terminator use

Guest
I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators? thanks jk
 
entropy429@yahoo.com wrote:
I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators? thanks jk
Click to expand...

The terminator? He's currently governor of California :)

It's rather simple: Connect a 43ohms resistor from the center conductor
of the coax to the scope input. If you have to do this a lot and the
cable has BNC it might pay off to grind off the corner of a 90 degree
BNC adapter until you've just separated the center. Solder the 43ohms in
series there and close it up so no dirt and grime gets in. Use a chip
resistor or at least one that is low in inductance.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
 
On Mon, 05 May 2008 14:18:39 -0500, entropy429@yahoo.com wrote:

I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators? thanks jk
Click to expand...

If the generator is really a 93 ohm source, you don't need a
terminator at the far end, no matter what impedance the scope is. Your
system is "source terminated" and any reflections from the scope will
be re-absorbed when they bounce back into the generator.

But if you want to soak up any reflections at the scope (say, the
generator isn't truly a 93 ohm source, and you don't want stuff
sloshing around) you need a series resistor of 43 ohms between the end
of the coax and the 50 ohm scope input... not a tee.


John
 
On Mon, 05 May 2008 14:18:39 -0500, entropy429@yahoo.com wrote:

I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators?
Click to expand...
---
If you have a 50 ohm scope, then putting a 93 ohm resistor in parallel
with its input (which is what you're doing if you're using a tee) will
get you an impedance of:

R1 * R2 50R * 93R
Rt = --------- = ----------- = 35.52 ohms
R1 + R2 50R + 93R

So you'll be mismatched with the cable.

You don't need a terminator, you need a minimum-loss pad to match the
93 ohm cable to your 50 ohm scope.

View in Courier:


93 OHM CABLE +------------+ 50 OHM CABLE
+-->>------------->>-|--[75R]--+--|-<<------------<<--+
|R1 | R2 | | |
[93R] GENERATOR | [62R]| SCOPE [50R]
| | R3| | |R4
+-->>------------->>-|---------+--|-<<------------<<--+
+------------+


It'll match pretty well, but you'll take about a 17dB loss in voltage
at the input of the scope.

For example, if you have a 1 volt output from the generator,
unterminated, with the scope and the pad in place the input voltage to
the scope will be:


Vin (R3||R4)
Vout = --------------------
(R1+R2) + (R3||R4)

1V * 28R
= --------------------- ~ 0.143V
93R + 75R + 28R


JF
 
On Mon, 5 May 2008 13:39:09 -0700 (PDT), BobG <bobgardner@aol.com>
wrote:

Hi John. Dont we need a transmission line of 1/4 wavelength of the
freq of interest before we start worring about termination?
Click to expand...
---
No.

Sourcing and terminating a line with its characteristic impedance will
make the line length unimportant.

JF
 
On Mon, 05 May 2008 13:10:02 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 05 May 2008 14:18:39 -0500, entropy429@yahoo.com wrote:

I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators? thanks jk


If the generator is really a 93 ohm source, you don't need a
terminator at the far end, no matter what impedance the scope is. Your
system is "source terminated" and any reflections from the scope will
be re-absorbed when they bounce back into the generator.
Click to expand...
Does source termination only refer to periodic signal or does it
also apply to one shot signals also?? What is source termination? I
haven't heard that before,( shows you my level) :) jf
But if you want to soak up any reflections at the scope (say, the
generator isn't truly a 93 ohm source, and you don't want stuff
sloshing around) you need a series resistor of 43 ohms between the end
of the coax and the 50 ohm scope input... not a tee.


John
Click to expand...
 
Hi John. Dont we need a transmission line of 1/4 wavelength of the
freq of interest before we start worring about termination?
 
On Mon, 05 May 2008 15:55:02 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Mon, 05 May 2008 14:18:39 -0500, entropy429@yahoo.com wrote:

I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators?

---
If you have a 50 ohm scope, then putting a 93 ohm resistor in parallel
with its input (which is what you're doing if you're using a tee) will
get you an impedance of:

R1 * R2 50R * 93R
Rt = --------- = ----------- = 35.52 ohms
R1 + R2 50R + 93R

So you'll be mismatched with the cable.

You don't need a terminator, you need a minimum-loss pad to match the
93 ohm cable to your 50 ohm scope.

View in Courier:


93 OHM CABLE +------------+ 50 OHM CABLE
+--<<------------->>-|--[75R]--+--|-<<------------>>--+
|R1 | R2 | | |
[93R] GENERATOR | [62R]| SCOPE [50R]
| | R3| | |R4
+--<<------------->>-|---------+--|-<<------------>>--+
+------------+


It'll match pretty well, but you'll take about a 17dB loss in voltage
at the input of the scope.

