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Interpreting the Direct RF Sampling Architecture and Its Advantages

For many years, digital transceivers have been used in many types of applications, including terrestrial cellular networks, satellite communications, and radar-based surveillance, earth observation, and monitoring. In the past, system engineers of transceivers used intermediate frequency architectures in these applications. Now, the latest developments in high-speed data converters have made new architectures based on direct RF sampling possible.

Converter technology is advancing every year. The sampling rates of analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) from major semiconductor companies are several orders of magnitude faster than those of a decade ago. For example, in 2005, the world's fastest 12-bit resolution ADC had a sampling rate of 250 MS/s; in 2018, the sampling rate of a 12-bit ADC has reached 6.4 GS/s. As a result of 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 sampling rate (mainly dynamic range) converters, this technology allows you to replace the widely used heterodyne RF architecture with a direct RF architecture to support specific applications. For example, for wideband RF applications that require a smaller form factor or reduced cost, direct RF sampling instruments with simplified front ends are ideal. In particular, this technology has been further developed in some 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 structure, the receiver receives a signal at an RF frequency, downconverts the signal to a lower intermediate frequency (IF), digitizes it, filters it, and demodulates it. Figure 1 shows the block diagram of a heterodyne receiver. As you can see, the RF front end of the instrument includes a bandpass filter, a low-noise amplifier, a mixer, and a local oscillator (LO).

1. This heterodyne receiver block diagram shows an instrument with an RF front end consisting of a bandpass filter, low noise amplifier, mixer, and local oscillator.

In contrast, a direct RF sampling receiver architecture consists of only a low-noise amplifier, appropriate filters, and an ADC. The receiver in Figure 2 does not require the use of a mixer and LO; the ADC directly digitizes the RF signal and sends it to the processor. In this architecture, you can implement many of the analog components of the receiver 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 of the analog filtering with digital filtering, except for antialiasing or reconstruction filters.

Since no analog 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 an ADC.

2. How to achieve direct sampling?

Before the rapid development of converter technology in recent years, direct sampling architectures were not practical due to the limitations of converter sampling rate and resolution. Semiconductor companies have used new technologies to improve resolution at higher sampling frequencies to reduce noise within the converter. With the advent of ultra-high-speed converters with higher resolution, RF input signals can be directly converted to signals of several gigahertz. The latest generation of ADCs from semiconductor companies such as Teledyne e2v (UK), TI, and ADI can meet this standard.

These conversion rates allow engineers to digitize at very high instantaneous bandwidths in the L-band and S-band. As converters continue to advance, direct RF sampling in other bands, such as C-band and X-band, is not out of the question.

3. When should you consider using a direct RF sampling architecture?

The main advantages of direct RF sampling are simplified RF signal chains, reduced cost per channel, and lower channel density. Instruments based on direct RF sampling architectures are typically smaller and more power efficient because they use fewer analog components. If you are building a high-channel-count system, direct RF sampling can reduce the system's 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 up to hundreds or even thousands of antennas. Since the same system contains multiple RF signal generators and analyzers, the size and cost of each channel becomes an important consideration.

In addition to size, weight, and power (SWaP) reduction, the simplified architecture eliminates possible noise, images, and other error sources within the RF instrument itself, such as LO leakage and quadrature impairments.

Finally, direct RF sampling architectures can also simplify synchronization. For example, to achieve phase coherence in an RF system, the internal clock and LO of the RF instrument must be synchronized. In direct sampling, where LO is not required, only the clock synchronization of the device is of concern. Similarly, for phased array radar applications that require multiple phase-coherent RF receivers, direct sampling architectures are an effective choice to simplify the design.

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With the advent of ultra-high-speed converters with higher resolution, RF input signals can be directly converted to signals of several gigahertz. The latest generation of ADCs from semiconductor companies such as Teledyne e2v (UK), TI, and ADI can meet this standard.