Dynamic Signal Analyser - Line Width?

[tec5c]

Hi guys,

I have been using a dynamic signal analyser for a low frequency noise measurement project and have run into something that I have been unable to find any info. about online.

I am interested in doing measurements as low in frequency as the DSA will allow (model SR780, stanford research systems). The manual states that the FFT span is 195.3mHz to 102.4kHz, so I naturally assumed I could take noise measurements as low as 195mHz. However, upon testing I have found that it seems that the setting of the 'Line Width' is what determines the lower frequency, i.e., I have a start of 0Hz, end of 1.6kHz, center of 800Hz and line width = 4Hz. The data output then displays down to 4Hz (I chop some of this off and start at 10Hz as there is "unreal data" between 4 and 10Hz due to the windowing [Hanning] of the instrument)

My question comes to, what exactly is line width? Now, its name makes it a bit obvious but in regards to going below the FFT span, is it safe to say that I can do measurements down to 488.3uHz (corresponding to start=0Hz, end=195.3mHz, center=97.65mHz... line width=488.3uHz)? Yes, this is going to take several days to complete!

Thanking you in advance 

Edit

My assumption is that for whatever span you have, say full span from 0 to 102.4kHz with a line width of 256Hz. The 256 is the number of points to be sampled, spaced evenly, across the span. AFAIK lines (or bins) are common to the "FFT language".  or  ?

[KE5FX]

"Line width" isn't a standard term in FFT analysis, so you'd ultimately want to ask SRS what they mean by that.  It's a property of the signal being analyzed, not the analysis process itself.

For a real-only FFT, the Nyquist frequency (fs/2) appears at the halfway point in the set of output bins.  The upper half contains no unique information, just a mirrored copy of the lower half, so it's discarded.  So if you start with a 256-point time record at fs=102.4 kHz, your FFT bins are 400 Hz wide.  Bin zero contains the DC component, and the Nyquist frequency at 51.2 kHz appears at bin 128.  Your signal should be at zero amplitude by the time that point is reached, so the useful information would be limited to bins 0-127.   "Line width" never comes into this picture, but bandwidth does.  The contents of a given bin represent not only energy at that bin's center frequency but leakage from the center frequencies of adjacent bins as well... along with all other possible frequencies in the vicinity. 

Your window function will determine the usable frequency resolution that you can achieve at a given record width.  As you've noticed the window function also has the effect of generating questionable data at the very beginning of the spectrum near DC.  Between the effects of the window function and the antialiasing filter, you often need to discard the first and last few bins in real-world FFT applications.

All of this stuff, of course, makes it harder to answer simple questions like, "Can I make noise measurements at 195 mHz?"    You will discover (if you haven't already) that the effective bin width due to the window function may be different for noiselike signals versus CW tones.  At the end of the day, the safest thing to do is acquire a long time record, move the data to your PC, and crunch it yourself in MATLAB or Octave.  Your only alternative is to become very familiar with exactly what your SR780 is doing.

[InterestedTom]

Surely for Noise analysis you should be using rectangular/uniform window?

If you programme the instrument over GPIB (if you can afford to) then you may be able to do several sweeps and string them together like you suggest and spend less time in the lab doing measurements. May still take some time though, depending on what programming language/environment you use.