Simplify communication system design while increasing available bandwidth

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1 Simplify communication system design while increasing available bandwidth Clarence Mayott - July 13, 2015 Introduction In modern communications systems, the more bandwidth that is available, the more information that can be transmitted. As the requirements for bandwidth increase, the need for faster and higher linearity ADCs (analog-to-digital converters) and amplifiers also increases. With increased bandwidth, more noise is introduced into the system which can overpower low level signals of interest. This places added requirements on the ADC and amplifier to have low noise. Also with increased bandwidth, the linearity of the system becomes more critical, especially in the presence of a strong interferer which can block other signals of interest. One approach to resolve these issues is to use a high speed, high resolution ADC with an equally fast amplifier driving it. This will allow better sensitivity and selectivity of the receiver, and will ultimately improve the quality of the system. Basic Introduction to Communications System Design There are several fundamental design trade-offs in any communications system. Bandwidth, spurious free dynamic range (SFDR) and sensitivity are all important factors in a communications system, but are difficult to achieve with a single solution. The usable bandwidth of a system is heavily dependent on the sample rate of the ADC. The bandwidth of the system cannot be greater than half the sample rate of the ADC. For more bandwidth, faster sampling is required, which limits the range of practical ADC options. High speed ADCs are typically lacking in terms of SFDR and signal to noise ratio (SNR), limiting the performance of the receiver. However, the LTC Msps 16-bit ADC sets a new level of performance with excellent linearity. With a sample rate of 210Msps, the usable data bandwidth is nearly 105MHz, and with an SNR of 79dB, very low level signals can be detected. The LTC2107, coupled with a high linearity amplifier like the LTC6409, improves the throughput of modern communications systems while simplifying the front-end design.

2 Trade-offs and Challenges A major challenge in high speed communications design is maintaining good SNR while maintaining wide bandwidth. Designers face trade-offs among the various approaches. One approach is to use slower sample rates and higher order filters to attenuate the out-of-band noise before the analog input signal is sampled. This requires a complex filter network with several stages of attenuation, requiring a significant number of components. High order filters also tend to ring in response to the sampling glitches of the high speed ADC. This problem is solved by using an ADC with a faster sample rate. This simplifies the analog filter design. Using a low order filter allows the sampling glitches to settle properly and improves the linearity of the system. Using a high speed ADC, such as the LTC2107, solves the design problem in a simplified and elegant way. With a sample rate of 210Msps, there is more than enough bandwidth for any demanding communications system. With a wider bandwidth, the anti-aliasing filter used to reject out-of-band signals becomes easier to design. With more bandwidth as a guardband, the filter used can be lower order, making it easier to design and reducing component count. A wider bandwidth allows more noise to be sampled by the ADC, increasing the need for an ADC with a high SNR. The SNR of the LTC2107 is 79dB. The device s improved SNR and SFDR allow the receiver to accept lower level signals that could be buried in the noise floor of noisier parts. This adds range to the receiver and allows signal transmission over longer distances. The high SNR and SFDR of the LTC2107 improve system performance without sacrificing bandwidth. Best Design Practices: Symmetry, Absorptive Network Best Design Practices: Symmetry, Absorptive Network High performance ADCs require a high performance environment in which to operate. With any direct sampling ADC, nonlinear charge is produced in the sampling process. This charge is reflected into the input network each time the sampling switches close. A highly absorptive network is required at the analog input to ensure that none of this charge is reflected back into the input network where it can potentially be re-sampled. The input network of the ADC should be as close to 50 ohms as possible to allow for maximum absorption of this nonlinear charge. If this charge is reflected in a less than ideal reflective network, it can be re-sampled by the ADC and result in simple distortion or intermodulation distortion. Another potential source of distortion is an asymmetrical layout of the input network. Nearly all high speed ADCs are differential by design. With an ideal layout, this allows excellent common mode rejection and very good 2 nd order harmonic distortion. Any deviation from perfect symmetry will cause a mismatch in the differential signals which will manifest itself as 2 nd order harmonic distortion. Even a simple design decision to flood copper closer on one side of the differential pair than the other side can cause a difference in ground current in the adjacent ground planes. This adds distortion to the system. Absolute symmetry is required for maximum performance. Figure 1 shows an example of good layout of a high speed ADC. The design files for the LTC2107 are available at This symmetry should extend several centimeters beyond the ADC or

