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Simplifying software-defined radio with filter

26 Apr 2016  | John Wendler

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A close examination of figure 2 shows some interesting features. On the carrier trace, the sampling-frequency spurs appear at ±19.2kHz away from the centre. The modulated spectrum is also interesting. The filter in the sigma-delta DAC causes the nearly vertical roll-off at approximately ±10kHz. The mounds that appear around ±12kHz and gradually roll off are spectral regrowth, which non-linearities in the high-power amplifier cause.


Figure 3: This circuit has excellent phase and amplitude balance between the output channels and eliminates some critical component-matching requirements.


Many moderate-bandwidth SDRs need a translator between the sigma-delta DAC's single-ended output and a typical balanced-input quadrature modulator. It is frequently desirable to follow up the DAC output with a hardware filter that removes the DAC's high-frequency noise and ensures compliance with spectral-mask requirements. Further complicating things, the optimal common- and differential-mode output voltages of the DAC are likely to differ from those that the modulator requires. An easy scaling factor does not relate common- and differential-mode voltages.

Handling all of these considerations with a conventional approach can require as many as four operational amplifiers with multiple filter sections per I or Q channel. The filters require close component matching to guarantee that carrier and single-sideband suppression—key measures of quadrature-modulator ideality—do not degrade as a function of base band frequency. The Linear Technology LTC1992, on the other hand, addresses the problem in a single section. Linear shows a fully balanced approach to the problem in its data sheet (Reference 1).


Figure 4: The measured frequency response of the positive channel with respect to ground shows an apparent 6-dB loss as a result of looking at only half the differential-output voltage; when you examine the full balanced output, the net gain is 0 dB.


It turns out, however, that a fully balanced approach is unnecessary. The circuit in figure 3 has excellent phase and amplitude balance between the output channels and eliminates some critical component-matching requirements. Pin 2 is set for the desired common-mode output voltage, and the DAC's midpoint voltage connects through an input resistor to Pin 8. Note that any mismatch between the input voltage and the midpoint voltage appears at the outputs and causes asymmetrical swing. This application bypasses Pin 7. The filter is a passive single-pole circuit cascaded with an inverting Sallen-Key filter, but other topologies are feasible.


Figure 5: The measured deviation from an ideal equal-amplitude,

1808 phase shift between the positive and the negative outputs shows agreement of less than 0.1 dB and 0.18 between 300Hz and 3kHz.


Figure 4 shows the measured frequency response of the positive channel with respect to ground. The apparent 6-dB loss is a result of looking at only half the differential-output voltage; when you examine the full balanced output, the net gain is 0 dB. Figure 5 shows the measured deviation from an ideal equal-amplitude, 180° phase shift between the positive and the negative outputs. The agreement in the critical 300Hz to 3kHz range is less than 0.1 dB and 0.1°. Even at 50kHz, the error is less than 0.5 dB and 1°.
Acknowledgment
The author gratefully acknowledge the assembly assistance of Deb Girard.


Reference
"LTC1992: Fully Differential Input/Output Amplifier/ Driver," Linear Technology, July 2003.


About the author
John Wendler contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on December 3, 2007 in EDN.com.


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