Features and Benefits of FPGA-Based DDCs

The DDC (Digital Downconverter) has become a cornerstone technology in communication systems. Similar to its analog receiver counterpart, the DDC provides the user with a means to tune and extract a frequency of interest from a broad radio spectrum. Over the past few years, the functions associated with DDCs have seen a shift from being delivered in ASICs (Application-Specific ICs) to operating as IP (Intellectual Property) in FPGAs (Field-Programmable Gate Arrays).

For many applications, this implementation shift brings advantages that include: design flexibility, higher precision processing, higher channel density, lower power, and lower cost per channel. With the advent of each new higher performance FPGA family during the past few years, these benefits continue to increase.

In this article, we will explore some of the key advantages of implementing DDC designs in FPGAs and will describe some of the situations when ASICs can still offer the best solution.

Digital Downconverter Fundamentals

To understand how FPGAs play a key role in implementing DDCs that perform the function of a receiver, it's important to break the DDC down into its individual functional blocks. Figure 1 shows a classic DDC. Regardless of whether it's implemented in an ASIC or an FPGA, this is the common architecture of the DDC function.

The first stage of the DDC uses a complex digital mixer to translate the frequency of interest down to baseband. It uses a pair of multipliers and a DDS (Direct Digital Synthesizer) as the NCO (Numerically Controlled Oscillator). This function enables the user to tune the receiver to the desired frequency of interest. The second stage of the DDC reduces the sampling frequency of the signal to match the desired output bandwidth. It uses a CIC (Cascaded Integrator Comb) filter to decimate the data.

A second CIC filter provides a coarse gain adjustment stage. The signal is then passed to a pair of additional polyphase filters. First a CFIR (Compensation Finite Impulse Response) filter then to a PFIR (Programmable Finite Impulse Response) filter. This filter pair provides additional decimation and final signal shaping prior to the rounding stage and final output.

When we get past all the acronyms, we realize that most of the individual function blocks of the DDC are implemented using multipliers. It thus becomes apparent how the DDC might map into current FPGA families. Most new FPGAs include a wealth of DSP function blocks which are primarily multipliers. The general purpose logic resource and on-chip memory of FPGAs also match the requirements of the DDC for implementing the required FIR filters and filter coefficient tables.

DDCs as Intellectual Property Cores

As part of their IP (intellectual property) library series, Xilinx provides a free DDC core. The core serves as a good general reference design, following the classic DDC architecture shown in Figure 1. While this core can be used as a building block for general purpose DDCs, the real advantages of an IP-based implementation can be best seen in optimized custom cores that are designed to match the requirements of a specific application.

IP Enables Software Radio Products

Pentek offers a series of high-performance IP- based DDCs, available preinstalled in software radio modules. Each is optimized to match a specific range of application requirements.

These cores range from the high-channel count/narrow bandwidth of the 430 Core installed in the Model 7141, to the wider bandwidths and excellent SFDR (Spurious Free Dynamic Range) of the core installed in the Model 7153.

Table 1 lists the range of DDC cores available from Pentek as software radio modules. For each core, pertinent specifications are listed. All products are available in industry standard PMC/XMC modules as well as 3U and 6U CompactPCI, PCI and PCI Express form factors. In addition to the IP-based solutions, a popular ASIC-based DDC solution from Texas Instruments, the GC4016, is included as a reference.

When compared on a size/power/cost per channel basis, it becomes apparent that narrowband, high channel-count DDC cores can be very efficiently implemented in FPGAs. Implementation of wideband DDCs consumes many more FPGA DSP and logic resources. As a result, the number of channels that can be fit into a single FPGA is limited. Even with less cost-effective wideband DDCs, the custom IP approach can sometimes provide the only viable solution when a specific performance characteristic is required. The improved SFDR of the Pentek 420 core is an example of such a requirement.

Flexible Implementation

An additional benefit of IP based solutions is the flexible nature of their implementation. The Models 7141-420 and 7141-430 are created by using the same hardware base with different installed IP cores. Similarly, the Models 7151, 7152 and 7153 are all based on the same 4-channel, 200 MHz, 16-bit A/D PMC/XMC with different FPGA IP cores. All share the same software base allowing migration between different applications to be accomplished with minimum software porting.

Additionally, some applications like JTRS (Joint Tactical Radio System), need to operate across a wide spectrum to handle the diverse signal types. Such applications can benefit greatly by IP based solutions. Figure 2 on the next page, shows the six optimized Pentek cores across a range of applications and the number of channels and bandwidth they typically require.

Again, this wide range of applications can be satisfied by using a small set of hardware with different, optimized IP cores. This is one of the fundamental concepts of SDR (Software Defined Radio), and it's difficult, if not impossible, to achieve with ASIC-based solutions.

System Level Savings

Let's now take a look at a complete receiver system. One common application is GSM 2G, a high channel count, low bandwidth system. An E-GMS receiver requires 174 channels spaced 200 kHz apart. Just three or four years ago, a viable solution would have used the TI/Graychip 4-channel GC4016 ASIC-based DDCs. A common board form factor for these types of application is PMC, such as the Pentek Model 7131. One PMC can house two 100MHz A/Ds and four GC4016s and all of the required interface and support circuitry. For a 174-channel system this would require 11 Model 7131's. By comparison, an IP DDC with 174 channels and similar performance to the 4016 can fit in a single Virtex-5 XC5VSX95T FPGA that can be housed on a single PMC, along with 2 channels of 200MHz A/Ds and all support circuitry such as the Pentek Model 7151. A visual comparison of these two solutions is shown in Figure 3.

Comparing FPGAs and ASICs

FPGAs continue to offer new possibilities and performance when addressing processing tasks like digital downconversion. With each new generation of higher performance FPGAs, processing precision continues to increase. This enables IP-based DDCs to outperform their ASIC-based cousins with specifications like better SFDR.

As shown in Figure 4, it's easy to understand how packing many channels of DDCs into one or two FPGAs can reduce the board count, power requirements and cost over a solution that requires 30 or 40 individual ASIC DDC chips. Additionally, FPGA solutions are extremely flexible since they can support vastly different signals with the simple loading of a different IP core while using the same hardware platform.

FPGA solutions are not a perfect match for all requirements. They show the greatest advantages in systems with high channel densities and, typically, narrower bandwidths. In systems with just one or two channels and bandwidths in the range of 100 MHz or greater, the higher cost of the FPGAs needed can quickly exceed the cost of designing the system with a single multichannel DDC ASIC. Again, while cost, size and power are important factors in designing a receiver system, ultimately the technical requirements may require the choice of an ASIC or FPGA solution.

Credits

This article is based on the paper Digital-Down Converter Implementation, FPGAs Offer New Possibilities by Richard Kuenzler rich.kuenzler@mrcy.com and Robert Scandurra bob.sgandurra@mrcy.com both of Pentek, Inc. The paper appeared in the September 2008 issue of Military Embedded Systems.