DSA815 as Oscilloscope? - Page 1
Forum section: electronics / testgear

author: josephny
author_rank: Contributor
timestamp: October 19, 2015, 04:40:25 pm
content: I just got a DSA815 and I remember seeing somewhere that it behaves like an oscilloscope when in zero span mode.I tried, but am having difficulty.I'm sure it's me, as I don't really know much about anything. 40 years of doing everything other than tinkering with electronics and I'm just now trying to learn.I injected a 1khz tone (Signal Generator app on iphone) and this is what I see. How can I make it show 1 or 2 full cycles of a stable (still) sine wave?

author: MadTux
author_rank: Frequent Contributor
timestamp: October 19, 2015, 06:34:50 pm
content: Maybe try some DC signal and watch the magic smoke disappear out of the first mixer?(Yes there is a reason why people have both spectrum analyzer and oscilloscope on the desk)OK, lets be nice here. A: bad idea to use Iphone signal generator because it may easily exceed the allowed input level of SA.Zero span => Demodulates the amplitude at given frequency, so it will show the AM-modulation (FM as down-converted frequency on IF output) of the signal. A scope will show you both the carrier frequency and the modulated amplitude, while the bandwith of SA is limited by video and RBW. So you won't see any demodulated signal above the video or RBW (a few Hz to maybe 1MHz). Thereby your SA is a pretty bad scope.Second reason why it is bad idea to use it as a scope is that you can easily kill it with DC or low frequency AC (a few Hz) signals, because the mixer diode(s)/inductors in first converter don't like that, since these are essentially a short for DC signals (unless your SA has gilbert cell mixer or is AC coupled, which Rigol SA both isn't)

author: josephny
author_rank: Contributor
timestamp: October 19, 2015, 06:42:41 pm
content: Confirming your message: don't try to use the SA as an oscilloscope.Right?

author: josephny
author_rank: Contributor
timestamp: October 19, 2015, 06:53:31 pm
content: Great, thank you for the detailed explanation.BTW, the signal generator app output is adjustable way down to what is says is -95dB but I think it's really -95dBmNow I need to figure out if I need a 50, 100, 200 or 40mhz scope.

author: MarkL
author_rank: Unknown
timestamp: October 19, 2015, 07:10:48 pm
content: An SA displays the frequency domain of a signal (in general). In zero span, it is displaying the amplitude of a specific frequency in the time domain.What you're seeing is not the input sine wave. Look at the dB scale, for one thing.The DSA800 user manual says it's ok for up to 50V DC input, but it's never good practice to put DC into any spectrum analyzer. Damage to the front end can be instant and expensive.Here's a couple of docs you might find useful, and I'm sure you can find similar ones from Rigol as well.Spectrum Analyzer Basics:http://literature.cdn.keysight.com/litweb/pdf/5952-0292.pdfSpectrum Analyzer Basics Seminar Slides and Notes:http://literature.cdn.keysight.com/litweb/pdf/5965-7920E.pdf

author: josephny
author_rank: Contributor
timestamp: October 19, 2015, 07:14:45 pm
content: Thank you for the links. I will read ASAPI think I understand about zero span showing a specific frequency over time. Is this different that seeing voltage over time?Could you help me understand how you see that it is not the sine wave I'm seeing and why I don't see it?

author: MadTux
author_rank: Frequent Contributor
timestamp: October 19, 2015, 07:59:47 pm
content: Yes, you see the sinewave from your Iphone. But you see it in log mode, as Mark noted. To see real sinewave, you have to change display to linear mode. And you only see it as a sinewave because it it low frequency, so that it passes through your RBW filter and video filter. Change any of both to say 100Hz and you won't see it anymore. But better stop until you have a good signal generator so that it doesn't get expensive.Secondly you will never ever be able to see broad band signals as with an oscilloscope. Say you set your signal generator to 1GHz. You will see a peak on your spectum analyzer. Now if you set center freq to somewhere inside that peak and set it to 0 span you will see a line, because it is not a modulated signal. With a good oscilloscope, you will see a 1GHz sinewave with 1ns period. Now if you modulate that signal with 1kHz AM sinewave, you'll see a 1kHz sinewave on SA, with 1kHz square wave AM, a square wave. On good oscilloscope you will see 1GHZ sinewave which has gets stronger and weaker @ 1kHz (called signal envelope). That envelope is a square or sinewave. See pics in attachment, how these signals look on real scope (used them when I sold that POS HP-8116 on ebay). Acutally look only on first one, since those signals are all sine modulated square, triangle and sine waves, basically the other way round

author: josephny
author_rank: Contributor
timestamp: October 22, 2015, 08:41:15 pm
content: I think I got it.Or, at least, I got it enough to go out and order a 1054 scope.So now I can play in the dark with yet another piece of test equipment.Now I need something to do with my exciting new set of toys.Thank you!