Microwave sensors are increasingly finding commercial applications, particularly in the automotive industry. Here, advanced features are used to distinguish vehicles, while safety is a major consideration in new car purchases.
Therefore, it is not surprising that radar technology has been adopted and implemented in adaptive cruise control (ACC) systems. However, in a highly competitive market the demand is for high performance, efficient, low cost solutions. These were the design considerations when a surface-mount, substrate-based Gunn diode voltage controlled oscillator (VCO) was developed for use in a 77 GHz frequency-modulated continuous wave (FMCW) radar sensor.
The advantage of this small VCO is that a planar circuit is used. This removes the need for complex automatic production equipment in the assembly and tuning process, which is necessary when using a waveguide cavity. Because the rate of change of frequency with voltage and its variation over temperature is critical to the overall performance of an ACC radar, a varactor diode is employed to tune the frequency output of the VCO. Another consideration is that it is easy to introduce a second microwave diode with a Gunn diode when both are used as naked semiconductor die.
Microstrip is the transmission medium for the oscillator circuit and fused silica is the substrate material, due to its dielectric and thermal properties, ready availability and compatibility with thin film processing. Also, an output in microstrip can be configured to give two output ports - one for the transmitter and the other as a local oscillator drive for a mixer. Figure 1 shows the relative sizes of the cavity type, the new VCO with microstrip output and the new VCO with waveguide output.
One of the key factors for automotive applications is the ability of components to operate over the broad temperature range of -40° to +85°C. To obtain good performance efficiency over this range a GaAs Gunn diode, with a graded AlGaAs injector, is employed as the millimeter-wave power source. Figure 2 is a schematic representation of the circuit, which is designed to operate with a fixed positive voltage applied to the Gunn diode, and a variable positive voltage (0 to 5 V) to the varactor diode to achieve a minimum of 300 MHz frequency tuning range.
The graded-gap Gunn diodes (shown in Figure 3 ) incorporate an integral heat sink structure to ensure effective heat transfer directly to the base of the VCO package. The package for volume manufacture is fitted with ceramic feed-throughs and is hermetically sealed. For good heat transfer the Gunn diode is thermosonically mounted to the gold-plated package floor, and alternative die mounting technologies are being developed, including the use of solder eutectic.
Epoxy is used to mount the varactor diode chip on to the floor of the package. Interconnection to the top contact on the Gunn diode is made using a solid gold Maltese cross structure, which is bonded using a thermosonic wedge bonder. Gold wire is used to make a similar connection to the varactor.
Being designed and constructed around the company's patented graded-gap Gunn diode the VCO exhibits excellent frequency stability against temperature and low noise characteristics, both for FM and AM. Not only is it operational across a wide range of temperatures but also a patented semiconductor structure enables cold start turn-on. To match its high efficiency the VCO also produces high power of greater than 30 mW at 77 GHz. It is also small in size and has a low component count.
Work on novel diode structures, especially graded-gap Gunn technology, continues in collaboration with a number of universities in the United Kingdom. It is expected that this will result in diodes that have improved output powers and an ability to work at higher fundamental frequencies. The VCO is being used as the key component in future ACC transceiver designs that are in development.
Conclusion
The Gunn diode VCO lays claim to being the smallest, assembly friendly, packaged oscillator available for use in ACC systems. Compatible with printed circuit board and RF module assembly techniques, it can be incorporated into a 77 GHz sensor without the need for high class clean room conditions. It represents a breakthrough for Gunn diode-based VCOs and offers a solution where the key drivers of size, cost, performance and versatility can be satisfied simultaneously. Featuring excellent phase noise performance, high temperature stability and output power to meet requirements of ACC systems, the new VCO has also been tested for cost, reliability and manufacturing capability.
Marconi Applied Technologies,
Chelmsford, UK +44 (0) 1245 493493,
www.marconitech.com
Circle No. 300