signal generator output

[Phil Allison]

John Larkin is a CRIMINAL LIAR "

Phil Allison

Sure as hell looks like standing waves at play to me.

Sure, but you don't see them at the scope.


** The LIAR claimed there were no standing wave effects - quote:

" No standing waves or resonances at all. "

Then he denied that the "quarter wave stub " was relevant to the case.

Wot a blatant fucking LIAR !!!


As I noted in a previous post, any standing wave effects
are buried inside the cable and invisible at the load.

** What the FUCKING NUT CASE LIAR actually wrote was:

" If there are any standing waves, they are somewhere inside the cable,
totally invisible from outside;"

Wot a BLATANT LIAR !!!!

I made no comments about a quarter-wave stub.

** To my Q:

Never heard of a " quarter wave stub " ??

The FUCKING LIAR responded:

" Good grief, you don't understand transmission lines. Too much audio, I
suppose."

Read your own posts. You said:

"The real situation is that of an unterminated transmission line of
operating over a very wide frequency range.

Will exhibit more peaks and dips than the Himalayas."

There are no such peaks or dips at the end of the line,


** But they ** ARE THERE ** at the generator end of the line !!

The discrepancy was the OP's original problem:

" The trouble is , the voltage readings on my scope do not match
my output readings on the signal generator., sometimes off by a
factor of 3 to 4. "

I supplied him with a 100% CORRECT answer.

YOU FUCKING NUT CASE LIAR !!!!!!!!!!!!

YOU need TEN bullets in the head.



.... Phil

 

[John Larkin]

On Sun, 12 Dec 2010 13:19:38 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

John Larkin is a CRIMINAL LIAR "

Phil Allison

Sure as hell looks like standing waves at play to me.

Sure, but you don't see them at the scope.


** The LIAR claimed there were no standing wave effects - quote:

" No standing waves or resonances at all. "

Then he denied that the "quarter wave stub " was relevant to the case.

Wot a blatant fucking LIAR !!!


As I noted in a previous post, any standing wave effects
are buried inside the cable and invisible at the load.


** What the FUCKING NUT CASE LIAR actually wrote was:

" If there are any standing waves, they are somewhere inside the cable,
totally invisible from outside;"

Wot a BLATANT LIAR !!!!

I made no comments about a quarter-wave stub.


** To my Q:

Never heard of a " quarter wave stub " ??

The FUCKING LIAR responded:

" Good grief, you don't understand transmission lines. Too much audio, I
suppose."


Read your own posts. You said:

"The real situation is that of an unterminated transmission line of
operating over a very wide frequency range.

Will exhibit more peaks and dips than the Himalayas."


There are no such peaks or dips at the end of the line,


** But they ** ARE THERE ** at the generator end of the line !!

Which the OP won't see at the scope. So they don't matter.

The discrepancy was the OP's original problem:

" The trouble is , the voltage readings on my scope do not match
my output readings on the signal generator., sometimes off by a
factor of 3 to 4. "

And standing waves have nothing to do with that. If he used 2 cm of
coax or 2 meters, the results will be the same, given a 50r generator
and a 50r cable.

I supplied him with a 100% CORRECT answer.

0%. You did nothing to explain his discrepency. And you still haven't.
All that stuff about standing waves makes no difference here.

John

 

[Phil Allison]

"John Larkin is an INSANE CRIMINAL LIAR "

Phil Allison

Sure as hell looks like standing waves at play to me.

Sure, but you don't see them at the scope.


** The LIAR claimed there were no standing wave effects - quote:

" No standing waves or resonances at all. "

Then he denied that the "quarter wave stub " was relevant to the case.

Wot a blatant fucking LIAR !!!


As I noted in a previous post, any standing wave effects
are buried inside the cable and invisible at the load.


** What the FUCKING NUT CASE LIAR actually wrote was:

" If there are any standing waves, they are somewhere inside the cable,
totally invisible from outside;"

Wot a BLATANT LIAR !!!!

** No answer NOTED = my point #1.
--------------------------------------------

I made no comments about a quarter-wave stub.


** To my Q:

Never heard of a " quarter wave stub " ??

The FUCKING LIAR responded:

" Good grief, you don't understand transmission lines. Too much audio, I
suppose."

** No answer NOTED = my point #2.
-------------------------------------------

Read your own posts. You said:

"The real situation is that of an unterminated transmission line of
operating over a very wide frequency range.

Will exhibit more peaks and dips than the Himalayas."


There are no such peaks or dips at the end of the line,


** But they ** ARE THERE ** at the generator end of the line !!

Which the OP won't see at the scope. So they don't matter.


