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PDSP16488A Datasheet(PDF) 8 Page - Mitel Networks Corporation |
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PDSP16488A Datasheet(HTML) 8 Page - Mitel Networks Corporation |
8 / 12 page PDSP16350 8 Modulated Frequency The output frequency can be modulated very simply, see Fig 8. Since the phase increment value can be loaded as a complete word every cycle, there is no need to provide internal double buffering to prevent spurious frequencies being gener- ated during the load operation. Binary Frequency Shift Keyed (BFSK) modulation can easily be implemented by externally multiplexing between two phase increment values represent- ing the two frequencies to be used. The value to be used can be instantaneously changed, thus maintaining phase coher- ence, whilst the bit to be transmitted changes from a mark to a space. Frequency hopping could also be simply effected by clocking a new random number into the DIN port once every thousand cycles, for instance. The output will reflect any change in the frequency after 31 system clock cycles. If the phase increment value on the DIN port is changed on each clock cycle, then the output frequency will change without introducing any dis- continuities. Thus, a linear frequency sweep can be achieved by incrementing the value on the DIN port by a fixed amount each cycle. Al- ternatively, a logarithmic sweep could be implemented by ‘walking’ a one across the DIN port. Shifting the input one place to the left every hundred cycles, for example, would double the frequency every time. Chirp generation for FM - CW Radar systems is a typi- cal example of the need for linear frequency sweeps. This application requires the gen- eration of quadrature chirp waveforms and is illustrated in simplified form by Fig. 7. One waveform is needed for Fig. 8 Frequency Modulation Timing Diagram 16 bit Cordic Sin / Cos Generator Gain Compensate ROM D17:0 D33:18 PDSP 16350 D/A D/A SIN COS SWEEP GENERATOR the transmitter, and the other for the receiver. The phase increment value is supplied by the counter block which simply increments at a rate determined by dividing down the time base clock. The synthesised frequency thus increases during the sweep period. A number of the more significant phase increment bits are used to supply the addresses to a PROM. The output of this PROM is used to amplitude modulate the sine and cosine waveforms. In this manner it is possible to compensate, at the source, for any poor frequency versus gain characteristics of analog circuits further along in the system. The digital outputs directly drive two D/A converters. Once in the analog world, it is necessary to remove the alias frequencies with low pass filters. The phase linearity and pass band ripple characteristics of these filters are very important, if the correct phase relationships are to be maintained be- tween the two waveforms. Fig. 7 Quadrature Chirp Generator 1 2 3 4 5 CLK RESET DATA IN RESULT 30 31 32 33 34 35 A B A B Device Reset Apply First Data First Result Available C D JUMP MODE C D |
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