The following is excerpted from Chapter 18: Software
Defined Radio from the Book, RF & Wireless Technologies
by Bruce Fette. If you order a copy of this book before December 31, 2007 you can receive additional 20% off. Visit www.newnespress.com or call 1-800-545-2522 and use code 91137.
Part 1 introduces the basics of SDR.
Part 2 examines architectures for SDR, focusing on the receiver.
This part covers transmitters for software defined radio.
Transmit functions for software defined radio (SDR) are also based on some form of super-heterodyne or direct conversion. Figures 18.8 and 18.9 illustrate these two options. The multicarrier option is best suited to single- and multicarrier applications, whereas the direct conversion offers an excellent, low-cost solution for single-carrier applications.
As integration technology improves, multicarrier direct conversion may become a possibility; however, such a transmit configuration requires sideband suppression that is about 15 dB better than the spurious requirements to prevent images on one side of the center frequency from overpowering a potentially weak carrier on the other.
In either application, a digital signal processor (DSP) or baseband ASIC is used to generate the modulated baseband data. This data is fed either directly to a pair of baseband digital/analog converters (DACs) (I and Q) for direct RF modulation or to a digital processor responsible for digitally translating them to a suitable digital intermediate frequency (IF).
Depending on the application, the DSP alone or in conjunction with a digital processor can be used to digitally predistort the baseband data in such a manner that distortion products generated later in the signal chain will be canceled. If an IF stage is employed, the baseband data generated by the DSP must be upconverted either digitally with an FPGA or ASIC or alternatively with a traditional mixer or modulator to the desired IF.
This traditional technique is being replaced by digital means because of the added flexibility offered through digital logic and the availability of good, cost-effective digital-to-analog converters. As with the related receive function, the purpose of this device is to shape the bandwidth of the desired channel and then upconvert by digital means to the desired IF frequency. If multiple channels are required, they can be synthesized on one chip. After translation, each of the channels can be summed together and
interpolated to the desired data rate and then sent to a DAC. If desired, digital predistortion can be added in conjunction with the DSP to correct for distortion later in the signal chain.
Either a mixer or a modulator is used for frequency translation to the final RF frequency. If direct RF modulation employed, an RF modulator will be used. If an IF is used (either directly from a DAC or a traditional IF upconversion), a mixer will be used to translate to the final RF frequency. As with the receive mixer/demodulator, it may be desirable to change the bias levels or the drive level of the data or local oscillator (LO) levels to optimize distortion.
18.8 Multichannel transmit with single up-convert super-heterodyne.
18.9: Single-carrier direct-conversion transmit.
As with the receive LO, the transmit LO is variable in frequency and easily rogrammable via software control using PLL or DDS techniques. Here, too, it may be desirable to change the LO drive level to optimize spurious performance under a variety of signal conditions. As with the single-band operation of the receiver, there may be cases where a fixed LO is required.
Such an example would be for operation within a single band where tuning is accomplished within the ASIC or FPGA. As with the receive path, the data converter or DAC is often the bottleneck. However, since dynamic range requirements for the transmit signal path are much lower (typically 25 to 45 dB) than the receive path, component selection is not quite as difficult. Many DACs are available that facilitate a wide range of adjustments, including gain and offset correction so that I/Q imbalances in the transmit signal chain can be minimized. Other desired features include data rate interpolation and I/Q phase correction.
Finally, power gain is achieved through a pre-amp and power amplifier (PA). Aside from the fact that these devices must operate across a wide range of frequencies, it is desirable to adjust the RF output power. There could be regulatory issues that require some frequencies to be transmitted at lower power than others. While the PA gain is usually fixed, the pre-amp may be in the form of a VGA.