The discrepancy was the OP's original problem:

" The trouble is , the voltage readings on my scope do not match
my output readings on the signal generator., sometimes off by a
factor of 3 to 4. "

** No answer NOTED = my point #3.
---------------------------------------------

And standing waves have nothing to do with that.

** MASSIVE FUCKING LIE !!!!

I supplied him with a 100% CORRECT answer.

0%. You did nothing to explain his discrepency.

** If his RF gen metering reads the ACTUAL output level it sure as HELL
does !!!!

All that stuff about standing waves makes no difference here.

** Oh - so NOW they really exist.

The cat is finally out of the fucking bag.

YOU VILE STINKING AUTISTIC LIAR !!!!!!!!!!!!!


...... Phil

 

[John Larkin]

On Sun, 12 Dec 2010 13:54:37 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Larkin is an INSANE CRIMINAL LIAR "


Phil Allison

Sure as hell looks like standing waves at play to me.

Sure, but you don't see them at the scope.


** The LIAR claimed there were no standing wave effects - quote:

" No standing waves or resonances at all. "

Then he denied that the "quarter wave stub " was relevant to the case.

Wot a blatant fucking LIAR !!!


As I noted in a previous post, any standing wave effects
are buried inside the cable and invisible at the load.


** What the FUCKING NUT CASE LIAR actually wrote was:

" If there are any standing waves, they are somewhere inside the cable,
totally invisible from outside;"

Wot a BLATANT LIAR !!!!


** No answer NOTED = my point #1.
--------------------------------------------


I made no comments about a quarter-wave stub.


** To my Q:

Never heard of a " quarter wave stub " ??

The FUCKING LIAR responded:

" Good grief, you don't understand transmission lines. Too much audio, I
suppose."


** No answer NOTED = my point #2.
-------------------------------------------


Read your own posts. You said:

"The real situation is that of an unterminated transmission line of
operating over a very wide frequency range.

Will exhibit more peaks and dips than the Himalayas."


There are no such peaks or dips at the end of the line,


** But they ** ARE THERE ** at the generator end of the line !!

Which the OP won't see at the scope. So they don't matter.


The discrepancy was the OP's original problem:

" The trouble is , the voltage readings on my scope do not match
my output readings on the signal generator., sometimes off by a
factor of 3 to 4. "


** No answer NOTED = my point #3.
---------------------------------------------

And standing waves have nothing to do with that.


** MASSIVE FUCKING LIE !!!!



I supplied him with a 100% CORRECT answer.

0%. You did nothing to explain his discrepency.


** If his RF gen metering reads the ACTUAL output level it sure as HELL
does !!!!

OK, that's a possibility. Most of the RF gens I've used level or
measure the output and then attenuate it, effectively or actually
putting 50 ohms downstream of a voltage source, and are calibrated (if
they are calibrated at all) in terms of voltage or power into an
assumed 50 ohm load.

For a generator that meters the actual output voltage, at the
connector, as he swept the frequency, he'd see the peaks and dips at
the generator but not on the scope at the other end of the cable. I've
never seen a signal generator that worked that way. Have you?

John

 

[John Fields]

On Sat, 11 Dec 2010 17:29:23 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 11 Dec 2010 17:41:22 -0600, John Fields
jfields@austininstruments.com> wrote:


---
An ideal transmission line will exhibit only delay and insertion loss,
the insertion loss qualifying it as an attenuator, albeit a tiny one
for a short line at a low frequency.

How could an ideal transmission line have loss? It only includes
inductance and capacitance, so it can't dissipate power.

---
Poor choice of terms, thanks.

I should've said "real" instead of "ideal".
---

An ideal 50 ohm transmission line, used to extend the output of a 50
ohm signal generator, adds time delay. Nothing else.

---

Not true.

It can also be used as an impedance transformer:

http://en.wikipedia.org/wiki/Quarter_wave_impedance_transformer

---

Hook that to a 15 pF scope input and you get a 1st
order lowpass response with corner around 200 MHz or so. No standing
waves or resonances at all.

---
At the capacitive end of the cable that's true, but you don't get the
-6dB rolloff that you'd expect, you get the corner at -3dB and the
skirt falls off at -3dB per octave.

Insane. Sorry to say so, but insane.

---
Sorry, my ass...

You revel in being insulting.


Take a look at TP3 of:

5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and tell me what you see.
---

Tricky subterfuge, but no cigar.

What you've done is taken a typical scope input of 15pF to ground, set
its reactance to 50 ohms and solved for the frequency at that point:

1 1
f = ---------- = ---------------------- ~ 212.3MHz.
2pi C Xc 6.28 * 15e-12F * 50R

Which _is_ around 200MHz if Xc equals about 53 ohms.


However, the capacitor is a _reactive_, not a _resistive_ load, so
some of the energy sent to it would be stored in its out-of-phaseness
and sent back to the source on a cycle-per-cycle basis, resulting in
reflections and standing waves, no matter what.

