven either directly by the QPUC I input (Mode 01) or by the output of the FIR shaping filter (Mode 10). The input data to the FM Modulator, is defined as d φ(n)/dt, where φ(nT) is the phase of a theoretical sinusoid described by: The block diagram is shown in Figure 10. The input to the FM modulator, d φ(n)/dt, is integrated via the NCO accumulator. The NCO accumulator output represents phase and is used to address a SIN/COS generator, synthesizing a sinusoid of the form described in Equation 1. The phase accumulator feedback of the NCO is 20 bits and 18 bits of the phase word are routed to the SIN/COS generator. Eighteen bits of amplitude are provided on the Sine and Cosine outputs. The transfer function of the FM modulator is defined by the change in degrees per sample value, d φ(nT)/dt, where d φ(nT)/dt is a 16-bit, twos complement, fractionally notated frequency control word with a range from -FSAMP/2 to +FSAMP/2. FSAMP is defined as the sample rate into the FM GAIN PROFILE TO HALFBAND SHAPING FILTER I FIGURE 7. QASK Q GAIN PROFILE FM MODULATOR I SHAPING FILTER FIGURE 8. FM WITH BANDLIMITING GAIN PROFILE SHAPING FILTER I FIGURE 9. FM WITH PULSE SHAPING FM MODULATOR sn () A (cos φ nT () ]+ j sin φ nT () []); A ≈ 1 in Modulator [ = (EQ. 1) MODE d φ(nT)/dt φ(nT) COS[ φ(nT)] FIGURE 10. FM MODULATOR BLOCK DIAGRAM SIN[ φ(nT)] 16 or 20 20 18 18 ∑ 01 OR 10 R E G > FM ISL5217 |
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