Next generation advanced radar systems demand increased resolution as well as increased sensitivity and tracking capability.
To meet this need, radar systems require more complex waveforms and ever increasing levels of control within the exciter signal chain. Adding to these needs in the exciter signal path is the drive towards economic enhancements; lower power, size and weight as well as more flexible and modular architectures.
Not as often discussed or analysed, as compared to the receiver signal chain, but is equally as important to the overall system operation and performance is pulsed waveform generation for the radar exciter. This excitation waveform generator is a critical aspect of beamsteering, non-linear and complex pulse generation methods.
These different methods require a wide variety of advanced architectures and components to generate the radar pulses supporting challenging phased array transmitter systems.
There is a range of technologies that are available in this area to support the radar exciter designer. Three particular technologies in this area are: direct digital synthesis (DDS), high speed radio frequency digital-to-analogue converters (DACs) and integrated fractional-N (Frac-N) synthesisers. Each approach has its own advantages.
DDS devices have long been used for radar Waveform generation. DDS devices provide a level of integration and abstraction requiring only a limited set of parameters to be written to the device to generate a linear frequency modulated sweep, a widely used pulse type within radar systems.
For example the AD9914/5 DDS device from Analog Devices is able to support frequency agile pulse generation across 1.4GHz of bandwidth.
The frequency tuning and control words are loaded into the AD9914 via a serial or parallel I/O port. Using the integrated sweep mode, the AD9914 can generate linear swept waveforms varying frequency, phase or amplitude.
A 32-bit parallel port is also available to support faster control parameter writing increasing the agility of the DDS output.
RF-DACs provide the most suitable solution in terms of complexity and wide bandwidth waveform generation. Typically the data is fed directly from an FPGA and/or memory source where complex waveforms can be recalled and “played back” through the DAC.
The AD9129 5.6Gsample/s DAC is capable of generating signals across 1.4GHz of bandwidth. Waveforms are pre-calculated and recalled from memory, so very complex waveforms can be generated including non-linear frequency modulated waveforms which are utilised in some of today’s latest systems.
Provided the frequency agility is retained within the DAC bandwidth, the hop rate, pulse length and pulse repetition frequency can all be varied to within a single DAC sample, or a resolution of 350ps.
For L and S band radar systems, DDS devices and RF-DACs may be used directly without the need for upconversion which significantly reduces the system complexity, i.e. – saving power, size, and weight.
The DAC, when used in mixed mode, can generate waveforms at up to 4.2GHz. However, for X band or higher operating frequency radar systems the DDS or RF DAC will require at least a single upconversion stage to appropriately position the waveform at the correct RF frequency.
DDS and DAC sample rates will continue to increase and it is expected that in the near future C and X band may be addressed without upconversion stages.
However, the latest Frac-N frequency synthesisers, such as the ADF4159, provide one option today for covering L to X band waveform generation by using only a single device. This can reduce the system size, weight and power consumption of the overall system as shown in Figure 1.
One drawback however, is that a Frac-N is not as flexible in terms of agility as an RF-DAC, but does offer both fast and slow waveform generation capability and yet only uses 89mW.
The Frac-N is similar to the DDS as it has a significant amount of abstraction that can be ascertained within the device via a simple SPI interface. Various waveforms such as sawtooth and triangular as well as sweeps can be generated or altered as needed to provide frequency agility.
The advancements in DDS, DAC and PLL devices provide the radar system designer with several different methods and technologies to choose from with the final choice very dependent upon the end application.
Continued growth in bandwidth and integration options within this signal chain will only help to reduce the complexity of overall radar system in general. Watch out as this trend continues to meet the next generation radar system requirements.
Authors are Duncan Bosworth segment marketing engineer and Rob Reeder senior systems application engineer with the I&I Segment/MIL-AERO Group at Analog Devices