December 2010

Lecroy 9450 FFT Configuration Tool...

Having used a Lecroy 9450A oscilloscope for several years I have yet to find a better all round general purpose digital 'scope in it's price range. $400 to $600 on eBay.

When equipped with the WP02 waveform processing option it can also function as a very competent spectrum analyzer with frequency resolutions ranging from 20μHz to 100MHz, and spans of 0.5mHz to 5GHz, these in a series of possible combinations based upon the timebase setting and selected number of sample points to be processed.

Here's a photo of my 9450A displaying the FM radio band spectrum as available here in Saint Augustine FL:

To gather this spectrum I wanted a 50kHz frequency resolution, and a 200MHz span.

Selecting the FFT frequency resolution is somewhat easy as it is the reciprocal of the timebase setting times 10:

Rearranging this to calculate the timebase setting from the desired resolution we get:

Not too bad...

However calculating the span requires that you know the sampling rate (a factor of timebase setting and RIS¹ or SS² mode), and the maximum number of sample points captured and number of points to be used in the FFT calculations:

where:

While both span and resolution are displayed on the FFT setup screen, configuring the timebase and number of points to get the resolution and span you seek can be a lengthy trial and error process. I wrote the LecroyFFT.exe application presented here to eliminate the need to use the formulae presented above and/or the trial and error process.

It looks like this, here showing the needed time/division and points for the 50kHz resolution and 200MHz span used to capture the FM Band spectrum shown above:

You will also see that the capture characteristics for the needed timebase setting are displayed at the bottom of the dialog. Shown are the seconds/division, the RIS samples/second and record length, and the SS samples/second and record length.

To use the application just select the desired resolution from the list at the left, this will cause the needed timebase setting to be displayed (2μs above) and load the span list with the spans available for that timebase setting.

Next select the desired span (and RIS mode if desired and available--more about this below), the required number of points to be used in the FFT calculation will be displayed (8000 above).

That's it, now just set the indicated time/division on the 'scope, and set the required number of calculation points in the FFT configuration menu.

RIS mode is required for timebase settings of 1ns to 5ns, available for settings of 10ns to 5μs, and not available from 10μs to 5000s. The application will show messages indicating the availability of RIS mode.

There is only one application configuration option, so to keep things simple I just included it of the About dialog:

Checking the Confirm Exit box will cause the application to pop-up a dialog when you close it (via the standard Windows Close button). The dialog will ask if you wish to close or minimise the application, I added this because I found myself closing the app when all I wanted to do was minimise it. Some find this sort of thing annoying so I made it configurable.

You can download the application in a Windows installation package by clicking here.

-----------------------------------------------------------------
¹ - RIS = Random Interleaved Sampling capture mode
² - SS = Single Shot capture mode