Topics

Tek TDS694C advice pls


mosaicmerc
 

Hi , In my research for a higher bandwidth scope option, I have placed this item on my wish list.
I see it uses 50 Ohm probes only.



Is there anyone on the forum with advice on this Scope as a high bandwidth option vs sampling scopes etc?


thx


 

Sampling oscilloscopes are not good for general use; where they shine
is applications that require either the highest bandwidth or best
overload recovery. Almost all sampling oscilloscopes also require a
pretrigger signal.

Both types of oscilloscopes will require 50 ohm probes. You will not
find high impedance passive probes on any oscilloscope input faster
than 300 to 500 MHz and passive probes high impedance probes at those
frequencies do not perform well compared to low-z probes which are
easy enough to make on the spot for work up to about 1 GHz.

On 10 Apr 2016 19:16:21 -0700, you wrote:

Hi , In my research for a higher bandwidth scope option, I have placed this item on my wish list.
I see it uses 50 Ohm probes only.

Is there anyone on the forum with advice on this Scope as a high bandwidth option vs sampling scopes etc?

thx


mosaicmerc
 

TDS694C isn't a sampling scope though, it's a real time one shotter.


 

The TDS694C is a 3GHz, 10 GS/s Real Time-only 'scope. It is best suited for one-shot applications and because of that is often used in combination with a Logic Analyzer, which provides the trigger signal.

Unfortunately, the TDS694C tends to suffer from overheating trigger ASICs, eventually causing their destruction. It contains four of them; one for each channel. It seems that their bottom (heat sinking) tab is not correctly fixed/soldered to the PCB - or the chip separates from the tab...

Hi-jacking the thread: If anyone can provide me with a few of these unobtanium chips (156-8278-00), I'd be very much obliged....

Classic sampling 'scopes are well suited to judging repetitive signals, including pulse edge shape. A 7000 mainframe with an S-6 or better yet, an S-4 vertical input head, provides 11 or 14 GHz BW. I prefer the S-4 mostly because it has an internal trigger pickoff, which is easier to work with and avoids distortion caused by extra (splitting) cabling if there's no separate trigger signal. Generally, because there's no delay line, for pulse work you need a pretrigger signal (or an accurate separate delay).

Using such a setup in a 7854 is a real treat.

Raymond


 

Using such a setup in a 7854 is a real treat.
Another good option is using a variable persistence mainframe but any (low-bandwidth) 7000 mainframe will do.

Raymond


John Miles
 

TDS 694Cs are decent scopes for the money, and they're also new enough to
avoid the infamous Tek capacitor plague. See
http://www.eevblog.com/forum/testgear/least-expensive-but-reliable-used-mark
et-scope-with-at-least-1-ghz/msg791170/#msg791170 for more on the trigger
chips.



They don't seem to run excessively hot as long as you either keep the cover
on the scope or aim a fan at the acquisition board when servicing it with
the power on. Two or three minutes without cooling would make me nervous on
behalf of the front end chips. The trigger hybrids are far from the hottest
ones on the board...



They aren't the greatest 3 GHz scopes on the planet, but they are the
cheapest, and they're far nicer to work with than any sampling scope.



-- john, KE5FX



From: TekScopes@... [mailto:TekScopes@...]
Sent: Monday, April 11, 2016 3:01 AM
To: TekScopes@...
Subject: [TekScopes] Re: Tek TDS694C advice pls





The TDS694C is a 3GHz, 10 GS/s Real Time-only 'scope. It is best suited for
one-shot applications and because of that is often used in combination with
a Logic Analyzer, which provides the trigger signal.

Unfortunately, the TDS694C tends to suffer from overheating trigger ASICs,
eventually causing their destruction. It contains four of them; one for each
channel. It seems that their bottom (heat sinking) tab is not correctly
fixed/soldered to the PCB - or the chip separates from the tab...

Hi-jacking the thread: If anyone can provide me with a few of these
unobtanium chips (156-8278-00), I'd be very much obliged....

Classic sampling 'scopes are well suited to judging repetitive signals,
including pulse edge shape. A 7000 mainframe with an S-6 or better yet, an
S-4 vertical input head, provides 11 or 14 GHz BW. I prefer the S-4 mostly
because it has an internal trigger pickoff, which is easier to work with and
avoids distortion caused by extra (splitting) cabling if there's no separate
trigger signal. Generally, because there's no delay line, for pulse work you
need a pretrigger signal (or an accurate separate delay).

Using such a setup in a 7854 is a real treat.

Raymond


Phil Hobbs
 

My 694C is my favourite scope. (I have 8 in use at the moment, including two 1180x sampling scopes and about 20 plugins.) If I could have only one, it would be the 694C.

