News Article

Pulse Generator Slashes The Size And Cost Of 10 Gbit/s Transmitters

Oscillators, mixers and RF switches add to the cost and complexity of 10 Gbit/s transmitters for last-mile networks. However, all of these components can be discarded by switching to a radical design that features a pulse generator and an amplifier, says a team of researchers at Fujitsu.

High-capacity wireless systems are a great option for data transfer over distances of up to a few kilometers. If they can operate at 10 Gbit/s or more, they can help to address the rocketing demand for capacity in mobile phone and internet networks, which is partly fueled by the growth of uncompressed high-definition video. On top of this, wireless promises to be a popular alternative in locations where it is particularly challenging to lay fiber-optic cables for trunk-line data network systems. Mountainous regions and areas with rivers, roads or rail tracks all exacerbate the difficulties that are associated with build-out of buried fiber-optic networks.

The best frequencies for delivering data transmission at speeds in excess of 10 Gbit/s are found in the millimeter band. This region within the electromagnetic spectrum is rarely used for commercial applications, which makes it relatively easy to secure wide swaths of bandwidth. A subset of this band, the "radio window" of 70–100 GHz, is also subjected to a relatively low level of attenuation by atmospheric gases. This increases the data transmission range and improves the link s robustness.

W-band transmission

The short wavelengths that are associated with transmission in this radio window lead to equipment benefits. Millimeter-band radios can employ small hardware alongside high-gain antennas that produce a highly directional beam. This type of antenna reduces multi-path fading and interference with neighboring radios.

Despite these advantages, it is difficult to develop small, cost-effective equipment for transmitting in the 70–100 GHz range. This is because this type of unit currently requires the manufacture of several discrete single-purpose electronic components.

To address this weakness, our research team at Fujitsu and Fujitsu Laboratories, in Kanagawa, Japan, has embarked on the development of a radical form of impulse-radio technology. No oscillators or mixers are needed, and the transmitter can be built from just two active components: a pulse generator and an amplifier (figure 1). The pulse generator s emission corresponds to a data stream from a baseband block. RF signals, known as wavelets, are then created by passing this data stream through a band-pass filter. This filters a wide frequency spectrum of pulses to match the spectrum mask.

Our simple transmitter will only be a viable option for last-mile wireless if it can generate pulses with sufficient energy. Meeting this need requires very short pulses – the smaller the pulse s full-width at half-maximum (FWHM), the greater the power that is emitted at higher frequencies. Our application demands pulses with a FWHM of less than 10 ps, which we have realized with our InP HEMT technology. GaAs HEMT and sub-100 nm silicon MOS technologies were not employed because they have a weaker high-frequency performance and an inferior noise figure.

One distinguishing feature of our HEMT is its Y-shaped gate (figure 2). This architecture helps to prevent the top of the gate from peeling off and leads to a high gate-electrode fabrication yield. Conventional T-shaped gates are impaired by the low physical strength at the junction between a large top and a narrow stem. The Y-shaped gate is formed with a lift-off process that includes electron-beam lithography and the evaporation of a Ti/Pt/Au contact. The typical gate length is 0.13 µm.

Y-shaped gates have the additional advantage of being able to improve the uniformity and stability in an InP wafer. This enables more than 1000 HEMTs to be integrated in a single chip. Thinning of the Schottky barrier layer in each of these transistors enhances performance, including an increase in maximum transconductance and cut-off frequency to 1.5 S/mm and 220 GHz.

We used triple-layer gold-plated interconnections and benzocyclobutene dielectric films with a low dielectric constant to integrate the HEMTs into circuits. Metal-insulator-metal capacitors and NiCr thin-film resistors are also formed on the InP substrate.

Very simple digital-based pulse generators can be created with this technology. These consist of input and output buffers, delay control (DC) buffers and a pulse-generator core (figure 3a).

Short pulses are created in the core by adjusting the DC buffer delay times, which in turn tunes the overlapping time between input signals A and B (figure 3b). The pulse-generator core behaves like a logical NAND circuit and creates a pulse width that is nearly equal to the overlap.

Shorter pulses can be created by increasing the response speed of the pulse-generator core or the NAND circuit. This led us to develop a new, balanced NAND circuit (figure 3c) that can emit ultrashort pulses with a FWHM of just 7.6 ps (figure 4) and a peak-to-peak amplitude of 0.8 V.

We believe that these are the shortest pulses that have ever been generated from a semiconductor transistor. Spectral measurements reveal that the pulse generator can carry enough energy for frequencies above 100 GHz, which means that it s possible to build impulse-radio systems operating in the 70–100 GHz band.

Making a transmitter

We have followed this up by building a millimeter-band pulse transmitter (figure 5). This transmitter features a pulse generator, a 78–93 GHz band-pass filter and a W-band waveguide. Feeding 5 GHz clock signals into this module produced wavelets with a typical FWHM of 100 ps (figure 6). Spectral measurements confirmed that the wavelets had frequencies spanning the 78–93 GHz range, which almost corresponded to the transmission spectrum of the band-pass filter.

These results represent the world s first impulse radio transmitting at more than 10 Gbit/s in the millimeter band (in our case, 78–93 GHz). This new transmitter greatly simplifies the design of the millimeter-band emitters by eliminating the need for an oscillator, mixer and some other components. The upshot is a more compact package that is 70% smaller than its predecessors.

Our next aim will be to unite the transmitter with a receiver and conduct transmission testing with a fixed target. Field testing should follow.

Further reading

Y Kawano et al. 2006 IEEE Trans. Micro. Theory and Tech. 54 4489.

Y Nakasha et al. 2007 Ext. Abstr. of the 2007 SSDM 792.

Y Nakasha et al. 2008 International Microwave Symposium Digest 109.


View pdf of article

AngelTech Live III: Join us on 12 April 2021!

AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!

Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.

2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.

We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.

We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.

Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.

Having attracted 1500 delegates over the last 2 online summits, the 3rd event promises to be even bigger and better – with 3 interactive sessions over 1 day and will once again prove to be a key event across the semiconductor and photonic integrated circuits calendar.

So make sure you sign up today and discover the latest cutting edge developments across the compound semiconductor and integrated photonics value chain.



Search the news archive

To close this popup you can press escape or click the close icon.
Register - Step 1

You may choose to subscribe to the Compound Semiconductor Magazine, the Compound Semiconductor Newsletter, or both. You may also request additional information if required, before submitting your application.

Please subscribe me to:


You chose the industry type of "Other"

Please enter the industry that you work in:
Please enter the industry that you work in:
Live Event