Radio transmitters can now be designed with direct digital modulation, due to the availability of high-speed digital-to-analog converters (DACs). This approach is already commonly used to generate multicarrier quadrature-amplitude- modulation (QAM) signals for data transmission over Hybrid Fiber Coax (HFC) links in cable-television (CATV) access networks, and is also used to synthesize microwave intermediate- frequency (IF) signals for instrumentation, radar, and high-speed military wireless communications links.

Advances in silicon CMOS with effective scaling to create smaller feature sizes have yielded low-cost, numerically controlled oscillators (NCOs) that can generate high-frequency local-oscillator (LO) signals with low power consumption. Silicon CMOS also supports the design of low-power quadrature modulators for generating the in-phase (I) and quadrature (Q) signals that form the RF digital modulation in many systems. In conjunction with high-speed RF DACs such as the MAX19692 from Maxim Integrated Products, designers now have the building blocks needed to implement highperformance digital RF transmitters and other systems.

The update rates of 12- and 14-b CMOS RF DACs are approaching 5 GSamples/s. In conjunction with nanometerfeatured CMOS signal-processing components, frequencies to 2 GHz and higher can be synthesized digitally, with low spurious and noise content. A flexible digital transmitter can be formed by teaming a high-speed DAC with a highperformance programmable digital modulator.

Digital transmitters and modulators overcome some of the limitations of analog transmitters. Digital synthesis can provide signals limited only by the finite digital word length. Digital synthesis can be performed without the temperature drift, conversion loss, and distortion of analog sources. Using digitally generated quadrature modulation, quadrature errors and LO feedthrough can be essentially eliminated.

A high-speed RF DAC allows instantaneous, phase-continuous frequency hopping. It can synthesize broadband signals with excellent gain flatness. For example, the downstream CATV spectrum from 50 MHz to 1 GHz can be synthesized by means of a single MAX5881 DAC from Maxim with better than 2-dB gain flatness across the full bandwidth. With digital modulators, the lossy broadband RF combining networks can be eliminated for increased density of CATV head-end equipment. Radar signals with GHz bandwidths can be synthesized without using frequency doublers, providing increased resolution. Multi-standard, multi-band transmitters can be implemented using programmable digital hardware. Compared to an analog modulator and LO, a digital transmitter is simpler and more predictable and the transmitter subsystem requires less board space and consumes less power.

A DAC provides a discrete time signal as an output. Images of the carrier that are created are aliased symmetrically around integer multiples of the DAC update (clock) rate. If the DAC is non-ideal, it creates harmonics of the output signal that are also aliased symmetrically at integer multiples of the DAC update rate. In the cases where harmonics or folded images of the harmonics may fall within the desired RF band, they can be filtered. However, if they fall in the desired RF band and if the power level is high enough, the band’s spurious limit may be violated. One way to avoid this problem is to push the DAC update rate high enough that the dominant harmonics fall out of band.

For CATV distribution in the 50-MHz-to-1-GHz band, Nyquist theory states that a DAC with sample rate higher than 2 GSamples/s should be adequate for synthesizing the entire CATV band. The dominant harmonic product for a DAC is typically the third harmonic (HD3), assuming a differential output where second harmonic content is strongly attenuated. If a DAC with sampling rate of 2.5 GSamples/s is used [Fig. 1(a)], the third-harmonic image for a real 1-GHz carrier will be located at 500 MHz. But if the DAC’s update rate is pushed beyond 4 GSamples/s [Fig 1(b)], this third-harmonic image will never fall within the 50-MHz-to-1- GHz band. Pushing the DAC update rate higher also simplifies the design of the reconstruction filter.

DACs such as the 12-b, 4.3- GSamples/s model MAX5881 can be used in CATV head-end equipment and can directly synthesize multiple QAM carriers in the 50-MHz-to-1- GHz band. The MAX5881 DAC features a doubled update rate in which the output update rate of the DAC is twice the DAC clock rate. It updates on both edges of the input clock to achieve twice the sample rate of conventional DACs. The device features excellent spurious, noise, and adjacent channel power (ACP) performance and can synthesize multiple carriers per the requirements defined in the Data-Over-Cable Service Interface Specification (DOCSIS, detailed in Downstream RF Interface Specification CM-SP-DRFI-110-10011, Cable Television Laboratories, January 6, 2006).

Another DAC from Maxim, the MAX19692, operates with 12-b resolution and 2.3-GSamples/s update rate with selectable frequency response. The default operating mode of the DAC is the conventional nonreturn- to-zero (NRZ) mode that has a familiar sinc shape, with zeroes at every multiple of the DAC update frequency as shown in Eq. 1:

A_NRZ = A₀|[sin(πf_outT)]/(πf_outT)| (1)
where
T = the clock period,
f_out = the output frequency,
A₀ = the DC amplitude, and
A_NRZ = the amplitude response in NRZ mode.

In RZ-mode, the DAC zeros itself in the time domain at every half clock cycle. The resulting frequency (amplitude) response, A_RZ, is described by Eq. 2. This frequency response is flatter than the NRZ-response in the three first Nyquist zones, and particularly in the second and third Nyquist zone, providing useful performance for synthesizing wideband signals in the second and third Nyquist zones.

The third mode of operation is called RF-mode. This is similar to mixing the DAC output with the DAC update clock. In this mode, the amplitude response, ARF, is described by Eq. 3.

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