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Simplifying software-defined radio with filter

26 Apr 2016  | John Wendler

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Software-defined radios (SDRs) offer enormous flexibility, permitting you to change modes or waveforms at will. This design idea focuses on the "exciter" portion of a moderate-bandwidth SDR (figure 1). The RF carrier or transmitter IF enters the quadrature modulator, and the modulated output exits for further frequency translation or amplification, depending on the details of the design. The DSP section generally works with analytic signals—in this case, signals with real and imaginary parts—at base band. These signals may have started out as a voice speaking into a microphone that attaches to an ADC, or they may have started out as data from a computer. Regardless of the signals' origin, the DSP performs calculations on the stream of numbers, performing filtering, perhaps adding signalling tones or packetising the data, and converting the stream into the final I and Q modulating signals. For moderate bandwidths, a stereo sigma-delta DAC or codec provides the conversion to analogue signals and performs some additional filtering on the signal. Such filtering is often necessary because the quadrature modulator comprises a pair of mixers. These mixers translate any noise at base band frequencies directly to the modulator's output.

Figure 1: A pair of DACs converts the base band signal from a DSP into I and Q signals for the quadrature modulator of a software-defined radio.

Output noise is problematic. The United States Federal Communications Commission (FCC) sets spectral masks or adjacent-channel-power-ratio requirements on some services, such as land mobile radio. These requirements govern the allowed spectrum of a transmission and vary according to the bandwidth of the channel and the frequency of transmission. Their function is always the same, however: They limit the interference to other users on nearby channels to the transmitter. Meeting the spectral mask is a regulatory requirement; you cannot certify a radio without proving that it meets this requirement, and, without this certification, you cannot legally sell it. Figure 2 shows a sample spectral mask, 47 CFR 90.210 G, with a normalized X axis to show the offset from the centre of the channel and a normalized Y axis to show the unmodulated carrier output. This mask applies to the 800MHz SMRS (specialised-mobile-radio service) in which channels are 25kHz apart but signals can occupy only 20kHz.

Figure 2: A sample spectral mask, 47 CFR 90.210 G, has a normalized X axis to show the offset from the centre of the channel and a normalized Y axis to show the unmodulated carrier output.

The unmodulated carrier first transmits at the centre of the mask, and the top of the mask adjusts to correspond with the output power of the transmitter. You then turn on the modulation, thereby spreading the spectrum. The resulting spectrum must fall below the mask line in all places.

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