Abstract— A 700-MHz to 1.6-GHz RF power digital-to-analog converter with programmable integrated harmonic cancellation and mixed-signal filtering is depicted. Harmonic cancellation is achieved by splitting the power amplifier into different parts, driving different segments of the PA with phase-shifted versions of the local oscillator(LO) signals, and adding at the output. Mixed-signal filtering is realized in a similar way but with sections driven with delayed versions of the input data. The data delays and phase shift are implemented to operate across a wide frequency range and are reconfigurable too. For boosting efficiency, 25% duty-cycle LO signals are used. A technique to correct for IQ constellation distortion made by these 25% duty cycle LO signals is introduced and confirmed in measurements. The transmitter(TX) operates at an extreme sampling rate of 500 Mega Samples per second and an output power of 25.6 dBm is achieved for an output load of 100 ohm when harmonic cancellation is empowered. The TX demonstrates 24dB to 42 dB of third harmonic cancellation for continuous wave signals across a 700-MHz to 2-GHz frequency range, achieving an HD3 as low as ?57 dB. The TX achieves an HD3 drop of 33 dB and an 18-dB notch at 40-MHz offset with 20-MHz long-term evolution (LTE) data.
Keywords—Active cancellation, CMOS, digital-to-analog converter (DAC), power amplifier (PA), RF, switched-capacitor PA (SCPA), wideband transmitter (TX).
Digital power amplifiers(PAs) have become progressively striking because of their technology quantifiability and the capability to use them as the core of a Radio Frequency digital-to-analog converter(DAC). RF DACs are demanding as they enable for digital input data to be changed directly to RF output signals 1 – 3. The direct conversion from digital to radio frequency signals permits the easiness in reconfiguring the transmitter(TX) to meet different standards. Current works on multi-standard digital PAs 2 has established digital PAs as a feasible option for flexible transmitters. However, the 2 major issues which must be kept in consideration before this becomes a hands-on and efficient solution are harmonic emissions and quantization noise.
Basically, digital PAs, which works as switching PAs, produce strong harmonic content. Digital PAs generate square wave (voltage or current) which then is filtered with an output network. The power of each harmonic, with n as the harmonic number, attenuates with 1/n2 for a square wave, meaning that without extra filtering, the third harmonic is only 9.5 dB lesser than the fundamental in power. Naturally, using a narrowband and high-order fixed filter these harmonics are suppressed. A reconfigurable transmitter(TX) would demand a large amount of these filters, which would be expensive and would add an additional loss from the switch network. Due to which, an integrated and reconfigurable resolution is necessary, in aggregation with a wideband output network.
Moreover, an RF DAC creates quantization noise, as is basically the case with all DACs. Which in turn can cause problems with coexistence in frequency division duplex systems, as a nearby receiver (RX) can be desensitized by this quantization noise. In the long-term evolution(LTE) standard, for eg., a receiver could be offset only by 40MHz. Adding resolution or oversampling to the DACs, is a commonly used practice to reduce quantization noise, but is generally pricey either in area or in power. If the RX frequencies are identified, filtering selectively at those frequencies could be a less expensive decision.