5G at microwave frequencies will need a high bandwidth mixer, says Linear

Bandwidth is rapidly expanding in the next generation wireless access to cope with the ever-increasing Internet traffic. The available spectrum simply cannot support the needed bandwidth.

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Wideband Output Spectrum shows all the spurious products which affect the output filter requirements

Higher frequency spectrum is being evaluated for suitability. New mixers with improved performance will be needed, write Bruce Hemp and James Wong.

Also 5G wireless is expected to deliver 1Gbit/s. In order to achieve such speeds, the bandwidth will need to be pushed upward to 1GHz or higher. Invariably, new spectrum needs to open up.

Linear Technology believes it can address this with the LTC5549, which is a passive double-balanced mixer that can function as either an up- or downconverter.

Designed to have a wide RF operating frequency range from 2GHz to 14GHz, the mixer offers linearity of 28.2dBm IIP3 at 5.8GHz, and 22.8dBm at 12GHz. This is important to improve the dynamic range of both transmitters and receivers alike.

The mixer has an integrated LO buffer that needs only a 0dBm drive level, and there is an on-chip, bypassible frequency doubler for the LO signal, which will help in the choice of low-frequency synthesisers, where applicable.

This is a SiGe BiCMOS chip and the monolithic die is flipped and soldered onto a tiny 3mm x 2mm lead-framed, plastic surface mount package. Bond wires are eliminated to greatly enhance the device’s microwave frequency performance without introducing bond wire inductances.

In receiver design, the mixer’s 22.8dBm IIP3 performance increases the robustness when in the presence of close-in high power interference, either from out-of-band unintentional emitter sources or self-induced (leak through from another transmitter in multi-sectored systems).

Higher dynamic range receivers provide added design margin and therefore are more forgiving in handling high blockers – as airwaves continually degrade over time with ever-increasing radio deployments.

Similarly for transmitters, a higher IIP3 (hence higher OIP3) mixer helps to produce less spurious products and thus improved the spectral purity and better ACPR performance. This is particularly important for radios that use higher order modulations which can push upward of 1024 QAM or higher.

The improved linearity helps to produce better definition of the constellation accuracy. Additionally, higher IIP3 allows the mixer to operate at elevated input power, and therefore more robust output power levels.

The LTC5549’s integrated LO amplifier effectively eliminates the +10dBm to +17dBm LO amplifier that is typically required to drive the traditional microwave passive mixers. So its 0dBm LO drive enables the LO to be driven directly from a PLL/synthesiser without buffering.

The low LO power can result in lower LO leakage to either the IF or the RF ports, so less external filtering is necessary to contain any out-of-band emission associated with such a high power source. Another benefit is not having a high power radiation source on the PC board.

The 3rd Order Intermodulation spurs are measured at -74 dBm, suggesting an IIP3 performance of +23.8dBm at the 12.6 GHz frequency.

The 3rd Order Intermodulation spurs are measured at -74 dBm, suggesting an IIP3 performance of +23.8dBm at the 12.6 GHz frequency.

The novel planar balun transformer designs support a wide operating bandwidth and balanced operation.

For example, the 50Ω matched RF port with its built-in transformer and a 0.15pF external capacitor exhibits return loss better than 10dB from 2GHz to 14GHz continuously. Similarly, by connecting a 0.15pF shunt capacitor and a series capacitor at the LO input, the port is 50Ω matched from 1GHz to 12GHz. Return loss is better than 10dB across that entire frequency range.

Bruce Hemp is applications section leader and James Wong, product marketing manager with Linear Technology


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