Any reflections will be absorbed by the generator impedance.

---
Yes, but in the process of being absorbed, the voltage across the
source resistance will be exalted or diminished by the current in the
wave reflected from the load, causing unexpected variations from
nominal in the voltage across the load when the exalting/diminishing
wave from the source returns to the load.

Nope.

---
Then how do you explain the -3dB knee and the -3dB/octave slope at TP3
of the 50R 15pF lowpass filter ?
---

If you
sweep the gen frequency, the scope will see a voltage that looks just
like a 1st order RC lowpass response, flat to about 200 MHz and then
rolling off at -6 dB per octave. If there are any standing waves, they
are somewhere inside the cable, totally invisible from outside; they
won't cause frequency peaks or dips at the scope.

LT Spice has a tranny model, so you can try it.

---
I did, and got some interesting results, among them that with the
capacitance you specified the knee was at about -3 dB at 200MHz and
the rolloff was -3dB per octave.

The 200 MHz corner was where I predicted.

---
Nope.

With 15 pF in there the -6db corner is at about 367MHz.
---

And I bet the voltage at the
scope was flat, no frequency whoopie-doos, from DC up to there. But it
is 6 dB/octave after the corner.

---
Not with 15pF in there it isn't.

See Tp3 of my sim.
---

Possibly because of the "feedback" from the reflections?

If it's source terminated, there are no secondary reflections. The
generator gobbles them all up.

---
So you believe that the current in the source resistance caused by
reflections from the mismatched end of the cable won't cause a voltage
drop in the source resistance? Curious.

Take a look at the source end of the mismatched instances and tell me
what you see.

Don't you think that'll affect the signal developed across the source
resistance?
---

I don't know. My models blew up after a while and left me with the
feeling that maybe I was hooking up to the coax incorrectly.

Spice seems to have two txline models, the ideal and the lossy. For
some reason, the lossy-line model seems to work better, even if you
crank the losses down to invisibility.

---
As far as I can tell, LTspice only has the lossless line.
---

Seems strange, since it worked for a while, so I sent a request for
clarification to the LTspice user group.

They're excellent with feedback, so when I get an answer I'll modify
my circuit, as required, and post it.
---

Also, what you neglected to mention was that, even if what you said
was true, it would only be true at one frequency and, even at that
frequency, the reading of the 8567A's attenuator would be wrong with
respect to power going into the load.

A good signal generator should be a 50 ohm source at all frequencies.

John

---
Yes, of course.

We all know that, but a signal generator with an attenuator designed
and legended to drive a resistive load will only be accurate at the
frequency where the capacitor's reactance is such that the voltage
across it will be half that of the open-circuit voltage of the
generator.

Gibberish.

---
TP1 on my sim shows the frequency response at the output of a properly
terminated line: flat.

TP3 and 4 show the response with different capacitive loads.

How, in all seriousness, can you believe that an attenuator designed
for use where the load impedance is constant and matched to the line
and source will function properly when the load impedance varies?
---

And, even at that, the attenuator will only be useful in determining
the voltage across the load, not the power into it, as displayed,
since the load is reactive.

Ditto.

---
Just for grins, assume that you've got a matched system and that the
attenuator is set to output 100 microvolts into the 50 ohm load at 200
MHz.

Now assume that the attenuator hasn't been touched, but that the 50
ohm load was replaced by a 27 pF capacitor, and that the voltage
across the cap at the 200MHz point is 100 microvolts.

Take a look at TP4 and tell me where else along the curve the voltage
across the cap is 100 microvolts, OK?

---
JF

 

[John Fields]

On Sun, 12 Dec 2010 12:04:34 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Fields"


** I set up the same conditions as the OP described - ie an RF generator,
1.5 meters of RG58 co-ax and a 60MHz scope with 1 Mohm //20pF input.

AS EXPECTED the input impedance of the cable dips sharply at the quarter
wave frequency, 22MHz for my set up - or 28 MHz if the cable is left
completely unterminated. This was determined by monitoring the signal at the
output connector of the RF gen with a 10:1 probe.

However, at the scope end of the cable there is little sign of any dipping
or peaking.

So, the cable acts like a quarter wave stub all right, but the unterminated
end has a fairly consistent signal level since peaking and dipping occur
simultaneously at opposite ends.

Sue as hell looks like standing waves at play to me.



.... Phil

---
I agree.

Run an AC analysis of:

news:5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and take a look at the inputs of the transmission lines for your
Himalayas!

---
JF

 

[John Larkin]

On Sun, 12 Dec 2010 05:05:05 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 12:04:34 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


"John Fields"


** I set up the same conditions as the OP described - ie an RF generator,
1.5 meters of RG58 co-ax and a 60MHz scope with 1 Mohm //20pF input.

