Microwave Engineering Europe - March 2009 - (Page 23) WIRELESS INFRASTRUCTURE 23 Figure 3: An 8-bit D/A converter provides versatile programmability of the filter bandwidth. Figure 4: The DAC provides nearly 2 decades of filter bandwidth control in 1/256 increments. 9th order designs. The LTC6602 combines a 5th order lowpass cascaded with a 4th order highpass to form a bandpass response. The highpass filter can be bypassed under software control, providing a lowpass-only function. Maximum operating frequency is 900 kHz, suitable for RFID readers and lower frequency communications receiver uses. In contrast, the LTC6603 has higher bandwidth with maximum cutoff up to 2.5 MHz. It is a true 9th order lowpass filter. So its roll-off skirt is significantly sharper, providing more than 45 dB rejection at one octave above the corner frequency. Figure 2 shows the filter characteristics of the two filters. Both the LTC6602 and LTC6603 are switched capacitor designs. Each has an internal oscillator whose frequency is set by an external bias resistor. Once set, this free-running oscillation frequency is used to divide down to one of three softwaresettable filter cutoff frequencies for each the highpass and lowpass sections, where each section is controlled by a 2-bit (four discrete states), user-settable register accessible via a SPI serial bus. Table 1 and Table 2 show the LTC6602 and LTC6603 bandwidth mapping, respectively, the filter divider ratios at each filter state, and the corresponding filter cutoff frequencies. This method provides a fairly coarse but predictable control of the frequency resolution. If finer frequency control is needed, an external clock can be applied to override the internal one. The fCLK can vary within a range of 25 MHz to 90 MHz. Combined with the SPI control, one can easily place the filter corner or its notch at a precise determined location. Users can also implement an alternative digital control with fine frequency increments over 2 decades range with an 8-bit DAC as shown in Figure 3. The figure shows a serial input 8-bit DAC, the LTC2621-1 which powers up and resets to one-half of the VREF voltage, biasing the DAC output conveniently within the linear region of the subsequent amplifier stage. The LTC6603’s internal RBIAS DC voltage is buffered by one-half of the LTC6078 dual op amp that sets the DAC’s VREF. In turn, the DAC’s output voltage drives a voltage-tocurrent converter formed by the other half of the LTC6078 driving an output FET in a feedback loop. This way, the DAC meters the current precisely into the LTC6603 RBIAS pin, and hence the filter cutoff frequency. Figure 4 shows the frequency response range under DAC control. This implementation provides nearly 2 decades of filter bandwidth control ranging from 30 kHz to 2.5 MHz. Conclusion The new family of broadband filters provides a higher level of programmability, versatility, accuracy and matching performance to a wide range of wireless communications equipment, industrial signal processing, high performance instrumentation, and many other signal processing systems. References • LTC6601-1 datasheet at www.linear.com • LTC6602 datasheet at www.linear.com • LTC6603 datasheet at www.linear.com MICROWAVE ENGINEERING EUROPE Free subscription at: www.mwee.com/subscribe Microwave Engineering Europe ● March 2009 ● www.mwee.com http://www.linear.com http://www.linear.com http://www.linear.com http://www.mwee.com/subscribe http://www.mwee.com
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