Re: S-1 transient response problems
The strobe is adjustable in several ways (avalanche volts, snap-off current, bridge volts). But wouldn't insufficient drive show up as an incorrect sample time, lack of sensitivity, or both?
As I mentioned, it works properly with my pulser (4 ns flattop pulse with 650 ps risetime). The sampler risetime is at least in the ballpark too (would expect to display sqrt(650^2+350^2) = 738 and that looked about right when I checked).
The problem I'm seeing is exactly consistent with the photos in the manual (setting transient response at 1 MHz). There is too much feedthrough aka blowby and the inverted signal through Q13 does not have enough adjustment range to null it out. I thought that double diodes would help this. How would going back to single diodes help, which will double the feedthrough capacitance? Please explain further, I'm learning as fast as I can.
Also it will be mechanically difficult for me to jumper one diode without damaging or destroying anything ;)
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Stephen,
The gr connectores will be the hard part. The 5ns cable I belive you can
use and "good quality" 50 ohm cable. The importent part is the impedance
match. 75 ohm and 50 ohm cable and jacks are easly confused and
substituded. I find my self using tek, amphional, and belkin cables for
genral lab use unless I need something more exotic based on speed or
frequancy. The speed of the cable is a property of the dielectric insulator
around the inner conductor. If memory serves air is best but difficult to
work with, next down the line is teflon.
Sorry if there is any spelling errors typed out on a phone.
Eric
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On Wed, Mar 24, 2021, 9:46 AM Stephen <stephen.nabet@...> wrote: Anyone has GR874 to BNC adapters, T¡¯s and 50ohm through terminations
they¡¯re willing to part with?
5ns cable as well?
Thanks
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Anyone has GR874 to BNC adapters, T¡¯s and 50ohm through terminations they¡¯re willing to part with?
5ns cable as well?
Thanks
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Re: How to explain how negative feedback lowers noise?
Hello folks,
I enjoy reading here very much. Indeed everyone can learn something.
One of my past job duties was to teach young professionals a basic understanding of aerodynamics dealing with mining ventilation systems. Trying to do so without starting with lots of maths put me often into trouble to find easy examples. I have burnt much midnight oil but frequently ended spreading calculations all over the board as there was no better proof of theory than figures.
I had to tell the guys: "You must stand this now ..."
Regards, Joe
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Re: How to explain how negative feedback lowers noise?
Hi Tom,
Thanks for your gracious reply. I do see your point now and it makes sense to me. Also, I agree, there are too many problems with the example I gave. Also, somehow I missed your "thermostat" example in my read of the thread. That is clearly a great example, and when I read it I regretted even more even posting mine. Should have just said "Thermostat example is the one!" :-) Also, my friend Matt's example of automobile steering seems like a useful deeper dive into the idea, and seems to hold up well - at least to me.
Wish I could delete my post about the kids...guess I'll have to figure out how to do that - but for future readers, if you still see it, skip it and read Tom and Matt's examples.
Love this forum. Not just people shouting nonsense back and forth, but actual dialogue and learning. (at least I'm learning :-)
Cheers to all,
Keith
CBG
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Re: 7A16A high frequency compensation.
Thank you Raymond. I just looked briefly at internet too see whether or not it could do random sampling.
Charles can forget my inappropriate warning.
Albert
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Re: Recapping Tektronix 2465
I do have kits available, which I custom assemble, to suit your EXACT machine.
I'll need CLEAR photos of your A1 A2 A3 A4 A5 A9 A14 PCB's
Saves you the time and hassle of sorting and sourcing, and wondering if you made the correct choices.
Here's the ebay ad
Forum members who buy via my contact directly here (NOT via ebay), get a 15% discount.
Yachadm AT gmail DOT com
CondorAudio
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Re: S-1 transient response problems
Using double diodes changes a lot of the conditions. Try going back to single diodes instead of series doubles, and see how it works. You can temporarily short one of the diodes in each pair for experimenting. There may not be enough strobe drive - it may be adjustable and fixed that way. Also, check the other adjustments and study their purposes and effects.
Ed
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S-1 transient response problems
You may recall that I have an S-1 sampling head with blown diodes, which I replaced with M/Acom MA4E1339 Schottky diodes in SOT-323 packages. I used both diodes in the package in series, max 1.2 pf each, to keep the capacitance down. They are mounted on small pieces of copper-clad and the package spans a gap in the copper. Hopefully low capacitance.