With the matching P6249 FET probes, the 694C becomes more or less a general purpose scope. You do have to watch out that you don't blow the probes up with overvoltage, and they're only 20k // 1 pF, but the combo is very powerful.

Cheers

Phil Hobbs


 

They don't seem to run excessively hot.
Thanks for the link, John, I'll have a look over there.

There's no heat spreader or anything on top of these chips but they seem to have a solid metal tab underneath.
In my TDS694C, two of them were too hot to touch for even 5 seconds after operating for less than a minute,
hence my theory that the tab has come loose (bad/non-existent solder flow) or the chip has come loose inside, as per one of the theories on the infamous U800 in the 24X5 series.

In my TDS694C, trigger chips for CH1 and 2 don't get very hot. CH3 and especially CH4 do. CH3 has become intermittent, CH4 is dead now. I only got this 'scope very recently and I opened it up because of the trigger problems. Putting a heat sink on top helped a little for a while.
I don't think that a non-cooling-related problem caused these chips to heat up in the first place.

None of the other chips on the acquisition board get very hot. A small fan while open suffices, but for the trigger chips.

Looking at a <= 200 ps edge with either a TDS694C or a 10 GHz+ sampling 'scope makes quite a difference.
With practice, a classic sampling 'scope is not too difficult to use, especially if you know what to expect...

Raymond


 

Thanks for the link, John, I'll have a look over there.
I "strongly doubt" that what i said in the EEVBLOG thread about the 2 "superfluous" trigger chips is correct.

Apart from this problem, I find the TDS694C a very nice 'scope. Wish I could repair the missing trigger channels OR trust the remaining two to go on living...

Raymond


John Miles
 

I can't believe it, either, but... if the two chips aren't superfluous, why
does the scope work well (on all four channels) without them?



Lots of retired Tek engineers around this neck of the woods. Maybe I'll be
lucky enough to run into somebody from the group responsible for the TDS694C
at one of the local hamfests...



-- john, KE5FX



From: TekScopes@... [mailto:TekScopes@...]
Sent: Monday, April 11, 2016 4:25 AM
To: TekScopes@...
Subject: [TekScopes] Re: Tek TDS694C advice pls





Thanks for the link, John, I'll have a look over there.
I "strongly doubt" that what i said in the EEVBLOG thread about the 2
"superfluous" trigger chips is correct.

Apart from this problem, I find the TDS694C a very nice 'scope. Wish I could
repair the missing trigger channels OR trust the remaining two to go on
living...

Raymond


 

My S-4s have an annoying amount of blow-by so I prefer the S-1 and S-2
which are also repairable.

The 7T11 is one of the rare exceptions of a sampling oscilloscope
which can display a pulse edge without a pretrigger signal or delay
line.

On 11 Apr 2016 03:00:54 -0700, you wrote:

...

Classic sampling 'scopes are well suited to judging repetitive signals, including pulse edge shape. A 7000 mainframe with an S-6 or better yet, an S-4 vertical input head, provides 11 or 14 GHz BW. I prefer the S-4 mostly because it has an internal trigger pickoff, which is easier to work with and avoids distortion caused by extra (splitting) cabling if there's no separate trigger signal. Generally, because there's no delay line, for pulse work you need a pretrigger signal (or an accurate separate delay).

Using such a setup in a 7854 is a real treat.

Raymond


 

The truth is .... you only need one, namely the one for the channel that you use to trigger the 'scope. You may e.g. display CH4 and trigger on CH1.
IOW, moving the chips isn't necessary, just choose your channel.

Raymond


mosaicmerc
 

Seems that heat sinking the TDS694 trigger IC's is a good idea. Kinda like the U800 in the 24xx series.

Ancel


John Miles
 

(Catching up on old mail...)



The actual truth is that the scope triggers just fine from all four channels with the two missing chips as shown in the EEVBlog thread. Obviously that's the first thing that anyone who isn't a complete idiot would verify after such an unconventional "fix."



I'm not sure what would happen with only one of the chips in place. Still hoping someone will smuggle the schematics for that board out someday, so that we can see what the designers were up to and determine what specs and/or features are impacted by the missing parts, if any.



-- john, KE5FX





From: TekScopes@... [mailto:TekScopes@...]
Sent: Monday, April 11, 2016 6:50 AM
To: TekScopes@...
Subject: [TekScopes] Re: Tek TDS694C advice pls





The truth is .... you only need one, namely the one for the channel that you use to trigger the 'scope. You may e.g. display CH4 and trigger on CH1.
IOW, moving the chips isn't necessary, just choose your channel.

Raymond


 

Obviously that's the first thing that anyone who isn't a complete idiot would verify after
such an unconventional "fix."
Firstly, I wouldn't call everyone making an operating error a complete idiot. Secondly, not everyone is immediately aware that trigger source selection allows each of the four channels as trigger source independently. Combined, one may see a triggered CH4, not realising for a moment that they're still triggering on CH1. You may call me a complete idiot if I'm wrong ;-).