AS EXPECTED the input impedance of the cable dips sharply at the quarter
wave frequency, 22MHz for my set up - or 28 MHz if the cable is left
completely unterminated. This was determined by monitoring the signal at the
output connector of the RF gen with a 10:1 probe.

However, at the scope end of the cable there is little sign of any dipping
or peaking.

So, the cable acts like a quarter wave stub all right, but the unterminated
end has a fairly consistent signal level since peaking and dipping occur
simultaneously at opposite ends.

Sue as hell looks like standing waves at play to me.



.... Phil

---
I agree.

Run an AC analysis of:

news:5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and take a look at the inputs of the transmission lines for your
Himalayas!

But not at the scope. One normally doesn't open up a coax and probe
points internally.

"Standing waves" do not explain the OP's issue. A 50 ohm generator,
driving a 50 ohm coax, into a capacitive load, is flat at the load, up
to the -3 dB corner, roughly 200 MHz for a typical scope Cin. Then it
rolls off nicely at 6 dB/octave. There are no frequency peaks or dips
at the load.

Below the corner frequency, the scope sees the same voltage as the
unloaded generator would put out at its connector, at all frequencies,
no matter how long the cable is.

If you add a 50 ohm feedthru terminator at the scope, the amplitude
drops in half and the corner frequency doubles, independent of cable
length. Still no whoopy-doos.

(I have described the asymptotic behavior, which is usual in this
business. Don't get preachy about a dB or so near the corner
frequency.)

John

 

[John Larkin]

On Sun, 12 Dec 2010 04:52:27 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 11 Dec 2010 17:29:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 11 Dec 2010 17:41:22 -0600, John Fields
jfields@austininstruments.com> wrote:


---
An ideal transmission line will exhibit only delay and insertion loss,
the insertion loss qualifying it as an attenuator, albeit a tiny one
for a short line at a low frequency.

How could an ideal transmission line have loss? It only includes
inductance and capacitance, so it can't dissipate power.

---
Poor choice of terms, thanks.

I should've said "real" instead of "ideal".
---

An ideal 50 ohm transmission line, used to extend the output of a 50
ohm signal generator, adds time delay. Nothing else.

---
Not true.

It can also be used as an impedance transformer:

http://en.wikipedia.org/wiki/Quarter_wave_impedance_transformer

The first thing you see on that wiki page, in huge print, is the
impedance equation. In our case Zin/Z0 = 1, so there is no impedance
transformation. The generator is 50 ohms. The cable is 50 ohms. The
end of the coax looks like a 50 ohm source to any load, just like the
generator looks at its output connector with no external coax in the
path. As seen at the load, the whole point of this thread, the coax
adds time delay, nothing else.

Invoking "standing waves" to explain the OPs observations is just
wrong.

---


Hook that to a 15 pF scope input and you get a 1st
order lowpass response with corner around 200 MHz or so. No standing
waves or resonances at all.

---
At the capacitive end of the cable that's true, but you don't get the
-6dB rolloff that you'd expect, you get the corner at -3dB and the
skirt falls off at -3dB per octave.

Insane. Sorry to say so, but insane.

---
Sorry, my ass...

You revel in being insulting.

You insist on lecturing about stuff you don't understand.

John

 

On Sat, 11 Dec 2010 08:34:56 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 10 Dec 2010 06:04:29 -0500, JW <none@dev.null> wrote:

On Thu, 09 Dec 2010 20:11:18 -0600 entropy4269@yahoo.com wrote in Message
id: <cj23g61a5u47t2785nfl53rnd08ndere4v@4ax.com>:

OK, this is real basic but I'm not getting it. I hooked up my HP
8657A signal gerator into my TDS 210 oscope with RG 58a/u ,which is 50
ohm to match my input and output. The trouble is , the voltage
readings on my scope do not match my output readings on the signal
generator., sometimes off by a factor of 3 to 4. The scope is set for
1X probe . Why the hugh mismatch? thanks jk

Since the TDS210 scope does not have 50 ohm input coupling (As many scopes
do) the voltage will be double what is expected.

Right. The end of the coax looks like a 50 ohm source, so a 50 ohm
load will form a simple voltage divider with gain of 0.5.

The frequency response is the same either way, flat until the scope
capacitance or the scope bandwidth starts to roll it off.

RF signal generators are usually spec'd/calibrated to deliver some
voltage or power into a 50 ohm load. Function generators are usually
spec'd/calibrated to deliver some voltage into an open circuit. The
difference is 2:1 voltage.

As Phil said, get a 50
ohm terminator to properly view the output. If you're not too concerned
with signal integrity, you can solder a 50 ohm resistor across a coax T
connector.

