The differential-signal advantage for communications system design

In the past, practical issues caused the performance advantages of fully differential signal chains to be trumped by single-ended options, but recent advances in integrated RF circuit technology and high-performance differential RF building blocks now allow differential architectures to be applied to high-performance receiver designs. This article discusses the performance and merits of differential signal chains in the context of 3G and 4G wireless applications.

Receiver Signal Chain
Figure 1, which shows a traditional superheterodyne receiver, will help illustrate the benefits of differential signal chains over single-ended signal chains. Regardless of the topology, the goal is to successfully deliver a desired signal to an analog-to-digital converter (ADC) for digitization. The signal path consists of several RF blocks: the antenna, filters, low noise amplifier (LNA), mixer, ADC driver amplifier, and ADC.

Figure 1: Receivers are evolving and are increasingly using differential components, a trend that began at the ADC and is gradually moving up the signal chain. Advances in integrated RF circuit technology and expansion of differential RF building blocks allow differential architectures to be applied for high-performance receiver designs.
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The LNA, which is the first block after the antenna, amplifies the signal above the thermal noise. Noise at this stage is critical as it will determine the sensitivity of the system, and amplification ensures that subsequent mixers and amplifiers do not add significant noise. Along the way, band-pass filters suppress out-of-band signals and reduce distortion and noise that was added by other stages.

Following the LNA, the mixer frequency translates the signal of interest, down-converting the high-frequency RF signal to a lower, more manageable, intermediate frequency (IF). The ADC driver amplifier and the anti-aliasing filter (AAF) prepare the signal to be digitized. The driver offers gain and the AAF suppresses signals outside the first Nyquist zone, including noise and out-of-band spurious components that would be delivered to the ADC. At the end of the analog signal path, the ADC digitizes the baseband information.

Ideally, only the signal of interest (shown in blue on the left side of Figure 1) would be delivered to the digital domain. A robust system is needed to process the desired signal, which may be small, while blocking the interfering signals, which may be larger. High sensitivity, input linearity, selectivity, and noise immunity are needed to design a robust system. Depending on the application and the architecture, the performance specifications will vary, but common considerations such as distortion, noise floor, and dynamic range are prevalent in most communication systems. Input third-order intercept (IP3) and 1-dB compression point (P1dB) must be high. Other considerations include low cost, low power, and small size.

The Differential Advantage
Figure 2 contrasts the basics between single-ended signals and differential signals. A generic gain block is used, but the same concepts apply to mixers and other devices in the signal chain. As single-ended and differential signals are compared, it is important to keep the system level performance metrics in mind for good overall receiver design.

Figure 2: The inherent cancellation benefit of differential signals provides noise and interference immunity while offering a cancellation effect of even-order harmonics.
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A single-ended signal, unbalanced by definition, is measured by the difference between the signal of interest and a constant reference point. The reference point, which is normally ground, serves as the return path for the signal. A problem can be encountered if an error source is introduced into the signal path. Because the ground reference will be unaffected by the injected error, the error is carried forward through the signal. Any signal variation introduced in a single-ended configuration will be difficult to remove without using overly complex cancellation techniques. Single-ended signals are thus more prone to noise and electromagnetic coupled interference.

Differential signals, on the other hand, are made up of pairs of balanced signals moving at equal but opposite amplitudes around a reference point. The difference between the positive and negative balanced signals corresponds to the composite differential signal. If an error is introduced to a differential system path, it will be added to each of the two balanced signals equally. Because the return path is not a constant reference point, the error will be canceled in the differential signal. Consequently, differential signal chains are less susceptible to noise and interference. This inherent error cancellation also provides better common mode rejection ration (CMRR) and power supply rejection ratio (PSSR).

Differential signal chains have another advantage over single-ended chains, as the composite signal swing can be twice that of the single-ended swing on the same power supply, increasing signal-to-noise ratio. Alternatively, the amplifier headroom can be increased on the same power supply, lowering distortion; or a lower power supply voltage can be used to provide the same signal swing, lowering power dissipation.

Figure 2 also highlights the inherent cancellation of even-order harmonics in a differential system. Non-linear devices, in this case a single-ended and differential amplifier, can be described by power-series expansion transfer functions given sinusoidal inputs. In the single-ended approach, a constant is tied to each frequency multiple of the output, both even and odd orders. In the differential block, even-order nonlinearities are canceled in the composite output response. Real devices will not achieve perfect cancellation, but they do benefit from lower even-order harmonics.

Figure 3: Though the distortion performance in single-ended mode is very low, there is a clear advantage in the even-order performance with differential operation. Approximately 6dB of improvement on output 1dB compression point and IP3 can also be expected for a differential topology on the same supply rails.
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Figure 3 shows the harmonic distortion of an ultra-low-distortion, low-noise differential amplifier optimized for driving high speed 8- to 16-bit ADCs. The plot shows the second- and third-order harmonics when the device is configured for single-ended and differential topologies. Although the distortion in single-ended mode is very low, with HD2 values of "82 dBc at 100 MHz, the even-order performance with differential operation is even better, with HD2 values less than "100 dBc at the same frequency. Across the signal chain, about 6 dB of improvement can be expected in P1dB and IP3 for a differential topology on the same supply rails.

Differential Signal Chain
As receivers evolve, differential components are increasingly being used to provide higher performance levels. The evolution began at the ADC and is gradually moving back through the signal chain.

In the past, signal application issues and limited differential RF building blocks lead to single-ended or partially differential signal chains. One example of a partially differential signal chain omits the differential ADC driver, instead using single-ended devices and a transformer to drive the ADC. While this offers a simple solution, the performance demands are simply pushed upstream. In addition to consuming more power, single-ended driver amplifiers tend to have worse even-order distortion, CMR and PSR.

As shown in Figure 1, the architecture commonly used for receivers has a single-ended RF input and a differential output. The dividing line between single-ended and differential operation seems to have settled at the mixer, with RF components such as the LNA still being offered as single-ended components. Most SAW filters and mixer cores are inherently differential circuits, but are converted to single-ended for application purposes.

For years, doubly balanced mixer topologies have been adopted for cellular applications due to their high linearity. Unfortunately, the traditional transformer networks used to couple the signals to the mixing core consume considerable board space and add significant cost to the design. Newer RF components, such as the ADL535x mixer family, integrate baluns and transformers, providing easy-to-use RF blocks with single-ended RF inputs and differential IF outputs.

Figure 4: Recent advances in integrated RF circuit technology have allowed for easy to use mixers with single-ended RF input to differential IF output. The differential advantage can be exploited at all three internal mixer ports while interfacing to the outside world with relative ease.
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Figure 4 shows that all three mixer ports are fully differential internally. For ease of use, the RF and LO ports are connected to the outside world using transformers, allowing a single-ended interface. By contrast, the IF output port, which includes a driver amplifier having 200-Ω output impedance, is left differential to facilitate connection to a differential SAW filter. The integration of the LO and RF baluns limits the operating frequency of the mixers, thus requiring a family of devices specified for operation across the cellular frequency ranges.

About the author

Carlos Calvo is an RF applications engineer for the RF Group at Analog Devices, Inc. He holds B.S. and M.S. degrees in electrical engineering from Worcester Polytechnic Institute, Worcester, MA and an MBA from the University of Massachusetts, Boston, MA. He has worked as an RF Applications Engineer in the Linear and RF Organization at Analog Devices since joining the company in 2002. He can be reached at carlos.calvo@analog.com.