[time-nuts] Chinese Scopes

John Lofgren jlofgren at lsr.com
Tue Apr 17 14:55:30 UTC 2012


One feature of the Agilent and Rohde scopes (maybe Tek, too?) that can help in some situations is segmented memory.  It allows you to capture periodic or random events with the full sample rate but to ignore all the dead time between events.  For each trigger it stores one sweep with a time stamp.  When you want to look at the record you can roll back through memory and look at each individual event with full resolution.

This isn't a cure-all because the time stamps will have limited resolution and some amount of jitter, but it can be helpful in some applications.  It also assumes that you know what you're looking for and can trigger on it :)

-John


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Jim Lux
Sent: Tuesday, April 17, 2012 9:27 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Chinese Scopes

On 4/17/12 6:56 AM, shalimr9 at gmail.com wrote:
> I agree that memory depth is an under appreciated parameter, but even 2,500 points like what's available on the cheap Tek scopes is quite useful.
>
> On the other hand, I had a few LeCroy with 50k deep memories and there are cases where that is very useful too. I can't imagine real life use cases when I would need multiple MB. It would be nice to have but seldom used.
>

oddly enough, I had a case where very deep memory was useful last fall. 
  It was an issue with logic that was switching from one clock source to 
another where the clocks were orders of magnitude different frequency 
(10Hz and 300kHz or something like that), and it was the relative timing 
of the edges that was important, so you needed a bunch of cycles of the 
low frequency clock (i.e. record length of half a second or so), but 
enough samples to see the timing of the 300kHz at the same time.

Another deep memory use was when I used a fast 20GHz sample rate Tek 
scope a few years back (2007) debugging a radar target simulator (for 
the landing radar that's going to be used to land on Mars in August) and 
deep buffers were nice there, because we essentially needed to capture 
multiple pulses that were 4 ns to several microseconds long.   The 
requirement was that the delta phase (and time) of successive pulses be 
within a certain value (the radar used what's known as "two pulse 
doppler") following a pre-programmed simulated descent profile.  We also 
wanted the pulse timing after the trigger to be accurate to, as I 
recall, 0.5 or 1 ns.

The PRF is pretty high, so you don't have time to unload the memory in 
between pulses.

So we did something like 500 pulses, captured 16,384 samples at a time 
at 20GS/sec to make a dataset of 16 million samples.

You learn a lot about what's hidden in the specs on inexpensive signal 
generators like the Agilent E4421B when you start comparing phase for 
1600 pulses 1 microsecond apart.

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