The waveform will be the same, just different amplitude.

John

Yes, I placed a 50 ohm terminator at the scope end and now thw rms on
the scope is reading same as the generator. Now I just have to figure
the best cable length for single pulses that measure 3.5 ns at FWHM,
so I can get a good picture on the scope of these pulses. Any thoughts
on what frequency/wavelength to assume? A sine approx with period of
7ns?? thanks jk

 

[Phil Allison]

<jfisher864@comcast.net>

Yes, I placed a 50 ohm terminator at the scope end and now thw rms on
the scope is reading same as the generator.

** Finally.

Now I just have to figure
the best cable length for single pulses that measure 3.5 ns at FWHM,
so I can get a good picture on the scope of these pulses.

** Not very likely with a 60MHz scope with a 6nS rise time.

You really are a babe in the woods.

BTW

It is bad manners to use more than one email addy on a newsgroup.



..... Phil

 

[John Larkin]

On Sun, 12 Dec 2010 16:41:59 -0600, jfisher864@comcast.net wrote:

On Sat, 11 Dec 2010 08:34:56 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 10 Dec 2010 06:04:29 -0500, JW <none@dev.null> wrote:

On Thu, 09 Dec 2010 20:11:18 -0600 entropy4269@yahoo.com wrote in Message
id: <cj23g61a5u47t2785nfl53rnd08ndere4v@4ax.com>:

OK, this is real basic but I'm not getting it. I hooked up my HP
8657A signal gerator into my TDS 210 oscope with RG 58a/u ,which is 50
ohm to match my input and output. The trouble is , the voltage
readings on my scope do not match my output readings on the signal
generator., sometimes off by a factor of 3 to 4. The scope is set for
1X probe . Why the hugh mismatch? thanks jk

Since the TDS210 scope does not have 50 ohm input coupling (As many scopes
do) the voltage will be double what is expected.

Right. The end of the coax looks like a 50 ohm source, so a 50 ohm
load will form a simple voltage divider with gain of 0.5.

The frequency response is the same either way, flat until the scope
capacitance or the scope bandwidth starts to roll it off.

RF signal generators are usually spec'd/calibrated to deliver some
voltage or power into a 50 ohm load. Function generators are usually
spec'd/calibrated to deliver some voltage into an open circuit. The
difference is 2:1 voltage.

As Phil said, get a 50
ohm terminator to properly view the output. If you're not too concerned
with signal integrity, you can solder a 50 ohm resistor across a coax T
connector.

The waveform will be the same, just different amplitude.

John


Yes, I placed a 50 ohm terminator at the scope end and now thw rms on
the scope is reading same as the generator. Now I just have to figure
the best cable length for single pulses that measure 3.5 ns at FWHM,
so I can get a good picture on the scope of these pulses. Any thoughts
on what frequency/wavelength to assume? A sine approx with period of
7ns?? thanks jk

If it's terminated at either end, or at both ends, the cable length
won't matter. Very long coax runs can degrade rise/fall times, but a
few feet of RG58 won't have any noticable effect at risetimes of, say,
1 ns.

You'll need a fast scope, 200 MHz at the very least, to measure a 3.5
ns wide pulse. 500 MHz would be better.

John

 

[Jamie]

John Larkin wrote:

On Sun, 12 Dec 2010 16:41:59 -0600, jfisher864@comcast.net wrote:


On Sat, 11 Dec 2010 08:34:56 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:


On Fri, 10 Dec 2010 06:04:29 -0500, JW <none@dev.null> wrote:


On Thu, 09 Dec 2010 20:11:18 -0600 entropy4269@yahoo.com wrote in Message
id: <cj23g61a5u47t2785nfl53rnd08ndere4v@4ax.com>:


OK, this is real basic but I'm not getting it. I hooked up my HP
8657A signal gerator into my TDS 210 oscope with RG 58a/u ,which is 50
ohm to match my input and output. The trouble is , the voltage
readings on my scope do not match my output readings on the signal
generator., sometimes off by a factor of 3 to 4. The scope is set for
1X probe . Why the hugh mismatch? thanks jk

Since the TDS210 scope does not have 50 ohm input coupling (As many scopes
do) the voltage will be double what is expected.

Right. The end of the coax looks like a 50 ohm source, so a 50 ohm
load will form a simple voltage divider with gain of 0.5.

The frequency response is the same either way, flat until the scope
capacitance or the scope bandwidth starts to roll it off.

RF signal generators are usually spec'd/calibrated to deliver some
voltage or power into a 50 ohm load. Function generators are usually
spec'd/calibrated to deliver some voltage into an open circuit. The
difference is 2:1 voltage.

