24 GHz to 44 GHz Wideband Integrated Upconverter and Downconverter Boosts Microwave Radio Performance While Reducing Size

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24 GHz to 44 GHz Wideband Integrated Upconverter and Downconverter Boosts Microwave Radio Performance While Reducing Size [Copy link]

24 GHz to 44 GHz Wideband Integrated Upconverter and Downconverter Boosts Microwave Radio Performance While Reducing Size

Analog Devices has introduced a pair of highly integrated microwave up-down converters, the ADMV1013 and ADMV1014. These two devices operate over a very wide frequency range, from 24 GHz to 44 GHz, and provide 50 Ω matching while supporting an instantaneous bandwidth greater than 1 GHz. The performance characteristics of the ADMV1013 and ADMV1014 simplify the design and implementation of small 5G millimeter wave (mmW) platforms, which include the 28 GHz and 39 GHz bands commonly found in backhaul and fronthaul applications, as well as many other ultra-wideband transmitter and receiver applications.

Each upconverter and downconverter chip is highly integrated (see Figure 1), consisting of an IQ mixer and on-chip orthogonal phase shifters, and can be configured in baseband IQ mode (zero IF, IQ frequency supports dc to 6 GHz) or in IF mode (real IF, IF frequency supports 800 MHz to 6 GHz). The RF output of the upconverter integrates a driver amplifier with a voltage-controlled attenuator (VVA), and the RF input of the downconverter includes a low-noise amplifier (LNA) and a gain amplifier with VVA. The local oscillator (LO) chain of both chips consists of an integrated LO buffer amplifier, a quadrupler, and a programmable bandpass filter. Most of the programmable and calibration functions are controlled through the SPI interface, which makes the IC easy to configure to excellent performance levels through software.

Figure 1. (a) Block diagram of the ADMV1013 upconverter chip.

(b) Block diagram of the ADMV1014 downconverter chip.

Internal View of the ADMV1013 Upconverter

The ADMV1013 provides two frequency conversion modes. One mode is direct upconversion from baseband I and Q to the RF band. In this I/Q mode, the baseband I and Q differential input signals range from dc to 6 GHz, such as those generated by a pair of high-speed digital-to-analog converters (DACs). The common-mode voltage range of the IQ input signals is 0 V to 2.6 V; therefore, they can meet the interface requirements of most DACs. When the common-mode voltage of the selected DAC is within this range, the registers of the upconverter can be configured to achieve the best match between its input common-mode voltage and the common-mode voltage of the DAC output, thereby simplifying the interface design. The other mode is a complex IF input (such as the signal generated by an orthogonal digital upconverter device) with single-sideband upconversion to the RF band. A unique feature of the ADMV1013 is its ability to digitally correct the dc offset errors of the I and Q mixers in I/Q mode, thereby improving the LO leakage of the RF output. After calibration, the LO leakage at the RF output can be as low as -45 dBm at maximum gain. One of the more difficult challenges that hinders zero-IF radio design is the phase imbalance of I and Q, which results in poor sideband suppression. Another challenge facing zero-IF is that the sidebands are often too close to the microwave carrier, making filters difficult to implement. The ADMV1013 solves this problem by allowing the user to digitally correct the I and Q phase imbalance through register tuning. During normal operation, the upconverter exhibits an uncalibrated sideband suppression of 26 dBc. Using the on-chip registers, its sideband suppression can be improved to approximately 36 dBc after calibration. Both calibration features are implemented through the SPI, requiring no additional circuitry. In I/Q mode, sideband suppression can also be further improved by adjusting the phase balance of the baseband I and Q DACs. These performance-enhancing features help minimize external filtering while improving radio performance at microwave frequencies.

Figure 2. The ADMV1013 in a 6 mm × 6 mm surface-mount package is shown on the evaluation board.

With the LO buffer integrated, the part requires only 0 dBm of drive power. Therefore, the device can be driven directly from a frequency synthesizer with an integrated voltage controlled oscillator (VCO), such as the ADF4372 or ADF5610, further reducing the number of external components. An on-chip quadrupler multiplies the LO frequency to the desired carrier frequency, and the unwanted harmonics of the doubler are then filtered out by a programmable bandpass filter placed before the mixer quadrature phase generation block. This layout greatly reduces spurious frequencies entering the mixer while allowing the part to operate with an external low-cost, low-frequency synthesizer/VCO. The modulated RF output is then amplified by a pair of amplifier stages with a VVA in between. The gain control block provides the user with a 35 dB adjustment range and a maximum cascaded conversion gain of 23 dB. The ADMV1013 is available in a 40-lead land grid array package (see Figure 2). These features combine to provide excellent performance, maximum flexibility, and ease of use while minimizing the number of external components required. This enables small microwave platforms such as small cell base stations.

Internal View of the ADMV1014 Downconverter

The ADMV1014 has some similar elements, such as an LO buffer, a frequency quadrupler, a programmable bandpass filter, and a quadrature phase shifter in its LO path. However, constructed as a downconverter (see the block diagram in Figure 1b), the ADMV1014 has an LNA installed in the RF front end, followed by a VVA and an amplifier. The continuous 19 dB gain adjustment range is controlled by the dc voltage applied to the VCTRL pin. The user can choose to use the ADMV1014 as a direct demodulator from microwave to baseband dc in I/Q mode. In this mode, the demodulated I and Q signals are amplified at their respective I and Q differential outputs. Their gain and dc common-mode voltage can be set by registers via SPI, allowing the differential signals to be dc coupled to, for example, a pair of baseband analog-to-digital converters (ADCs). Alternatively, the ADMV1014 can be used as an image-reject downconverter for a single-ended complex IF port. In either mode, I and Q phase and amplitude imbalances can be corrected via the SPI, improving image rejection when the downconverter is demodulating to baseband or IF. Overall, the downconverter provides a 5.5 dB total cascaded noise figure and 17 dB maximum conversion gain over the 24 GHz to 42 GHz frequency range. The cascaded NF remains firmly at 6 dB when operating close to the baseband edge (up to 44 GHz).

Figure 3. The ADMV1014 in a smaller 5 mm × 5 mm package is shown on the evaluation board.

Dramatically improving 5G mmW radio performance

Figure 4 shows the measured performance of the downconverter at 28 GHz using a 5G NR waveform with four independent 100 MHz channels, each modulated to 256 QAM at -20 dBm input power. The measured EVM result is -40 dB (1% rms), which supports demodulation of the higher order modulation schemes required for mmW 5G. With >1 GHz bandwidth capability of the upconverter and downconverter, as well as 23 dBm OIP3 of the upconverter and 0 dBm IIP3 of the downconverter, the combination can support higher order QAM modulation, thus achieving higher data throughput. In addition, the device also supports other applications such as satellite and ground station broadband communication links, secure communication radios, RF test equipment and radar systems. Its excellent linearity and image rejection performance are impressive, and when combined with the compact solution size, small form factor, high performance microwave links, it can realize broadband base stations.

Figure 4. Measured EVM performance (rms percentage) vs. input power at 28 GHz and corresponding 256 QAM constellation diagram.

By James Wong, Kasey Chatzopoulos, and Murtaza Thahirally, Analog Devices