Ajay Kuckreja, Principal Technical Staff at Maxim Integrated explores how RF DACs are an essential component in a direct RF radio transmitter
Wireless radio transmitters have evolved over the years from real IF (intermediate frequency) transmitters, to complex IF transmitters, to zero-IF transmitters. However, there are still limitations associated with these commonly used architectures.
A newer approach, a direct RF radio transmitter, can overcome the limitations of traditional transmitters. An RF DAC, such as the 14-bit 2.3Gsps MAX5879 from Maxim Integrated, is an integral component for the direct-to-RF architecture. This DAC transmits in the second and third Nyquist zones so can perform RF synthesis at output frequencies as high as 3GHz.
Traditional transmitter architectures have been implemented over the last few decades based on the super-heterodyne principle, where an intermediate frequency (IF) is generated using a local oscillator (LO) and a mixer. The mixer typically creates two images, known as sidebands, around the LO. The wanted signal is then obtained by filtering out one of the sidebands.
Modern radio transmitters, specifically the ones used in wireless base transceiver stations (BTS), commonly use complex in-phase (I) and quadrature phase (Q) symbols at baseband for a digitally modulated signal.
A complex baseband digital signal thus has two paths at baseband, I and Q. There is an advantage to using two signal paths in this manner: when the two complex IF signals are combined using an analogue quadrature modulator (MOD), one of the IF sidebands is eliminated. However, because of asymmetries in the I and Q paths, an ideal cancellation of the modulator image is never achieved.
This complex IF architecture is shown in Figure 1(B). Here the complex baseband I and Q signals are interpolated (by a factor R) and modulated to complex IF carriers using a digital complex modulator and a numerically controlled oscillator (NCO) that acts as an LO. The dual DACs then convert the digital I and Q IF carriers to analogue and feed it to the modulator. To further increase attenuation of the undesired sideband, a bandpass filter (BPF) is used.
In the Zero IF (ZIF) transmitter shown in Figure 1(A), the digital complex signal at baseband is simply interpolated to ease filtering requirements and then fed to the DACs. The complex analogue output of the DACs, still at baseband (DC), is fed to an analogue quadrature modulator. The ‘magic’ of using complex signals is readily apparent with the ZIF architecture, as the entire modulated signal is converted to an RF carrier at exactly the LO frequency. However, imperfections such as LO feedthrough and asymmetries in the I and Q paths result in an LO spur and a reversed signal image that falls within the transmitted signal.
This, in turn, degrades the bit error rate of the signal. In multicarrier transmitters, the images may be adjacent to the carriers and then in-band spurious emissions result. In the direct RF transmitter shown in Figure 1(D), the quadrature demodulator is implemented in the digital domain and the LO replaced by an NCO.
This results in near-perfect symmetry in the I and Q paths with virtually no LO feedthrough. The output of the digital modulator is thus a digital RF carrier that is fed to a very high-speed DAC. Since the output of the DAC is in discrete time, an aliased image is also created equidistant to the DAC clock frequency (CLK). The DAC output is filtered by the BPF to select the RF carrier and then fed to the variable gain amplifier (VGA).
This scheme for the direct RF transmitter can also be used to generate a high, ‘real’ (as opposed to complex) digital IF carrier, as shown in Figure 1(C). The DAC here converts the digital IF to an analogue IF carrier.
Since the analogue quadrature modulator and LO can be implemented in an FPGA or ASIC with a digital quadrature modulator and an NCO, the direct RF architecture eliminates the I and Q imbalance errors and the LO feedthrough. A very high-performance DAC is essential for the direct RF architecture to be a feasible alternative to traditional radio transmitters.
Maxim Integrated