As Phil said, get a 50

ohm terminator to properly view the output. If you're not too concerned
with signal integrity, you can solder a 50 ohm resistor across a coax T
connector.

The waveform will be the same, just different amplitude.

John


Yes, I placed a 50 ohm terminator at the scope end and now thw rms on
the scope is reading same as the generator. Now I just have to figure
the best cable length for single pulses that measure 3.5 ns at FWHM,
so I can get a good picture on the scope of these pulses. Any thoughts
on what frequency/wavelength to assume? A sine approx with period of
7ns?? thanks jk


If it's terminated at either end, or at both ends, the cable length
won't matter. Very long coax runs can degrade rise/fall times, but a
few feet of RG58 won't have any noticable effect at risetimes of, say,
1 ns.

You'll need a fast scope, 200 MHz at the very least, to measure a 3.5
ns wide pulse. 500 MHz would be better.

John

That is why I save my 500 Mhz Old Tek scope for things like that

 

[John Fields]

On Sun, 12 Dec 2010 10:16:50 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 05:05:05 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 12:04:34 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


"John Fields"


** I set up the same conditions as the OP described - ie an RF generator,
1.5 meters of RG58 co-ax and a 60MHz scope with 1 Mohm //20pF input.

AS EXPECTED the input impedance of the cable dips sharply at the quarter
wave frequency, 22MHz for my set up - or 28 MHz if the cable is left
completely unterminated. This was determined by monitoring the signal at the
output connector of the RF gen with a 10:1 probe.

However, at the scope end of the cable there is little sign of any dipping
or peaking.

So, the cable acts like a quarter wave stub all right, but the unterminated
end has a fairly consistent signal level since peaking and dipping occur
simultaneously at opposite ends.

Sue as hell looks like standing waves at play to me.



.... Phil

---
I agree.

Run an AC analysis of:

news:5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and take a look at the inputs of the transmission lines for your
Himalayas!


But not at the scope. One normally doesn't open up a coax and probe
points internally.

---
Depends.

Ever heard of a slotted line?
---

"Standing waves" do not explain the OP's issue. A 50 ohm generator,
driving a 50 ohm coax, into a capacitive load, is flat at the load, up
to the -3 dB corner, roughly 200 MHz for a typical scope Cin. Then it
rolls off nicely at 6 dB/octave. There are no frequency peaks or dips
at the load.

---

From the 200MHz corner it rolls off at 3db per octave until it gets to
400MHz, and _then_ it rolls off at 6dB per octave after that.
---

Below the corner frequency, the scope sees the same voltage as the
unloaded generator would put out at its connector, at all frequencies,
no matter how long the cable is.

---
Well, being picky, and since the lowpass isn't a brickwall filter, not
really.

The RC forms a voltage divider, so as the reactance of the cap changes
with frequency, so also does the voltage into the scope even if the
output amplitude from the generator remains constant.

of course the change would be very small, far from the corner
frequency, but the output voltage from the generator and the output
from the divider still wouldn't be the same.
---

If you add a 50 ohm feedthru terminator at the scope, the amplitude
drops in half and the corner frequency doubles, independent of cable
length. Still no whoopy-doos.

---
Driving the line through, and terminating it with its characteristic
impedance makes it transparent, except for delay and attenuation
losses with regard to its length, so there's no surprise there.

I was somewhat taken aback by the change in the terminated VS
unterminated corner frequency until I realized that the current
divided between the cap and the load resistor.

Thanks for that.
---

(I have described the asymptotic behavior, which is usual in this
business. Don't get preachy about a dB or so near the corner
frequency.)

John

---
Your admonition falls on deaf ears and since, if it pleases me to do
so, why shouldn't I?

---
JF

 

[John Fields]

On Sun, 12 Dec 2010 10:35:39 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 04:52:27 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sat, 11 Dec 2010 17:29:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 11 Dec 2010 17:41:22 -0600, John Fields
jfields@austininstruments.com> wrote:


---
An ideal transmission line will exhibit only delay and insertion loss,
the insertion loss qualifying it as an attenuator, albeit a tiny one
for a short line at a low frequency.

How could an ideal transmission line have loss? It only includes
inductance and capacitance, so it can't dissipate power.

---
Poor choice of terms, thanks.

I should've said "real" instead of "ideal".
---

An ideal 50 ohm transmission line, used to extend the output of a 50
ohm signal generator, adds time delay. Nothing else.

---
Not true.

It can also be used as an impedance transformer:

http://en.wikipedia.org/wiki/Quarter_wave_impedance_transformer

The first thing you see on that wiki page, in huge print, is the
impedance equation. In our case Zin/Z0 = 1, so there is no impedance
transformation. The generator is 50 ohms. The cable is 50 ohms. The
end of the coax looks like a 50 ohm source to any load, just like the
generator looks at its output connector with no external coax in the
path. As seen at the load, the whole point of this thread, the coax
adds time delay, nothing else.