Anyhow, it works fine on fast pulses, and at DC too. But I ran a 1 MHz square wave through it (something I would not normally use a sampling scope for) and found that the transient response is waaaaay undercompensated. There's a bad overshoot, then undershoot, finally recovery. Per the S-1 manual that is the input to use while adjusting the transient response. But there isn't enough range on the pot (R13) to get it near flat, although with it all the way "up" it's better. I'll post display pics tomorrow.
I conclude that either my mount has excessive capacitance causing blowby in that frequency range, or possibly Q13 is bad. Although the trigger pickoff does function (Q13 drives Q17 which sends the trigger to the 3S2). It will be difficult to make a mount with much less C... the diodes themselves, at 0.6 pf, should not be a problem.
So I could use some suggestions. If I have to modify the circuit to allow for more out-of-phase signal to bypass the sampling bridge, what would be easiest to change with minimal side effects? Thanks.
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Re: FG502 doesn't start at some specific settings
Both marked 820 ohms (grey-red-brown); initial measure in circuit R155 real
value 1053 ohms (same value when removed), R290 real value 832 ohms and they
look like 0.5W (if not more). Now the two have places switched, but, as I
said, no difference in behavior of the stage
It is better to place the proper value, 820-ohm nominal, in both places. Voltages look OK ballpark. Perhaps some of the transistor gains decreased over time and now positive feedback coming back through C247 is not strong enough. I assume your serial number doesn't have the R247 resistor parallel to C247 given R155/R290 were 820-ohm nominal. You could try adding another 2.7pF in parallel with C247 to see if it helps. Note that C247 was selected so its value is critical to proper operation. At the higher frequency settings (higher charge/discharge currents) slight mismatches in source and sink current may be flowing into R155/Q292 junction and place the operating point in the bad stable condition. Looking at the min/max voltages at Q292 collector in normal operation in those high frequency settings may give other clues. Ozan
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Re: 485 super weak brightness control
Tomorrow I'm going to make a table with gate times both A and B at each
setting.
I observed today the gate width was same as 1ns on 10ns and 100ns, there
must be some logical explanation for that? Can't think of an easy explanation. When you are making the table could you also mark down both A gate width and B gate width while you are measuring B sweep timing? Also please observe if you really see a fast sweep when gate is shorter than expected or if the gate is cut short. If you apply a signal you can tell from the screen whether sweep is complete but fast or if it is truncated. Ozan
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Re: 7A16A high frequency compensation.
On Tue, Mar 23, 2021 at 11:44 PM, Albert Otten wrote:
The PM3340 only does sequential sampling and requires signal delay..
My PM3340 has a signal delay line... Trigger pickoff right at the N-socket and a nice blue Suhner coax cable after that for each channel. Nice but very heavy beast. Raymond
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Re: 7A16A high frequency compensation.
Hi Max,
It's nasty that the PG506 trigger output is not usable as pretrigger. Also (e.g. in my PG506) there is too much between-trigger jitter to view the fast edge of the next pulse. That's why a 7M11 delay line is used in the calibration section. But the 7M11 is not ideal. Hence the PG506 signal output waveform is compared with a faster TD pulser waveform. The PM3340 only does sequential sampling and requires signal delay..
BTW I found that the fast-rise rise time depends a lot on the amplitude, something like 0.75 ns for low to 1 ns for max amplitude.
Albert
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On Tue, Mar 23, 2021 at 10:23 PM, unclebanjoman wrote:
Hi Albert,
you made me doubt about the PG506 now :-)
I would not like to have calibrated the 7A16A trying to compensate for the
aberrations of the PG506 itself.
In the weekend I'll check the fast rise output of my PG506 using the
3S11/3T77A and also with a 2 GHz Philips PM3340 that I received a month ago
(fully functional, like new).
I'll post some photos as soon as possible.
Max
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Re: 7A16A high frequency compensation.
Hi Albert,
you made me doubt about the PG506 now :-)
I would not like to have calibrated the 7A16A trying to compensate for the aberrations of the PG506 itself.
In the weekend I'll check the fast rise output of my PG506 using the 3S11/3T77A and also with a 2 GHz Philips PM3340 that I received a month ago (fully functional, like new).
I'll post some photos as soon as possible.
Max
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Interesting HP production test fixture, even Tek guys will like it.
