Re: 7T11 horizontal memory


cmjones01
 

On Wed, Dec 5, 2018 at 08:17 AM, Albert Otten wrote:

I might have good news for you.
The real time multivibrator doesn't work when you don't have a 7S11 adjacent
to (or otherwise connected to) the 7T11. So then the memory gate is always
blocked and you have no sweep output at the front panel.
Aha! Your news is indeed good :-) I put the 7T11 in to a slot next to my 7S11 and I now have a horizontal sweep in the rea ltime ranges.

You mean U512 I suppose.
It was U512 when I killed it, but I swapped it with U572 to check that it was actually faulty. My 741 kludge seems to work but I'm not convinced it's perfect. The Tek chip seems specified quite fast (80MHz gain-bandwidth product!) and I think the rather pedestrian 741 may be running out of speed (or slew rate) in the faster real-time ranges.

In real time ranges the horizontal amplifier now has
one continuous ramp as input during the whole sweep of the 7T11. It depends on
the time/div setting which outputs of U512 are really used in the amplifier
chain. Not used outputs can be overdriven within short time without any
consequence, and stay at some very high maximum level during the rest of the
sweep . It's what I see happening.
Yes, I've observed that in the U512A/B/C/D chain too. The ramp only really makes sense properly after the last stage.

The good news is that, coupled with my 7S11, I now have something resembling a working sampling system. I have three rather tired-looking sampling heads in unknown condition (an S-1 and two S-2s, one labelled 50ps risetime and the other labelled 75ps). I have only tried the S-1 but it does actually work. Observations so far:
- equivalent time mode works a lot better than real time mode. At least, the waveform I see on the screen in equivalent-time mode looks like what I'd expect. I haven't tried a really fast edge in to the sampling head yet to see how the performance is at the really short time'div settings.
- in real time mode, it's hard to get a trace that makes sense. presumably the 50kHz-ish clock from the real time multivibrator limits the display to about one sample every 20us, so some of the faster sweep speeds don't show a lot. Even on the slower sweeps, the edges of my test squarewave exhibit terrible risetime (about half a division) which is puzzling, and very different to the behaviour in equivalent time. Maybe it's something to do with the recovery time of the sampling head, or there's something amiss with the 7S11. I tried adjusting the 'dot response' control and making sure smoothing was switched off, but it didn't improve much.

Thank you for your help so far. You can probably tell that I'm new to the 7000-series sampling world. I regularly use my CSA803A/SD24 sampling setup, but that makes it all so easy...

Chris

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