Invoking "standing waves" to explain the OPs observations is just
wrong.


---


Hook that to a 15 pF scope input and you get a 1st
order lowpass response with corner around 200 MHz or so. No standing
waves or resonances at all.

---
At the capacitive end of the cable that's true, but you don't get the
-6dB rolloff that you'd expect, you get the corner at -3dB and the
skirt falls off at -3dB per octave.

Insane. Sorry to say so, but insane.

---
Sorry, my ass...

You revel in being insulting.

You insist on lecturing about stuff you don't understand.

---
Perhaps, but if, at the corner frequency, the amplitude of the signal
is down 3 dB and at twice the corner frequency it's down by another
3dB, how is that 6dB per octave?

---
JF

 

[John Larkin]

On Sun, 12 Dec 2010 19:23:27 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 10:16:50 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 05:05:05 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 12:04:34 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


"John Fields"


** I set up the same conditions as the OP described - ie an RF generator,
1.5 meters of RG58 co-ax and a 60MHz scope with 1 Mohm //20pF input.

AS EXPECTED the input impedance of the cable dips sharply at the quarter
wave frequency, 22MHz for my set up - or 28 MHz if the cable is left
completely unterminated. This was determined by monitoring the signal at the
output connector of the RF gen with a 10:1 probe.

However, at the scope end of the cable there is little sign of any dipping
or peaking.

So, the cable acts like a quarter wave stub all right, but the unterminated
end has a fairly consistent signal level since peaking and dipping occur
simultaneously at opposite ends.

Sue as hell looks like standing waves at play to me.



.... Phil

---
I agree.

Run an AC analysis of:

news:5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and take a look at the inputs of the transmission lines for your
Himalayas!


But not at the scope. One normally doesn't open up a coax and probe
points internally.

---
Depends.

Ever heard of a slotted line?
---

"Standing waves" do not explain the OP's issue. A 50 ohm generator,
driving a 50 ohm coax, into a capacitive load, is flat at the load, up
to the -3 dB corner, roughly 200 MHz for a typical scope Cin. Then it
rolls off nicely at 6 dB/octave. There are no frequency peaks or dips
at the load.

---

From the 200MHz corner it rolls off at 3db per octave until it gets to
400MHz, and _then_ it rolls off at 6dB per octave after that.

It rolls off at all possible slopes from udB/octave far below the
corner to almost 6 dB/octave far above. I was describing the
asymptotic bahavior, which is the way people talk in this business.

---

Below the corner frequency, the scope sees the same voltage as the
unloaded generator would put out at its connector, at all frequencies,
no matter how long the cable is.

---
Well, being picky, and since the lowpass isn't a brickwall filter, not
really.

The RC forms a voltage divider, so as the reactance of the cap changes
with frequency, so also does the voltage into the scope even if the
output amplitude from the generator remains constant.

of course the change would be very small, far from the corner
frequency, but the output voltage from the generator and the output
from the divider still wouldn't be the same.
---

If you add a 50 ohm feedthru terminator at the scope, the amplitude
drops in half and the corner frequency doubles, independent of cable
length. Still no whoopy-doos.

---
Driving the line through, and terminating it with its characteristic
impedance makes it transparent, except for delay and attenuation
losses with regard to its length, so there's no surprise there.

I was somewhat taken aback by the change in the terminated VS
unterminated corner frequency until I realized that the current
divided between the cap and the load resistor.

Thanks for that.
---


(I have described the asymptotic behavior, which is usual in this
business. Don't get preachy about a dB or so near the corner
frequency.)

John

---
Your admonition falls on deaf ears and since, if it pleases me to do
so, why shouldn't I?

Because it makes you look ignorant.

Did you ever have a formal EE education? They teach this stuff, and
establish the terminologies that most EEs use.

John

 

[John Larkin]

On Sun, 12 Dec 2010 19:33:19 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 10:35:39 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 04:52:27 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sat, 11 Dec 2010 17:29:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 11 Dec 2010 17:41:22 -0600, John Fields
jfields@austininstruments.com> wrote:


---
An ideal transmission line will exhibit only delay and insertion loss,
the insertion loss qualifying it as an attenuator, albeit a tiny one
for a short line at a low frequency.

How could an ideal transmission line have loss? It only includes
inductance and capacitance, so it can't dissipate power.

---
Poor choice of terms, thanks.

I should've said "real" instead of "ideal".
---

An ideal 50 ohm transmission line, used to extend the output of a 50
ohm signal generator, adds time delay. Nothing else.

---
Not true.