Every so often, I get some intriguing test gear from inside places that make test gear. It's fascinating to see what they use themselves. I just overhauled and posted such an item, an HP K09-0982B, a variable 115/230V AC Power supply, used to check the effects of different AC supply voltages on finished gear. Since many here often discuss slowly ramping up the AC to items that have been sitting a long time, I thought it might be of interest. you can see it here:
I also have some other strange HP fixtures used to control a large series of AC outlets with serial commands, for controlled power cycling, presumably for reliability testing. I haven't posted it up yet, but let me know if anybody finds that interesting. There's lots of weird microwave bits on the stuff season page too, so wander around, you may find something you like, which will make Susan very happy if it leaves here.
Let me know any requests, always happy to put up things people are actually interested in.
all the best,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.
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Re: 485 super weak brightness control
History is entirely unknown. I bought it for equivalent of about $20 from
guy who does disposal for universities and state own research institutes
etc. It was heading for a scrap pile.
Tomorrow I'm going to make a table with gate times both A and B at each
setting.
I observed today the gate width was same as 1ns on 10ns and 100ns, there
must be some logical explanation for that?
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On Tue, 23 Mar 2021, 20:55 Ozan, <ozan_g@...> wrote:
I must have a failed relay in the B board. Any idea what these are? They
are labeled tektronix but are they replaceable with something commonly
obtainable?
The relay doesn't change state until you hit 200ns. It could still be bad
but I don't see how it could create the behavior you are seeing. The good
news is it is on a socket, you can easily pull it off and test. While the
relay is off check for any continuity between C1443/C1442 pins to ground.
As I mentioned earlier they are three terminal caps with shield connected
to ground, which is not shown in the schematic. On my 485 there was a short
to shield.
Still the best explanation is somehow your timebase shaft is misaligned.
What is the history of this unit? May be previous owner tried to repair and
placed the shaft wrong.
Ozan
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Re: 485 super weak brightness control
I must have a failed relay in the B board. Any idea what these are? They
are labeled tektronix but are they replaceable with something commonly
obtainable?
The relay doesn't change state until you hit 200ns. It could still be bad but I don't see how it could create the behavior you are seeing. The good news is it is on a socket, you can easily pull it off and test. While the relay is off check for any continuity between C1443/C1442 pins to ground. As I mentioned earlier they are three terminal caps with shield connected to ground, which is not shown in the schematic. On my 485 there was a short to shield. Still the best explanation is somehow your timebase shaft is misaligned. What is the history of this unit? May be previous owner tried to repair and placed the shaft wrong. Ozan
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Re: 485 super weak brightness control
On Tue, Mar 23, 2021 at 08:29 PM, Ondrej Pavelka wrote:
I must have a failed relay in the B board. Any idea what these are? They
are labeled tektronix but are they replaceable with something commonly
obtainable?
K1242: They are well-known points of failure, which has been described several times, can't point you somewhere though. Have a look at TekWiki. Raymond
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Re: 485 super weak brightness control
On Tue, Mar 23, 2021 at 08:26 PM, Raymond Domp Frank wrote:
A time/div, pulse length B gate (A and B interlocked):
1 ns - 75ns
2 ns - 80 ns
5 ns - 120 ns
10 ns - 120 ns
20 ns - 230 ns
50 ns - 58 ns
100 ns - 1.1 us
200 ns - 2.2 us
500 ns - 5.5 us
1 us - 11 us
2 us - 22 us
5 us - 55 us
10 us - 110 us
20 us - 215 us
50 us - 540 us
100 us - 1.1 ms
200 us - 2.2 ms
500 us - 5.4 ms
1 ms - 11 ms
Raymond
Sorry: 50 ns - 580 ns Raymond
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Re: 485 super weak brightness control
I must have a failed relay in the B board. Any idea what these are? They
are labeled tektronix but are they replaceable with something commonly
obtainable?
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Show quoted text
On Tue, 23 Mar 2021, 20:26 Raymond Domp Frank, <hewpatek@...> wrote: A time/div, pulse length B gate (A and B interlocked):
1 ns - 75ns
2 ns - 80 ns
5 ns - 120 ns
10 ns - 120 ns
20 ns - 230 ns
50 ns - 58 ns
100 ns - 1.1 us
200 ns - 2.2 us
500 ns - 5.5 us
1 us - 11 us
2 us - 22 us
5 us - 55 us
10 us - 110 us
20 us - 215 us
50 us - 540 us
100 us - 1.1 ms
200 us - 2.2 ms
500 us - 5.4 ms
1 ms - 11 ms
Raymond
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Joe
Raymond Domp Frank