Did you try logic triggering, using all 4 channels for establishing conditions?

My TDS694C won't trigger on CH3 nor CH4 after trigger chips 3 and 4 died. I wrote about this elsewhere and earlier.

My observation while they were still alive but overheating was that forced cooling #3 or #4 (cold spray) temporarily returned CH3 and CH4 respectively back into operation. I don't think that "blowby" of the cold spray distorted these observations.

If what is stated in the EEVBlog post is correct, CH3 and CH4 would be served by trigger chip #3; In the 'blog, Chip 2 was moved to the #1 position and chip #4 to the #3 position.

If #3 serves CH3 and CH4, that would be good news. I'd only need someone who can safely move my #2 chip to position #3...

Raymond


John Miles
 

No, the idea isn't to call anyone in particular an idiot, but to point out
that someone would have to be one in order to claim that the modification
works without bothering to verify that all four channels can still serve as
trigger sources.



If someone who's running two chips as in the photo is unable to use any of
the four channels as a trigger source, then something else is wrong with the
scope.



I think I've used combinatorial logic triggering once in my life, and that
was on another scope. I agree that's a good candidate for the no-free-lunch
rule violation, as it's pretty difficult to test exhaustively.



-- john, KE5FX



From: TekScopes@... [mailto:TekScopes@...]
Sent: Tuesday, May 31, 2016 3:53 AM
To: TekScopes@...
Subject: [TekScopes] Re: Tek TDS694C advice pls





Obviously that's the first thing that anyone who isn't a complete idiot
would verify after
such an unconventional "fix."
Firstly, I wouldn't call everyone making an operating error a complete
idiot. Secondly, not everyone is immediately aware that trigger source
selection allows each of the four channels as trigger source independently.
Combined, one may see a triggered CH4, not realising for a moment that
they're still triggering on CH1. You may call me a complete idiot if I'm
wrong ;-).

Did you try logic triggering, using all 4 channels for establishing
conditions?

My TDS694C won't trigger on CH3 nor CH4 after trigger chips 3 and 4 died. I
wrote about this elsewhere and earlier.

My observation while they were still alive but overheating was that forced
cooling #3 or #4 (cold spray) temporarily returned CH3 and CH4 respectively
back into operation. I don't think that "blowby" of the cold spray distorted
these observations.

If what is stated in the EEVBlog post is correct, CH3 and CH4 would be
served by trigger chip #3; In the 'blog, Chip 2 was moved to the #1 position
and chip #4 to the #3 position.

If #3 serves CH3 and CH4, that would be good news. I'd only need someone who
can safely move my #2 chip to position #3...

Raymond


 

John,
the idea isn't to call anyone in particular an idiot,
I meant this tongue-in-cheek and tried to use an emoji ( ;-)). No harm done!

If someone who's running two chips as in the photo is unable to use any of
> the four channels as a trigger source, then something else is wrong with the
> scope.
Evidently!


mosaicmerc
 

Well my TD694C arrived today, the cal sheet claims they used several Fluke and Agilent instruments to verify cal. Axiom test. http://www.axiomtest.com/
Haven't had time to set it up yet though.

Any advice for a noob user of the TDS?
thx
Ancel


mosaicmerc
 

Ok, I also seems to have trigger issues.
1) Selecting either Ch1 or 2 and hitting autoset allows a stable trigger.
Ch3 or 4 freezes the acq. and DOES not do any more acq while the trg is detected. if I vary the trig outside the signal (1V P-P 10Mhz) the auto kicks in and the untriggered waveform is visible. Move the trigger back into range and everything freezes - zero acq happens on all channels.

If I set the trigger to stop on a 'single acq' punching the run/stop will grab a new single frozen sample at the trigger point. This is only in channels 3 or 4. Setting to stop sampling when run/stop is pressed does zippo, once a trig happens all sampling acq. halts once CH3 or 4 is set as their triggers.

Is this 'normal', does ch3 & 4 operate in this one shot trigger mode only and then cease all acquisitions?


James Martin
 

masiacmerc- I have exactly the same triger issue you describe. Additionally, my unit has gotten progressively worse.
Works perfect at turn-on, and after some time it shows same symptoms as you describe.
Over the past couple of days the length of time it operates OK after turn-on gets shorter and shorter.
One other symptom I have that you did not list is that after some time left on in this state, the on screen clock will stop.


Never had reason to open unit before, but did so yesterday and found that the scope had been modified, someone pulled 2 of the 156-827-800 chips off the signal board.


A serious amount of trying to locate thermally impaired components has not yielded anything. Have worked on a variety of TDS scopes in the past, but never my '694c.


James