Up to now, we have considered the challenges associated with operation across multiple frequencies. However, the design of reconfigurable systems that operate in a single band also presents level planning challenges. As the air interface changes, the headroom between signal levels and compression points must vary so that the various distortion and signal-to-noise targets can be achieved.

Consider a software radio design example in the context of the requirements on the IQ modulator. Assume a common transmitter that can switch between the GSM and WCDMA air interfaces operating at 1960 MHz or 2140 MHz. In the context of this discussion, this range can still be considered “narrowband.” Figure 6 shows a representation of the power, distortion and noise levels at the output of the modulator along with the requirements from the GSM and WCDMA air interface standards.

For this example, an IQ modulator which is optimized for operation in the 1.5 GHz to 2.5 GHz range (FMOD-2) should be chosen. For GSM operation, the first step is to choose an output power level. An output power level of +5 dBm, which is well below the ADL5372's +12 dB output compression point would be conservative value. Examining the spectral mask at 400 KHz offset from the carrier, it is apparent that there is a comfortable margin of 11 dB on the 60 dBc requirement. Note that in practice, the modulator could operate quite a bit closer to the compression point since the GMSK carrier has a constant envelope and its spectral mask shows little sensitivity to headroom.

More critical is the noise spectral density at 6 MHz offset from the carrier. In a typical +47 dBm transmitter, the requirement at the antenna is a noise level of less than -36 dBm peak hold in 100 kHz measurement bandwidth. With the modulator running at +5 dBm output power, this corresponds to a noise spectral density of -140 dBm/Hz (or -145 dBc/Hz). With the IQ modulator delivering only -152 dBm/Hz (-157 dBc/Hz), there is once again plenty of margin.

For WCDMA operation, a significantly lower output power level must be chosen to provide more headroom to the modulator's output compression point. This will have a direct impact on adjacent-channel leakage ratio (ACLR). While the requirement for this specification is 45 dBc at the antenna, components at this point in the signal chain are generally expected to dramatically exceed this requirement. In this case, a single-carrier output power level of -10 dBm is chosen, resulting in an ACLR of -75 dBc.

The maximum broadband noise that the WCDMA standard will tolerate at the antenna is -30 dBm, measured in a 1 MHz bandwidth. Assuming that the system operates at an output power of +45 dBm and the modulator is running at -10 dBm, this corresponds to a noise power level at the modulator output of -85 dBm in 1 MHz bandwidth or -145 dBm/Hz. At -157 dBm/Hz, we have 12 dB margin.

In the GSM and WCDMA case, the broadband noise has significant margin on the overall requirement at the antenna, even with the carriers generously backed off from the modulator's compression point. Therefore, it is arguable that a transmitter could be built with limited noise filtering. As noted before, some baseband filtering will always be required to filter out DAC sampling images. At the modulator output, however, only the harmonics of the LO and receive-band noise require filtering, because the unfiltered broadband noise is already well below the required limit.

Significant obstacles still stand in the way of mass manufacture and deployment of infrastructure-grade software-defined radios. However, advances in IQ modulators are bringing this goal closer. The increased dynamic range of modern IQ modulators allow for transmission of various air interfaces at different power levels while maintaining adequate noise and distortion margin. Broadband frequency agility can be achieved by choosing one of a family of pin-compatible devices during manufacture. Alternatively, by choosing an IQ modulator with a 2XLO, broadband operation can be achieved across multiple octaves with a single device, bringing the goal of a true SDR closer to reality.

Eamon Nash is applications engineering manager for RF standard products at Analog Devices. He has worked at Analog Devices for 16 years, first as a field applications engineer, based in Germany, covering mixed signal and DSP products, then as product-line applications engineer specializing in RF building-block components for wireless applications. He holds a Bachelor of Engineering degree in electronics from the University of Limerick, Ireland. He can reached at (781) 937-1239 or eamon.nash@analog.com.

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