For example, if you have a 1 volt output from the generator,
unterminated, with the scope and the pad in place the input voltage to
the scope will be:


Vin (R3||R4)
Vout = --------------------
(R1+R2) + (R3||R4)

1V * 28R
= --------------------- ~ 0.143V
93R + 75R + 28R


JF
Click to expand...
 
entropy429@yahoo.com wrote:
I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators? thanks jk
Click to expand...
Is the scope terminated at 50 ohms, or is it just a 50 ohm BNC jack into
the usual high-impedance scope input?

If it's the high-impedance input and you _really_ need the line
terminated, then use a 93 ohm termination. You can roll your own with a
91-ohm resistor, that'll be plenty close enough.

If the scope really does present a 50 ohm termination, then you have
plenty of suggestions already.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
On Mon, 05 May 2008 16:10:51 -0500, jfisher864@comcast.net wrote:

On Mon, 05 May 2008 13:10:02 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 05 May 2008 14:18:39 -0500, entropy429@yahoo.com wrote:

I'm confused about the use of terminators. I'm sending pulse signals
from some 93 ohm output equipment through 93 ohm coax to my scope
which is 50 ohm. I have a t at the scope. Should I be puting a 93 ohm
terminator on the other end of the t or a 50ohm? Is there a rule of
thumb where to place terminators? thanks jk


If the generator is really a 93 ohm source, you don't need a
terminator at the far end, no matter what impedance the scope is. Your
system is "source terminated" and any reflections from the scope will
be re-absorbed when they bounce back into the generator.

Does source termination only refer to periodic signal or does it
also apply to one shot signals also?? What is source termination? I
haven't heard that before,( shows you my level) :) jf
Click to expand...

Any signal.

If the generator behaves like a real 93 ohm source....


___________
| |
| 93r gen | ------------------------ any
| |--| 93r coax |----------load
| | |------------------------| |
| |--+ -----------+
___________


Then, for any signal waveform and for any coax length, the waveform at
the load is exactly the same as if there was no coax present....


___________
| |
| gen |
| |------load
| | |
| |-------+
___________


except for the coax time delay, of course.


That's "source termination", namely driving an X ohm coax from an X
ohm generator.

That's handy in fast logic systems. If gate A drives a 50 ohm trace
that runs to gate B, and you end terminate at B with 50 ohms to
ground, gate A sees a 50 ohm load, AC and DC, and that's hard to
drive. But if you put a series 50 ohm resistor at A, before the trace,
and don't terminate at B, it works just as well, except that driver
gate A now sees a 100 ohm AC load and an infinite DC load, which saves
a lot of power.

John
 
On Mon, 5 May 2008 13:39:09 -0700 (PDT), BobG <bobgardner@aol.com>
wrote:

Hi John. Dont we need a transmission line of 1/4 wavelength of the
freq of interest before we start worring about termination?
Click to expand...
If a line is 1/4 wavelength, and the far end is open, it looks like a
dead short to a sinewave drive. Numbers like 1/20 are closer to
invisible. It really depends on the impedances and how much precision
you want.

John
 
In article <am9124p6ki036mo9bt1d9vrsbie86mu8is@4ax.com>,
jjlarkin@highNOTlandTHIStechnologyPART.com says...
On Mon, 5 May 2008 13:39:09 -0700 (PDT), BobG <bobgardner@aol.com
wrote:

Hi John. Dont we need a transmission line of 1/4 wavelength of the
freq of interest before we start worring about termination?

If a line is 1/4 wavelength, and the far end is open, it looks like a
dead short to a sinewave drive. Numbers like 1/20 are closer to
invisible. It really depends on the impedances and how much precision
you want.

John


I expected at least 1 bad John Conner joke out of this thread (did I
Click to expand...
miss it?).
Instead I actually learned something useful.
Boy, this newsgroup sure has gone downhill!
LOL

Jim
 
On Tue, 6 May 2008 16:14:19 -0400, James Beck
<jim@reallykillersystems.com> wrote:

In article <am9124p6ki036mo9bt1d9vrsbie86mu8is@4ax.com>,
jjlarkin@highNOTlandTHIStechnologyPART.com says...
On Mon, 5 May 2008 13:39:09 -0700 (PDT), BobG <bobgardner@aol.com
wrote:

Hi John. Dont we need a transmission line of 1/4 wavelength of the
freq of interest before we start worring about termination?

If a line is 1/4 wavelength, and the far end is open, it looks like a
dead short to a sinewave drive. Numbers like 1/20 are closer to
invisible. It really depends on the impedances and how much precision
you want.

John


I expected at least 1 bad John Conner joke out of this thread (did I
miss it?).
Instead I actually learned something useful.
Boy, this newsgroup sure has gone downhill!
LOL

Jim
Click to expand...


--------------------------------+
|
|
/ But there's no reflection!
\
/ OF COURSE. I'M A TERMINATOR.
\
|
|
gnd
 
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