开云体育

7T11 Sequential/Random Problem

David C. Partridge Member Profile All Messages By This Member #70076 Hi folks, Please could you take a look at the pictures in here: <> All working fine in sequential mode, but in random mode only the RHS of the display is correct? Does anyone have any ideas what could be causing this? Thanks Dave
Albert #70079 Hi David, The RHS is also far from good. At those 20 MHz you should see no quality difference between sequential and random. I can more or less simulate your random waveform by playing with the trigger Level (also with LHS dim and/or reduced size, RHS noisy but more normal). So I guess it's a triggering problem. Random is much more sensitive to missed or wrong time trigger events. I used 20MHz from SG503, internal triggering, range 500 ns, speed 50 ns/div. Albert toggle quoted message Show quoted text Hi folks, Please could you take a look at the pictures in here: <> All working fine in sequential mode, but in random mode only the RHS of the display is correct? Does anyone have any ideas what could be causing this? Thanks Dave
Mike All Messages By This Member #70082 David, Albert- I have vaguely similar issues with both my 7T11 and 7T11A in random mode. I see large gaps in the horizontal dot density, and even misplaced (time-reversed!!) dots. For example by moving 7S11 delay, 7T11 time offset, and trigger position I can display a 100 ps step with positive, zero or even negative risetime (all with clean repeatable triggering). Since the incoming trigger is shared with sequential mode, I think I must be noise or crosstalk from the sample clock getting into the delta time converter or the random sampling strobe. Perhaps some bad decoupling caps? This problem renders random mode all but useless for risetime measurements, but I haven't had time (or enough extenders) to dig into it. So far I've just gotten by with sequential mode which is fine. If you have a fast pulse source, I'd be interested to see if you can check how random mode works with that. With CW sines it's hard to interpret where the trigger is occurring (they all look alike). I won't have access to my rig until next week but can put up some pictures then if it helps. Mike toggle quoted message Show quoted text --- In TekScopes@..., "Albert" <aodiversen@...> wrote: Hi David, The RHS is also far from good. At those 20 MHz you should see no quality difference between sequential and random. I can more or less simulate your random waveform by playing with the trigger Level (also with LHS dim and/or reduced size, RHS noisy but more normal). So I guess it's a triggering problem. Random is much more sensitive to missed or wrong time trigger events. I used 20MHz from SG503, internal triggering, range 500 ns, speed 50 ns/div. Albert Hi folks, Please could you take a look at the pictures in here: <> All working fine in sequential mode, but in random mode only the RHS of the display is correct? Does anyone have any ideas what could be causing this? Thanks Dave
David C. Partridge Member Profile All Messages By This Member #70083 Hi Albert, Pointing me to triggering was an excellent judgement call. I fiddled a bit with the bias controls on the various TDs and managed to get a stable display at some trigger levels, but it was very "twitchy". Using the classic finger test, it looked awfully like the arming TD was oscillating. I shortened the top lead to the diode and readjusted the arming bias, and I've now got a fairly stable trace but the dot is elongated to about 0.4 vertical divisions at the zero crossing points when in manual scan mode, is this normal or do I still have an excessive amount of trigger jitter? When I replaced the arming and trigger diodes, the tinning on the square pad to which they were mounted came off with the solder and I had stick a small square of copper foil onto that to allow me to solder them in place :( Maybe that's added some undesirable stray capacitance to the circuit. I still can't get rid of the left shift of the trace on the equivalent time ranges when I switch from sequential to random (about 2.5 divisions) or as I turn the time/div control clockwise within a sweep range - clearly I have one of the calibration adjustments incorrect (but which), or there's another problem. I've added a couple of pictures to the album showing the current state of the 20MHz signal in random mode (including left shift) and of the slightly elongated dot at the zero crossing in manual scan mode. Regards, David Partridge toggle quoted message Show quoted text -----Original Message----- From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Albert Sent: 20 January 2012 13:39 To: TekScopes@... Subject: [TekScopes] Re: 7T11 Sequential/Random Problem Hi David, The RHS is also far from good. At those 20 MHz you should see no quality difference between sequential and random. I can more or less simulate your random waveform by playing with the trigger Level (also with LHS dim and/or reduced size, RHS noisy but more normal). So I guess it's a triggering problem. Random is much more sensitive to missed or wrong time trigger events. I used 20MHz from SG503, internal triggering, range 500 ns, speed 50 ns/div. Albert Hi folks, Please could you take a look at the pictures in here: < st> All working fine in sequential mode, but in random mode only the RHS of the display is correct? Does anyone have any ideas what could be causing this? Thanks Dave ------------------------------------ Yahoo! Groups Links
