LeCroy DDA-125/LC684DXL sampling and BW

[rhb]

The specifications claim a BW of 1.5 GHz, except that in 4 channel mode it is only sampling at 2 GSa/S.  In 2 channel mode it will sample at 4 GSa/S.

Interestingly, the step response for a <40 pS rise time square wave is the same at either sample rate at around 250 pS rise time.  So in 4 channel mode with Nyquist  at 1 GHz and the anti-alias filter corner at 1.5 GHz there's significant aliasing. 

Is anyone familiar with these?  What's the idea?  Was it expected to only be used in 4 channel mode with the 200 MHz BW?  Overall it seems like a pretty nice scope for the price.

[DaJMasta]

That is accurate, when all four channels are enabled, the maximum frequency you can discern from the inputs is 1GHz - I don't remember if they have a hardware bandwidth limiter or they just allowed the aliasing, but if I remember right, they maxed the FFT window out at 1GHz with all four channels enabled.  I didn't do much analysis of it when I had it, but since it also has a mode where it can be used as a sampling scope, they may just leave the full channel bandwidth on and you just have to remember to be careful.

Still, you can use two channels at 4GS/s, and with the PP06 adapter, or with the upgraded internal routing board between the frontends and the ADCs, you can get the full 8GS/s on a single channel.  I've attached an FFT capture of the full 8GS/s mode on one channel on the scope I had, at 500MHz per division, the rolloff starts early but there's some bandwidth yet beyond the specified 1.5GHz.


It's a pretty easy to use general scope, though I think more modern UIs are faster to setup for basic measurement, it's got the usual LeCroy suite of analysis features which layer in a logical way (but require a lot of clicks to setup), and then there's several saved functions that you can switch between, so at least it's easy to jump between a couple visualization modes.  One of the weird things for me was that the default display was linear interpolation between points, sin(x)/x is available but only as a math function, so zoomed in far it looks much more jagged and variable than you'd expect even on a sine wave, even though with the interpolation it looks fine.  Worth mentioning that at the very highest zoom levels, the internal timebase is a little jittery so the waveform moves around a bit.  If you lock to a good external clock the scope's time system is plenty stable to show a stable waveform, but on its own there was some variability in triggering for the display.  This is really only an issue when you're in 4+GS/s mode with only a few dozen points on the screen, and again, only if you're not on an external reference clock.


It's physically large, but not too heavy (has the space for the DDA-120/LC584 CRT but instead uses an LCD), and is a little noisy with the default fan, but that's a single 120mm that can be replaced without too much poking around.  Boot time is pretty good and I don't remember any particular issues in use, so if you like the older style UI, it can certainly be a general use sort of scope.

[rhb]

Thanks.

I dropped a screw into the main board compartment.  Foolishly I shook it to try to get the screw out.

BIG mistake!!!

I knocked a trim pot off the first channel.  Which I only discovered when I'd partially reassembled it.  On the 2nd disassembly I broke off an external clock line socket.  So I got to do my first SMD repairs.

I think they must just be allowing the aliasing to take place.  I'd expected to see a change when I went from 4  to 2 channels.

While working on it, I got to understand it better. The anti-alias filters are on paired boards, 2 channels per board.  Physically it's a large expanse of PCB area.  I'm rather interested in getting the 8 GSa/S gadget.  That would justify building new anti-alias filters that were 750, 1500 & 3000 MHz LP   with nice edges and  a pass thru option for RIS.