Over the years, digital transceivers have been used in many types of applications, including terrestrial cellular networks, satellite communications and radar-based surveillance, earth observation and surveillance. In the past, system engineers for transceivers have used IF architectures in these applications. Now, recent developments in high-speed data converters enable novel RF direct-sampling based architectures.
Converter technology evolves every year. The sampling rates of analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) from major semiconductor companies are orders of magnitude faster than those available a decade ago. For example, in 2005, the sampling rate of the world's fastest 12-bit resolution ADC was 250 MS/s; by 2018, the sampling rate of 12-bit ADC has reached 6.4 GS/s.
Thanks to these performance improvements, converters can directly digitize signals at RF frequencies and provide sufficient dynamic range for modern communications and radar systems.
Although there are trade-offs when using high sample rate (mainly dynamic range) converters, this technique allows you to replace the widely used heterodyne RF architecture with a direct RF architecture to support specific applications. For example, a direct RF sampling instrument with a simplified front end is ideal for wideband RF applications that require a smaller form factor or reduced cost.
In particular, the technology has been further developed in several defense and aerospace applications such as radar and electronic warfare.
1. What is direct RF sampling?
If you want to understand direct RF architecture, you need to understand how it differs from other RF architectures. In a heterodyne configuration, a receiver receives a signal at an RF frequency, downconverts the signal to a lower intermediate frequency (IF), digitizes, filters, and demodulates it. Figure 1 shows the block diagram of a heterodyne receiver. It can be seen that the RF front end of the instrument includes a bandpass filter, low noise amplifier, mixer and local oscillator (LO).
1. The heterodyne receiver block diagram shows an instrument with an RF front-end consisting of a band-pass filter, low-noise amplifier, mixer, and local oscillator.
Whereas a direct RF sampling receiver architecture consists only of a low noise amplifier, appropriate filters and ADC. The receiver in Figure 2 does not require the use of mixers and LOs; the ADC directly digitizes the RF signal and sends it to the processor. In this architecture, you implement many of the receiver's analog components on a digital signal processing (DSP) chip. For example, you can use direct digital conversion (DDC) to isolate the signal of interest without using a mixer. Also, in most cases you can replace most analog filtering with digital filtering, except for anti-aliasing or reconstruction filters.
Since no analog to frequency conversion is required, the overall hardware design of a direct RF sampling receiver is much simpler, allowing for a smaller form factor and lower design cost.
Figure 2. A direct RF sampling receiver architecture can consist of only a low noise amplifier, appropriate filters, and ADC.
2. How to implement direct sampling?
Before the rapid advances in converter technology in recent years, direct sampling architectures were not practical due to converter sampling rate and resolution limitations. Semiconductor companies are using new techniques to increase resolution at higher sampling frequencies to reduce noise within converters. With the advent of ultra-high-speed converters with higher resolution, RF input signals can be directly converted to multi-gigahertz signals. At present, the latest generation ADCs of semiconductor companies such as Teledyne e2v (UK), TI, and ADI can meet this standard.
These slew rates allow engineers to digitize at very high instantaneous bandwidths in both L-band and S-band. Direct RF sampling in other frequency bands such as C-band and X-band is also possible as converters continue to evolve.
3. When should a direct RF sampling architecture be considered?
The main advantage of direct RF sampling is that it simplifies the RF signal chain, reduces cost per channel, and reduces channel density. Instruments based on direct RF sampling architectures are typically smaller and more power efficient because they use fewer analog components. If building a high-channel-count system, direct RF sampling can reduce system footprint and cost. This is especially important when building systems such as fully active phased-array radars, which form beams by phase-shifting signals transmitted from as many as hundreds or even thousands of antennas. Since multiple RF signal generators and analyzers are included in the same system, the size and cost of each channel becomes an important consideration.
In addition to the size, weight, and power (SWaP) reduction, the simplified architecture eliminates possible noise, imaging, and other sources of error, such as LO leakage and quadrature impairments, within the RF instrumentation itself.
Finally, the direct RF sampling architecture also simplifies synchronization. For example, to achieve phase coherency in an RF system, the internal clock and LO of the RF instrument must be synchronized. In direct sampling where no LO is required, only the clock synchronization of the device is of concern. Likewise, for phased-array radar applications that require multiple phase-coherent RF receivers, a direct-sampling architecture is an effective choice to simplify the design.
Source: Organized from NI website
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