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Keysight S-Series oscilloscopes vs the RTO2000 – Part 1

Blog Post created by melissakeysight Employee on Nov 16, 2016

In today’s world, it is hard to buy a really bad oscilloscope. But how do you choose the very best oscilloscope, especially when messaging and datasheet specs look so similar between different vendors? I’m here to give you an insider view on the differences between the Keysight S-Series oscilloscopes and the Rohde & Schwarz RTO2000 oscilloscopes. 

 

The Keysight S-Series are digital oscilloscopes with models from 500 MHz to 8 GHz bandwidth. The Rohde & Schwarz RTO2000s are digital oscilloscopes with models from 600 MHz to 4 GHz bandwidth. Both oscilloscopes claim to provide incredible signal integrity.  

 

S-Series oscilloscope materials position this scope as “the new standard for superior measurements” and offering “the industry’s best signal integrity” referencing its incredible 10-bit ADC.  Meanwhile, the RTO2000 materials talk about “excellent signal fidelity” with “up to 16-bit resolution.”  So the question becomes, which of these oscilloscopes truly offers the best signal integrity?

 

First, let’s look at what the Keysight S-Series oscilloscopes offer.

 

The S-Series oscilloscopes contain an incredible amount of innovation, including hardware, software, and the GUI. For hardware innovation, Keysight designed their very own 10-bit ADC to ensure the oscilloscope could offer 10 bits at up to 8 GHz of bandwidth. This custom-designed ADC gives the S-Series oscilloscopes a system ENOB (effective number of bits) of up to 8.1 bits. Additionally, Keysight designed correction filters to run constantly within the FPGA so that magnitude and phase are continuously and properly corrected. This ensures peak-to-peak voltages and rise-times of the measured signal are accurate and consistently displayed.  Plus, the front end was designed for ultra-low noise so that all the benefits of these internal custom components are realized in the measurements.  These oscilloscopes are designed from start to finish to measure and display the signal on your test board, not any unnecessary noise introduced by the measurement system. 

 

This oscilloscope was a revolutionary introduction to the oscilloscope world bringing to market cutting edge, exclusive technology to make cleaner measurements than ever before.

 

Now, I can’t say what went into designing the Rohde & Schwarz RTO2000, but what I can do is make measurements and observe the performance of the final product. So let’s look at how the S-Series and RTO2000 performance compare.

 

For starters, the RTO2000 only has an 8-bit ADCThe S-Series’ 10-bit ADC offers 4X more resolution.  While R&S provides an ENOB specification for their ADC (>7 bits), they don’t publish their system ENOB.  So we should ask, “Why don’t they publish the system ENOB? Isn’t that what really matters when it comes to my measurements?”

 

Your measurements are only as good as the weakest link of your measurement system. So does it matter what the ADC ENOB is if the rest of the oscilloscope design reduces the effective number of bits of the system, as it inevitably will?  In the S-Series oscilloscopes the ADC has 8.7 ENOB, but the important specification is that the S-Series system ENOB is up to 8.1.  

 

To be completely transparent, here is the system ENOB for each model of the S-Series:

Keysight S-Series oscilloscopes ENOB plots

 

The ENOB on the 4 GHz RTO2000 sits right below 6 bits for the entire frequency sweep from 0 – 4 GHz.  The S-Series stays right around 7 ENOB for the frequency sweep from 0 – 4 GHz.   The S-Series has at least one more effective bit across the entire bandwidth of the oscilloscopes. This means that the S-Series has ½ the noise & distortion of the RTO2000.

 

“But what about the 16 bits that were advertised on the RTO2000?” you may ask. This is available with the RTO2000 16-bit high definition mode, option RTO-K17.  First, I suggest you get a quote for how much this option will cost in addition to the cost of the oscilloscope.  It may be upwards of $3,000 USD since the same option on their lower performance RTE digital oscilloscopes (200 MHz to 2 GHz), is priced at $3,175 USD. 

 

And here is what the RTO high definition option provides compared to the free S-Series high resolution mode:

 

Bandwidth

RTO2000 High Def Mode

S-Series High Res Mode

10 kHz – 50 MHz

16 bit

13 bit

100 MHz

14 bit

13 bit

200 MHz

13 bit

13 bit

300 MHz

12 bit

13 bit

500 MHz

12 bit

12 bit

1 GHz

10 bit

11 bit

 

 

You can see that this expensive upgrade only provides more bits than the S-Series from 10 kHz – 100 MHz. And in the plot above, you can see that it only gives you lower noise on the RTO than the S-Series in the same limited bandwidth. In all comparable high resolution modes of operation, the S-Series has lower noise. So, is it worth it to pay extra for the RTO high definition option?

 

Now let’s look at how the noise of the oscilloscopes compare, without the high resolution options turned on.  Below is a graph that shows the noise of both oscilloscopes as the vertical scale (volts on screen) is changed.

 

 

Keysight S-series clearly has much lower noise at all settings than the R&S RTO2000, meaning the signal you measure with the S-Series is much more similar to the signal on your DUT than what the RTO2000 would display.

 

Looking at the test results, my conclusion is the following: the Keysight S-Series has higher ENOB and are much lower noise oscilloscopes than the RTO2000 scopes.  The RTO2000’s signal integrity message does not hold up under testing. If you’re looking for a low noise scope with superior signal integrity, the S-Series oscilloscope is the best scope for you.

 

Coming soon will be Part 2 of the S-Series vs RTO2000 which will look at more features of the oscilloscopes, including other key specs, the GUI, probes, and applications. 

 

 

Specifications pulled from “R&S RTO Digital Oscilloscope Specifications” data sheet version 04.00, June 2016.

Measurements were made on an S804A with firmware version 5.70 and an RTO2044 with firmware version 3.30.1.1

R&S messaging pulled from R&S RTO2000 product page,  Nov. 3, 2016: https://www.rohde-schwarz.com/us/product/rto-productstartpage_63493-10790.html

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