Tektronix, Enabling TechnologyFeatured in the National Instruments PXIe-5185/86 Digitizers

Over the past 65 years, Tektronix has established its place as the world’s leading manufacturer of oscilloscopes. As a result, eight of every 10 engineers in the world today use a Tektronix oscilloscope. The company’s instruments include the industry’s highest sample rate at 100 GS/s in the flagship product family, the DPO70000C Series.

The Tektronix commitment to meeting customer needs, as evidenced by continuous investment in proprietary analog-to-digital converters (ADCs), amplifiers, digital-to-analog converters (DACs), and demultiplexing and multiplexing ASICs, has played a significant role in establishing the company as the leading provider of oscilloscopes. As part of this commitment, Tektronix has worked with IBM for over 15 years to develop ASICs on many of IBM’s SiGe process nodes, and currently has taped out and received first silicon of multiple 8HP ASICs to continue to extend the bandwidth and sample rate barrier.

As a collaborator in the design of the National Instruments PXIe-5185/86 digitizers, Tektronix helped

1. Design a high-bandwidth, fully featured analog front end

2. Integrate the Tektronix high-sample-rate ADC into the PXI architecture

Analog Front End DevelopmentOne of the Tektronix core technology investment areas in analog ASICs is in the design of front end ASICs such as pre-amplifiers and voltage limiting circuits. To develop an analog front end that is capable of meeting a broad range of measurement applications, from precision scientific measurements in large physics experiments to high throughput production test environments to even wideband military and aerospace applications such as radar, the design must meet a lengthy list of requirements:

a. The front end should filter out higher frequency components that alias back into the real signal. While these aliased signals could be identified in the frequency domain, they are indistinguishable from the real signals in the time domain. The highest bandwidth oscilloscopes and digitizers use filters that roll off very quickly at the specified bandwidth to prevent aliasing. A filter with a fast roll-off presents ringing in the step response (see item c below). Designing a high-performance front end requires delicate trade-offs between these considerations.

b. The frequency response, from DC to the maximum bandwidth, should be as flat as possible. A flat frequency response guards against the artificial gain or attenuation of signal components at various frequencies.

c. The step response should have minimal overshoot and ringing.

d. The noise floor should be as low as possible. This is important for frequency-domain users, who often need as much dynamic range as possible to identify small signals; it is also important for time-domain users, who cannot sacrifice vertical resolution. The theoretical lowest noise floor is a function of thermal noise, which tends to have a constant density across frequency and is dependent upon temperature. At 25 deg C, the thermal noise power spectral density is -173.9 dBm/Hz. Average noise density represents the noise power across a specified bandwidth.

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With these points in mind, Tektronix developed an analog front end ASIC and front end module circuitry to provide solid performance in all of these areas.

Comparison in Frequency Response FlatnessIt can be difficult for engineers to determine the frequency response flatness information for digitizers and oscilloscopes, but if they can find it, it is worthwhile to take a look. For illustration purposes, consider the frequency response flatness of a competitor’s digitizer in the 3U PXI form factor, referred to as Digitizer A. Table 1 shows the key specifications of Digitizer A.1

Digitizer A

Analog Bandwidth >1.4 GHz

Sample Rate 4 GS/s

Vertical Resolution 10-Bit ADC

Sampling Jitter 1200 fs RMS

RMS Noise 0.5% Full Scale

ENOB Not Specified Above 410 MHz

Form Factor 3U PXI

▶ Table 1. High-Level Specifications for Digitizer A

The manufacturer of Digitizer A does not provide frequency response flatness specifications for the product. The data shown here was taken from a representative sample of Digitizer A.2

▶ Figure 1. Digitizer A Frequency Response Flatness Profile

The graph in Figure 1 shows that Digitizer A meets its -3 dB bandwidth at 1.5 GHz. The frequencies leading up to the full bandwidth show that the frequency response stays relatively flat but is down by 1 dB at 700 MHz. At this point, some peaking has been implemented to extend the digitizer’s bandwidth. There is a 2 dB difference in the frequency response from 700 MHz to 1.25 GHz. At around 1.25 GHz, the response begins to roll off, and it reaches its -3 dB point at 1.5 GHz.

Above 1.5 GHz, there is another concern. The frequency response stays nearly flat up through 2.25 GHz, where it then peaks again before hitting -10 dB at 2.8 GHz. Signal content above 2 GHz, which is still between -3 dB and -10 dB, remains considerable, and, when digitized by the 4 GS/s ADC, it can be aliased back into the signal.

To make that comparison easier and to demonstrate the performance achieved using the Tektronix front end ASIC, Figure 2 shows the measured frequency response of the National Instruments PXIe-5186 5 GHz digitizer. The Figure 2 graph is also included in the digitizer’s specifications document.

▶ Figure 2. Frequency Response Flatness Profile of the National Instruments PXIe-5186 5 GHz Digitizer

The front end ASIC used in the National Instruments PXIe-5185/86 digitizers as well as in Tektronix oscilloscopes provides a flat frequency response that stays within 1 dB up to the 5 GHz bandwidth. National Instruments selected a front end filter that would provide good

1 Digitizer A specifications taken from the digitizer’s publicly available data sheet.

2 Digitizer A frequency response flatness chart produced from evaluation data taken in a lab at National Instruments. Measurements were taken after product warm-up, with self-calibration passed. Signal generator configured to output a sine wave generated from a Rohde & Schwarz SMA100 at 71% of full scale and 1 Vpp input range. Power levels validated using Rohde & Schwarz NRP-Z91 power meter. National Instruments used the same procedure to validate performance of the NI PXIe-5185/86 digitizers.

