Rohde & Schwarz - 2017 Press Clippings Book 3 of 3

TEST SYSTEMS

Entry-level digital scopes Q High vertical resolution analogue-to-digital converters provide the signal detail for everyday devices, writes Andreas Grimm

Today, even the most commonplace electronic designs involve highly integrated components and an array of once-specialist technologies. To measure small parts of signals, in the presence of large amplitudes, oscilloscopes with high vertical resolution and wider bandwidth are needed. The incorporation of

The same ADC in the R&S RTB2000 makes it possible to achieve a higher vertical resolution compared with oscilloscopes that have an 8-bit ADC. 7R PDNH H൵HFWLYH XVH RI WKHVH smaller signal details, the oscilloscope must also reduce the front-end noise. A 10-bit ADC achieving 1mV UHVROXWLRQ ZRXOG R൵HU QR DGYDQWDJH LQ an oscilloscope with 4mV noise. The impact of noise is often reduced either by averaging the signal or employing high-resolution (hi-res) acquisition modes. Reducing noise levels One of the two most common methods for increasing resolution is the use of averaging. This is typically provided as a mathematical functionality on the oscilloscope, which relies on multiple consecutive acquisitions. The more acquisitions are averaged, the better the resolution. A further advantage of averaging is that it also reduces wideband noise, giving a more accurate representation of the measured signal. However, there are two important disadvantages of this method: Q It takes time to acquire and process the data – several hundred times to get the required resolution Q It only works with repetitive signals, such as clock signals. Most signals do not repeat periodically, so the method of averaging over several acquisitions cannot be used. To overcome this drawback oscilloscopes increasingly support special acquisition (sometimes also called decimation) modes. One example is the hi-res acquisition method which exploits oversampling: resulting from a higher ADC sampling rate than the minimum required for accurate signal reconstruction, keeping in line with the Nyquist criteria. Additional samples are averaged and the resulting waveform is displayed. Since averaging is done within a single

8-bit ADC

+*-bit ADC ) +* = +,*)L steps

) 8 = )K steps

Quantization steps (resolution) Minimum resolution for a +V signal

O.P+mV *.P,,mV

Table B: 9-bit ADC vs B6-bit ADC resolution

10-bit analogue-to-digital converters (ADCs), as well as averaging and oversampling modes in new, entry-level digital oscilloscopes can help everyone see signal details much more precisely. When selecting digital oscilloscopes for electronic research, education and industry, inevitably users must square the technical requirements – bandwidth, sample rate and memory depth, for example – with the realities of productivity, usability and budget. In the past they have had to compromise at the lower end of the performance spectrum, typically with a ceiling of 8-bit resolution. These days, that is often not enough. In modern devices voltage levels are progressively falling and power-supply H൶FLHQF\ UHTXLUHPHQWV FRQWLQXH WR ULVH For the test and measurement engineer,

We have the technology Fortunately, the technology is now available for making higher-resolution ADCs with the necessary sample rate and low noise. State-of-the-art PDQXIDFWXUHU VSHFL¿F $'&V FDQ be produced for the same cost as R൵ WKH VKHOI PRGHOV EULQJLQJ D four-fold improvement in resolution to the value end of the market. The game is changing, with the introduction of instruments like the new R&S RTB2000 oscilloscope. This uses a proprietary 10-bit ADC, which Rohde & Schwarz originally developed for its Scope Rider, an isolated portable, multifunctional oscilloscope that provides options such as MSO and serial triggering, and decoding of serial protocols.

this means tolerances when measuring electronic circuits are becoming even tighter. With greater accuracy comes the need for higher resolution on measurements. Moving up from 8-bit to 10-bit ADCs makes a substantial step in the right direction, as shown in Table 1 ( above ). The example calculation for a 1V signal yields a resolution of about 1mV for the 10-bit converter, compared with nearly 4mV for the 8-bit ADC. Developers who want to characterise signals of power electronics or who work with high dynamic ranges, can EHQH¿W IURP IRXU WLPHV EHWWHU UHVROXWLRQ The downside is that until recently, $'&V R൵HULQJ ELW ZHUH RQO\ available in instruments costing €5,000 ($5,500) and more.

Figure ': Comparison of the HMO'6'7 with 9-bit ADC and the RTB'6'7 with B6-bit ADC

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September )*+, | www.electronicsweekly.com

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