Hello, I'm Joel Woodward, an engineer at Agilent's oscilloscope division. Do you have applications
that require more than the 8 bits of resolution that traditional oscilloscopes provide? On
the market today you have two different oscilloscopes with A to D converters of greater than 8 bits.
One of them is the LeCroy HDO which is an off-the-shelf A to D converter that runs at
2.5 gigasamples and has bandwidths of up to gigahertz. The other is Agilent's S-Series oscilloscope
with a A to D converter that runs internally at 40 gigasamples and can
be deployed at 2 channels times 20 gigasamples or 4 channels times 10 gigasamples. And the
scope itself has bandwidths from 500 megahertz up to 8 gigahertz.
What I'd like to do is show you some differences between the two in terms of measurements.
The NMA of resolution is noise. If the noise floor isn't sufficiently low, the additional bits of resolution are
really used just for digitizing noise. Let's look at both scopes at one gigahertz bandwidth.
The noise floors on both scopes are almost identical at all vertical settings enabling
the 10 bits to have the same value as 12 bits. The next graph shows effective number of bits
commonly referred to as ENOB. ENOB combines a variety of different vertical as well as
horizontal attributes to give you a feel for the effective number of bits that the oscilloscope provides.
As you can see from this graph, the Agilent S-series has a higher ENOB across
the entire bandwidth versus the 12 bit LeCroy HDO. What I'd like to do is show you a quick
example of a sine wave from a low noise PSG that's input into both the HDO from LeCroy
as well as the S-Series from Agilent. I have a 100 megahertz sine wave. And when I put
it into both scopes, they actually show a nice representation of the signal. Let's go
ahead and increase the frequency of the sine wave. In fact, I'm going to take it to 500 megahertz.
Both scopes are still showing a nice representation of the sine wave.
How fast of a sine wave should you be able to put in an oscilloscope that has one gigahertz bandwidth?
You would expect one gigahertz, right, and that the anti-aliasing filters
and other attributes of the scope would allow you to see that accurately. I'm going to go ahead
and increase the frequency from the PSG to 700 megahertz. And let's take a look at what
happens on both scopes. While the Agilent S-Series still presents a nice, crisp representation
of the 700 megahertz sine wave, the LeCroy HDO now is showing some distortion. And in
fact, if we increase a little bit more to 900 megahertz, you can see, even more
distortion. Why the difference? For the Agilent S-Series, we've designed an oscilloscope to
support up to the full bandwidth with anti-aliasing filters and other key signal integrity attributes.
From a LeCroy HDO perspective, this is something that you can try on your own and engage with
your LeCroy representative to ask them why it's like this.
We can gain some additional insight from this measurement by looking at both scopes in the frequency domain. The PSG
is outputting a single tone. And if we do an FFT on Agilent, we can see that single
tone as expected all the way up to a gigahertz sine waves. While on the LeCroy HDO, we see
two distinct tones at the higher frequencies while at lower frequencies we only see one tone.
This would indicate a potential anti-aliasing problem perhaps associated with the lower
ADC sample rate which is close to the Nyquist criteria.
For more information on Agilent's S-Series oscilloscopes with the industry's very best signal integrity, visit our website.
And thanks for watching.