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anti-aliasing when sampling at 12.5 GS/s. Figure 2 shows how the front end fi lter rolls off very quickly, reaching -10 dB at 6.25 GHz, and rolls off sharply past that point.

High-Sample-Rate ADC DesignThe ADC used in the National Instruments PXIe-5185/86 digitizers is the currently shipping ADC for Tektronix oscilloscopes and was designed in the IBM 7HP (High Performance) SiGe process node. It is

a quad-interleaved, pipelined, 8-bit ADC that incorporates a proprietary Successive Approximation architecture with a single clock input and a maximum sample rate of 12.5 GS/s. Using this ASIC, a digitizer can be created with either 1 or 2 input channels, operating at sample rates of 12.5 GS/s (1 ch) or 6.25 GS/s (2 ch), with the full 5 GHz of analog bandwidth on both channels. This allows for 2nd Nyquist sampling for I/Q demodulation applications.

The performance of the Tektronix ADC can be compared against commercially available alternatives using the industry-standard fi gure of merit for high-speed ADC technology – effective number of bits (ENOB). The comparison in Figure 3 features data gathered by Bob Walden of The Aerospace Corporation in his recent survey of ADC technology.

▶ Figure 3. This ADC survey, included with permission from Bob Walden of The Aerospace Corporation, compares resolution as measured by effective number of bits (ENOB) versus signal frequency for a single tone input.

In Figure 3, note that in the gigahertz range of analog input frequencies, the Tektronix 8-bit ADC architecture has superior resolution compared to competitors’ offerings, including some ADCs that boast higher bit resolution, and is the only ADC design in volume production that exceeds the 150 GHz aperture ambiguity boundary (indicated by the green-dashed line). The underlying transistor performance and hybrid ADC architecture enables more than 6 bits of ENOB for an input single tone signal bandwidth of 6 GHz.

When incorporating the Tektronix ADC ASIC into the digitizer design, the fi rst challenge was to preserve the low noise performance of the ADC. The front end ASIC dominates noise performance at low signal frequencies and provides the digitizer’s best case ENOB. The National Instruments PXIe-5185/86 digitizers have an impressive 6.5 bits of resolution for signals below 10 MHz, which is close to the maximum ENOB of the ADC ASIC. The second challenge, which was signifi cant, was to minimize the noise performance of the digitizers for high-frequency input signals.

The noise performance of the digitizers for high-frequency input signals is dominated by sampling jitter. Sampling jitter, which can also be referred to as phase noise, characterizes the timing deviations in the analog-to-digital

conversion process and dominates at higher frequencies. These timing deviations can result from inaccuracies inside the ADC, clock jitter, and system design issues.

To view the impact of sampling jitter, refer back to the ADC survey (Figure 3) conducted by Bob Walden. As an example, consider Digitizer A, which specifi es a 1.5 GHz bandwidth and 10-bit vertical resolution. Taken alone, these appear to be outstanding banner specifi cations; however, they do not account for the noise degradation caused by jitter.

Digitizer A’s 1200 fs RMS of sampling jitter is a more accurate indicator of its true performance.

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Starting at the vertical axis, on the 10-bit line, Figure 3 shows that with over 1 ps of sampling jitter, the 10-bit specification is valid only up to 100 MHz, where the 10-bit line intersects with the 1 ps red-dashed jitter line. Now note the red-dashed line down and to the right. At the stated 1.5 GHz bandwidth, the 1200 fs sampling jitter has degraded the digitizer’s actual ENOB performance to a maximum of 6 bits. When accounting for wideband noise, the actual ENOB delivered by Digitizer A, which uses a 10-bit ADC, drops to only 5 bits at 1.5 GHz.

The high ENOB of the Tektronix ADC ASIC is maintained in the National Instruments PXIe-5185/86 design because of the incredibly low sampling jitter of the digitizers. The digitizers’ very low 500 fs RMS integrated jitter results in a remarkable 5.5 ENOB at 5 GHz.3

NI PXIe-5186 Digitizer A

Analog Bandwidth 5 GHz >1.4 GHz

Sample Rate 12.5 GS/s 4 GS/s

Vertical Resolution 8-Bit ADC 10-Bit ADC

Sampling Jitter 500 fs RMS 1200 fs RMS

RMS Noise 0.35% Full Scale 0.5% Full Scale

ENOB6 Bits at 2.5 GHz5.5 Bits at 5 GHz

Not Specified Above 410 MHz

Form Factor 3U PXI Express 3U PXI

▶ Table 2. Comparison of Noise, Jitter, and ENOB Performance Between the National Instruments PXIe-5186 Digitizer and Competitor’s Digitizer A

Trusted Signal Fidelity with Tektronix,

Enabling TechnologyThe analog front end and ADC ASICs used in the National Instruments PXIe-5185/86 are proprietary Tektronix technologies designed to offer superior signal fidelity for oscilloscope and digitizer applications. For high-speed digitizers and oscilloscopes, ENOB offers the best figure of merit for comparing noise performance between alternatives. The Tektronix analog front end and ADC ASICs have been designed to offer best-in-class noise performance, resulting in high ENOB across the full digitizer bandwidth.

3 National Instruments PXIe-5185/86 specifications taken from NI PXIe-5185/5186 Specifications Manual, http://digital.ni.com/manuals.nsf/websearch/148683C5194E43828625784F0077F2A8.

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