3 to the nearest amplifier or balun transformer. Figure 1: Proper layout for the LTC2107 Driving the ADC: How this Helps with Trade-offs and Challenges To minimize the effects of the direct sampling process on the source of the analog signal, an amplifier can be used to absorb the charge from the sampling process. Since many feedback amplifiers have low output impedance, the nonlinear charge is attenuated before returning to the ADC. If the amplifier is located as close as possible to the ADC, these reflections can be reflected a number of times before the sampling period is over, thus reducing the impact of the glitches on the SFDR of the ADC. To take advantage of the available bandwidth, an amplifier with a large gain bandwidth product is required. With 10GHz gain bandwidth, the LTC6409 is an ideal choice to drive a high speed ADC. It has distortion products better than 90dB out to 100MHz and an input noise density of 1.1nV/rtHz. Combined with the LTC2107, the LTC6409 provides the system with low noise and distortion while maintaining a wide signal bandwidth. The interface between the LTC6409 and the LTC2107 can be minimal, further simplifying the design of the system. Being a feedback amplifier, the gain of the LTC6409 can be changed by simply changing the value of the feedback. With the low noise density of the amplifier, even high levels of gain can be achieved while still maintaining good SNR at the ADC. The increased gain and high linearity of the amplifier ensure detection of low level signals, which would be obscured by the noise with other solutions.

4 Since the LTC6409 has such low noise and high linearity, it can be used to drive an ADC with very little filtering between the ADC and amplifier, as shown in Figure 2. The bulk of the filtering should be placed before the LTC6409 to limit the frequency content at its input and to allow for more traditional filter topologies that would not work well between the amplifier and ADC. The LTC6409 also does a single-ended to differential translation of the input signal. This allows for filters at the input to be designed in a single-ended form, reducing component count by half compared to differential filters. Results Results Figure 2: Schematic of the LTC6409 and LTC2107 The combination of the LTC6409 and the LTC2107 produces excellent results at practical frequencies for communications systems. While simple harmonic distortion is a typical metric for system performance, two tone test is a better indicator of the true linearity of a system. A single tone test pushes the high order harmonics out into the region where a filter can attenuate the actual level of these harmonics. By driving the amplifier with two tones with a modest spacing between the tones, distortion products land in band and cannot be filtered out. The lower these signals, the more linear the system is and the better the SFDR of the system. In a communications system there will be rogue offenders and blockers that can appear in band at any

5 time. If the amplifier or ADC has poor linearity, then these unwanted and unanticipated tones can produce distortion products much higher than the signal of interest, causing the receiver to lose the signal of interest. The more linear the system, the less likely the interferer will cause a loss of reception. The results of the combination of the LTC6409 and the LTC2107 are shown in Figure 3. In this example, the tones are 10MHz apart and the distortion products are down by 95dBc. This setup resembles a system where a wide bandwidth signal is used and the intermodulation products land in band. The gain of the system is 21dB, which allows low level signals to be received by the ADC. Figure 3: Two tone test results. F1=50MHz, F2=60MHz, Fs=210Msps Conclusion The LTC2107 and LTC6409 combination solves many problems in modern communications systems. The high sampling rate of the LTC2107 and high gain-bandwidth product of the LTC6409 allow signal bandwidths up to 100MHz with no degradation in linearity. Ultimately, this improves the sensitivity of the receiver and the ability of the receiver to receive more data and improve throughput. The excellent performance of these Linear Technology devices ultimately simplifies the complexity of the system by reducing the need for complicated filters and signal processing. Schematics and design files for the LTC2107 and the LTC6409 are available at