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Switching a Tek TDS 5204 to 50 Ohms input impedance - Strange!


Jokken Feldhaar
 

Hi all,
last week I encountered a strange behaviour when operating a Tek TDS 5204 Scope. We were measuring the output of a photo diode that was connected to an input channel. It is operated from a 12 Volts battery, which gives a little more than 5 Volts in our setup - at 1 MOhms inout impedance.
We then wanted to switch to 50 Ohms input impedance, and always got a warningthat the 50 Ohms was disabled due to too high input voltage. When disconnecting the external cable, then switching to 50 Ohms, and reconnecting the cable, all was OK without the error message.
Do these Tek Scopes REALLY measure only the 1 MOhms voltage to decide whether to switch to 50 Ohms input impedance? In earlier Scopes and some competitor's scopes the voltage after switching is evaluated, and only then is the 50 Ohms termination disabled. The same goes for my cherished 485.
Did the engineers at Tek really commit this big time error? I cannot believe this!


Ed
 

When the photo diode output is connected to 50 ohms vs 1megohm, I would expect the voltage to drop unless there is an amplifier in line. And even if there is, it would have to have a low output impedance to drive 50 ohms with the same voltage as 1 megohm.

ed


Jokken Feldhaar
 

Hi Ed,

I think you missed my main point, the photo diode is quite high impedance.

The problem is, that at 1M impedance I get 5+ Volts on the input,
decreasing to 4 mV when switched to 50 Ohms. But the Scope measures the
voltage at 1M, and decides that this is too high to switch to 50 Ohms.
It completely ignores the impedance-induced voltage change.

So I have to manually disconnect the diode, switch to 50 Ohms, and then
reconnect the diode. Tek could have done better, IMHO.

Grgards, Jochen DH6FAZ

Am 15.06.2020 um 13:13 schrieb Ed:

When the photo diode output is connected to 50 ohms vs 1megohm, I would expect the voltage to drop unless there is an amplifier in line. And even if there is, it would have to have a low output impedance to drive 50 ohms with the same voltage as 1 megohm.

ed


Brian
 

Hi , sorry to but in but I think Tek have got it right . The scope has no idea if the voltage measured is from a hi-impedance current limited source or a low-impedance constant voltage source with little or no current limit . The 50R termination in the scope is expensive and difficult / costly to repair so they play safe to protect it . I would not be complaining about that .
regardsBrian Skilton

On Monday, 15 June 2020, 15:15:13 BST, Jokken Feldhaar <jochen_feldhaar@...> wrote:

Hi Ed,

I think you missed my main point, the photo diode is quite high impedance.

The problem is, that at 1M impedance I get 5+ Volts on the input,
decreasing to 4 mV when switched to 50 Ohms. But the Scope measures the
voltage at 1M, and decides that this is too high to switch to 50 Ohms.
It completely ignores the impedance-induced voltage change.

So I have to manually disconnect the diode, switch to 50 Ohms, and then
reconnect the diode. Tek could have done better, IMHO.

Grgards, Jochen DH6FAZ

Am 15.06.2020 um 13:13 schrieb Ed:
When the photo diode output is connected to 50 ohms vs 1megohm, I would expect the voltage to drop unless there is an amplifier in line. And even if there is, it would have to have a low output impedance to drive 50 ohms with the same voltage as 1 megohm.

ed



 

On Mon, Jun 15, 2020 at 04:27 PM, Brian wrote:


Hi , sorry to but in but I think Tek have got it right . The scope has no idea
if the voltage measured is from a hi-impedance current limited source or a
low-impedance constant voltage source with little or no current limit . The
50R termination in the scope is expensive and difficult / costly to repair so
they play safe to protect it . I would not be complaining about that .
regardsBrian Skilton
These were my thoughts exactly. I wanted to add that the 50 Ohm input may be protected but for a fast 'scope, the circuits would be tiny, so difficult to fully protect. Unfortunately, I couldn't find (the BW of) a TDS 5204 so decided not to write my reaction. Probably an MSO5204?

On Sun, Jun 14, 2020 at 06:26 PM, Jokken Feldhaar wrote:

Did the engineers at Tek really commit this big time error? I cannot believe this!
So, not an error at all, just clever design!

