FBH demonstrates fully digital transmitter chain
In common transmitter systems the baseband signal is usually generated in the digital domain, utilizing the powerful but yet low-cost digital signal processors that are available today. Still, up-conversion and amplification are carried out in the analog domain. In previous works, FBH has shown that it is beneficial to convey more components of the transmitter chain into the digital domain. More flexible and power efficient amplifiers were demonstrated; also a modulator which conditions the baseband signal for the purely digital amplifier has been developed.
Recently, a fully digital transmitter chain was demonstrated, combining the modulator and amplifier into one coherent system. This achievement is an important step towards the usability of such systems in real-world applications. Using a realistic test signal the overall system performance could be demonstrated meeting the high standards regarding signal quality and linearity.
The presented system is designed to be a drop-in replacement for existing, analog transmitter chains so that existing applications can be easily converted without requiring changes on the receiving side (Fig. 1). The switch-mode amplifier MMIC used in this system is fabricated on FBH’s in-house 0.25 µm GaN-HEMT process (Fig. 2). The modulator is implemented in Matlab, and an arbitrary waveform generator is used to create the final bitstream for the amplifier. While a common transmitter chain has to utilize a power-hungry digital predistortion (DPD) to correct non-idealities of the amplifiers, our digital PA modulator already incorporates provisions to eliminate such distortions without any additional building blocks. For the estimation of the correction parameters a sample of the live signal at the system’s antenna output is recorded using an oscilloscope and is then further processed within Matlab.
For demonstration purposes, a 5 MHz WCDMA-like signal with 6.5 dB PAPR was used on a carrier of 900 MHz. Initially, an ACLR of only 25 dB was measured. However, after enabling the aforementioned inbuilt correction the ACLR easily exceeded 51 dB in the adjacent and 55 dB in the alternating channel (Fig. 3). The EVM was reduced from 17% to only 0.6%. Even though the amplifier used in this setup was not built to be very efficient but to have steep signal slopes, 42% drain efficiency was measured at 6.5 dB PAPR.
The overall spectrum at the antenna output using only a simple lumped element LC filter as a bandpass is depicted in Fig. 4. As can be easily seen, there are no significant tones close to the wanted signal at 900 MHz; the next significant contribution is at two times the carrier frequency with less than -20 dBc. These low requirements on the steepness of the filter enables using easy-to-construct filters with very low insertion loss.
This work is funded within the competitive procedure of the Leibniz Association under contract SAW 2015-FBH1.
F. Hühn, A. Wentzel, W. Heinrich, "A new modulator for digital RF power amplifiers utilizing a wavetable approach", Int. J. Microwave Wireless Technolog., vol. 9, no. 6, pp. 1251-1260 (2017).
A. Wentzel, F. Hühn, W. Heinrich, "The Digital Power Amplifier for the Wireless Infrastructure: Status and Prospects", IEEE Topical Conference on RF/Power Amplifiers for Wireless and Radio Applications (PAWR), digest, Phoenix, USA, Jan. 15-18, pp. 14-17 (2017).
F. Hühn, A. Wentzel, W. Heinrich, "GaN-Based Digital Transmitter Chain Utilizing Push-Pull Gate Drivers for High Switching Speed and Built-In DPD", 47th European Microwave Conference (EuMC), 2017.