Albert #70090 Hi David, It's not an elongated dot. It's simply a small segment of the sine waveform. If you scroll through the scan positions, you will see that "dot" follows the shape of the sine wave. Therefore maybe jitter is a nasty term (it does not degrade the REP scan). Specific causes are repetition period jitter (hum) in your signal generator, and jitter in the delay times produced by the Rate meter. Both are "corrected" to show the dot at the proper horizontal position, i.e. the dot position is determined according to the recorded time difference between strobe and (nearby) trigger event. Albert toggle quoted message Show quoted text .... and I've now got a fairly stable trace but the dot is elongated to about 0.4 vertical divisions at the zero crossing points when in manual scan mode, is this normal or do I still have an excessive amount of trigger jitter? --- David Partridge
Albert #70092 Hi Mike, Strange things in the dot pattern happen indeed in Random mode, but I don't remember a case of time-reversal. I think much depends on the repetition rate of your signal generator. I think the 264 and S-52 gave me no problems. I don't have faster repetition fast pulses. Long ago I have seen negative rise and fall time with 3S76/3T77A. And that was sequential mode while showing the square wave output of a 284. Albert toggle quoted message Show quoted text --- In TekScopes@..., "Mike" <zuckerme@...> wrote: David, Albert- I have vaguely similar issues with both my 7T11 and 7T11A in random mode. I see large gaps in the horizontal dot density, and even misplaced (time-reversed!!) dots. For example by moving 7S11 delay, 7T11 time offset, and trigger position I can display a 100 ps step with positive, zero or even negative risetime (all with clean repeatable triggering). --- Mike
Mike All Messages By This Member #70094 I recall the behavior's independent of rep rate (at least between 1 kHz or so and 1 MHz). I initially saw it with my homebrew avalanchers, but it also was apparent with 067-0681-01 (driven at whatever rate you please), HP213B, 284, and S-52. I never tested it on my 109 because life is too short. The effect was not visible for risetimes longer than about 500 ps. BTW a 74LS14 with a resistor and mica capacitor provide a general-purpose 100 ns pretrigger delay. It seems stable to better than 20 ps rms if you happen upon a "good" inverter. I also have a 7M11 delay line, but it's sluggish. I think the cable shield is corroded. All in all I would love to fix my 7T11 and 7T11A but not clear where to start. toggle quoted message Show quoted text --- In TekScopes@..., "Albert" <aodiversen@...> wrote: Hi Mike, Strange things in the dot pattern happen indeed in Random mode, but I don't remember a case of time-reversal. I think much depends on the repetition rate of your signal generator. I think the 264 and S-52 gave me no problems. I don't have faster repetition fast pulses. Long ago I have seen negative rise and fall time with 3S76/3T77A. And that was sequential mode while showing the square wave output of a 284. Albert --- In TekScopes@..., "Mike" <zuckerme@> wrote: David, Albert- I have vaguely similar issues with both my 7T11 and 7T11A in random mode. I see large gaps in the horizontal dot density, and even misplaced (time-reversed!!) dots. For example by moving 7S11 delay, 7T11 time offset, and trigger position I can display a 100 ps step with positive, zero or even negative risetime (all with clean repeatable triggering). --- Mike
David Hess Member Profile All Messages By This Member #70095 Is this the kind of application where the differences between saturating (TTL) and non-saturating (ECL) logic would apply? I wonder if the extra jitter is from poor power supply rejection. toggle quoted message Show quoted text On Fri, 20 Jan 2012 19:57:34 -0000, "Mike" <zuckerme@...> wrote: BTW a 74LS14 with a resistor and mica capacitor provide a general-purpose 100 ns pretrigger delay. It seems stable to better than 20 ps rms if you happen upon a "good" inverter.
Albert #70097 Probably meaning it's only there at the fast ranges, 500 ns and 50 ns? The Rate meter can cope with an enormous range of repetition rates, split in 3 parts IIRC (more or less logarithmic). Because of that it's very well possible that the effect is confined to the faster timing ranges. Apart from a dozen other possible reasons of course. Albert toggle quoted message Show quoted text The effect was not visible for risetimes longer than about 500 ps.

Previous Topic Next Topic