Raymond


Brian
 

Hi Raymond , I have a couple of TDS's , a 744 original version and a 420A . Both are protected on the 50R input setting so I would expect all others to be the same . I have noticed this behaviour when using the 50R input and until I got used to it having wondered what was happening at first I much prefer it to the action taken by the 7A29 7k plug-in which just disconnects the input - and you need to disconnect to reset it.Regrettably I am just a hobbyist pensioner nowadays so my experience with anything newer than a TDS744 is zero , give me 500 series or 7K series even a 661 and I am happy
Brian Skilton

On Monday, 15 June 2020, 23:04:51 BST, Raymond Domp Frank <@Raymond> wrote:

On Mon, Jun 15, 2020 at 04:27 PM, Brian wrote:


Hi , sorry to but in but I think Tek have got it right . The scope has no idea
if the voltage measured is from a hi-impedance current limited source or a
low-impedance constant voltage source with little or no current limit . The
50R termination in the scope is expensive and difficult / costly to repair so
they play safe to protect it . I would not be complaining about that .
regardsBrian Skilton
These were my thoughts exactly. I wanted to add that the 50 Ohm input may be protected but for a fast 'scope, the circuits would be tiny, so difficult to fully protect. Unfortunately, I couldn't find (the BW of) a TDS 5204 so decided not to write my reaction. Probably an MSO5204?

On Sun, Jun 14, 2020 at 06:26 PM, Jokken Feldhaar wrote:

Did theĀ  engineers at Tek really commit this big time error? I cannot believe this!
So, not an error at all, just clever design!

Raymond


 

On Tue, Jun 16, 2020 at 12:27 AM, Brian wrote:


Hi Raymond , I have a couple of TDS's , a 744 original version and a 420A .
Both are protected on the 50R input setting so I would expect all others to be
the same . I have noticed this behaviour when using the 50R input and until I
got used to it having wondered what was happening at first I much prefer it to
the action taken by the 7A29 7k plug-in which just disconnects the input - and
you need to disconnect to reset it.Regrettably I am just a hobbyist pensioner
nowadays so my experience with anything newer than a TDS744 is zero , give me
500 series or 7K series even a 661 and I am happy
Brian Skilton
Hi Brian,
I have had several TDS 4xx and 5xx 'scopes and never experienced this behavior. I'm *that* careful - or lucky...
Nowadays, my only 'scope in that range is a TDS3054 (actually, a modified TDS3032) and I've never tried...
The 7A19 blows an input fuse if overloaded, the 7A29's behavior is much more user-friendly, being resettable.
I love 7K mainframes and I would never think "*regrettably*, I'm a hobbyist pensioner"...
Of course, some things can only be done with the newer stuff but I'm quite happy improvising.

Raymond


Brian
 

Hi Raymond , I have several 7A19's and so far not blown one of the input fuses , I hope I dont ever as I gather that they are rather hard to find , fortunately all the 7A19's I have purchased have had intact fuses and came complete with the spare ones inside if memory serves me well . I've not had that much luck with 7A29's , even with ones that still function , the input attenuator is often damaged such that the input inpedance is no longer constant when changing sensitivity . Fortunately my 7A29P that I purchased for my 7912AD doesnt suffer from that .
I am not 100% sure that the TDS series would exactly follow that original observation but I feel certain that it would in order to protect the input termination , its the only way that gives best protection for it . Later instruments are always more expensive so I would expect the cost of repairing the input termination out of warranty to match the cost of a new instrument . For a modest TDS744 diy replacement of the input hybrid costs in the region of 100GBP , without labour or factory overheads , if they would still do it -- imagine the likely cost if TEK did it


Brian

On Monday, 15 June 2020, 23:45:45 BST, Raymond Domp Frank <@Raymond> wrote:

On Tue, Jun 16, 2020 at 12:27 AM, Brian wrote:


Hi Raymond , I have a couple of TDS's , a 744 original version and a 420A .
Both are protected on the 50R input setting so I would expect all others to be
the same . I have noticed this behaviour when using the 50R input and until I
got used to it having wondered what was happening at first I much prefer it to
the action taken by the 7A29 7k plug-in which just disconnects the input - and
you need to disconnect to reset it.Regrettably I am just a hobbyist pensioner
nowadays so my experience with anything newer than a TDS744 is zero , give me
500 series or 7K series even a 661 and I am happy
Brian Skilton
Hi Brian,
I have had several TDS 4xx and 5xx 'scopes and never experienced this behavior. I'm *that* careful - or lucky...
Nowadays, my only 'scope in that range is a TDS3054 (actually, a modified TDS3032) and I've never tried...
The 7A19 blows an input fuse if overloaded, the 7A29's behavior is much more user-friendly, being resettable.
I love 7K mainframes and I would never think "*regrettably*, I'm a hobbyist pensioner"...
Of course, some things can only be done with the newer stuff but I'm quite happy improvising.

Raymond