It can also be used as an impedance transformer:

http://en.wikipedia.org/wiki/Quarter_wave_impedance_transformer

The first thing you see on that wiki page, in huge print, is the
impedance equation. In our case Zin/Z0 = 1, so there is no impedance
transformation. The generator is 50 ohms. The cable is 50 ohms. The
end of the coax looks like a 50 ohm source to any load, just like the
generator looks at its output connector with no external coax in the
path. As seen at the load, the whole point of this thread, the coax
adds time delay, nothing else.

Invoking "standing waves" to explain the OPs observations is just
wrong.


---


Hook that to a 15 pF scope input and you get a 1st
order lowpass response with corner around 200 MHz or so. No standing
waves or resonances at all.

---
At the capacitive end of the cable that's true, but you don't get the
-6dB rolloff that you'd expect, you get the corner at -3dB and the
skirt falls off at -3dB per octave.

Insane. Sorry to say so, but insane.

---
Sorry, my ass...

You revel in being insulting.

You insist on lecturing about stuff you don't understand.

---
Perhaps, but if, at the corner frequency, the amplitude of the signal
is down 3 dB and at twice the corner frequency it's down by another
3dB, how is that 6dB per octave?

6 dB/octave is the macroscopic slope of the rolloff, the zoomed-out,
ultimate slope as you go up and up in frequency. The asymptotic
frequency response is what EEs usually describe in words like "flat to
200 MHz, then rolling off at -6 dB/octave." Everybody knows that, if
you zoom in on that, it's actually a soft corner, not a sharp change
of slope at 200 MHz.

It's like saying "this road is flat for a mile, then slopes up at 5
degrees." The transition isn't knife-sharp, but the description is
useful.

It's 6 dB/octave because the reactance of the cap is inverse on
frequency. That's all that 6 dB/octave means: above the corner, the
voltage goes as 1/f. You've got to get away from the immediate
vicinity of the 3 dB point for that to be mainly true.

I didn't make any of this up. It's the way EEs are taught and the way
we talk.

http://en.wikipedia.org/wiki/Bode_plot

John

 

[John Larkin]

On Sun, 12 Dec 2010 19:48:27 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

John Larkin wrote:

On Sun, 12 Dec 2010 16:41:59 -0600, jfisher864@comcast.net wrote:


On Sat, 11 Dec 2010 08:34:56 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:


On Fri, 10 Dec 2010 06:04:29 -0500, JW <none@dev.null> wrote:


On Thu, 09 Dec 2010 20:11:18 -0600 entropy4269@yahoo.com wrote in Message
id: <cj23g61a5u47t2785nfl53rnd08ndere4v@4ax.com>:


OK, this is real basic but I'm not getting it. I hooked up my HP
8657A signal gerator into my TDS 210 oscope with RG 58a/u ,which is 50
ohm to match my input and output. The trouble is , the voltage
readings on my scope do not match my output readings on the signal
generator., sometimes off by a factor of 3 to 4. The scope is set for
1X probe . Why the hugh mismatch? thanks jk

Since the TDS210 scope does not have 50 ohm input coupling (As many scopes
do) the voltage will be double what is expected.

Right. The end of the coax looks like a 50 ohm source, so a 50 ohm
load will form a simple voltage divider with gain of 0.5.

The frequency response is the same either way, flat until the scope
capacitance or the scope bandwidth starts to roll it off.

RF signal generators are usually spec'd/calibrated to deliver some
voltage or power into a 50 ohm load. Function generators are usually
spec'd/calibrated to deliver some voltage into an open circuit. The
difference is 2:1 voltage.

As Phil said, get a 50

ohm terminator to properly view the output. If you're not too concerned
with signal integrity, you can solder a 50 ohm resistor across a coax T
connector.

The waveform will be the same, just different amplitude.

John


Yes, I placed a 50 ohm terminator at the scope end and now thw rms on
the scope is reading same as the generator. Now I just have to figure
the best cable length for single pulses that measure 3.5 ns at FWHM,
so I can get a good picture on the scope of these pulses. Any thoughts
on what frequency/wavelength to assume? A sine approx with period of
7ns?? thanks jk


If it's terminated at either end, or at both ends, the cable length
won't matter. Very long coax runs can degrade rise/fall times, but a
few feet of RG58 won't have any noticable effect at risetimes of, say,
1 ns.

You'll need a fast scope, 200 MHz at the very least, to measure a 3.5
ns wide pulse. 500 MHz would be better.

John

That is why I save my 500 Mhz Old Tek scope for things like that

We have one working 7104 (1 GHz, analog, microchannel plate) scope we
use occasionally. All our other analog scopes are retired.

We have a bunch of Tek 11801 sampling scopes, with sampling heads that
range from 3 GHz to 40 GHz. They are fairly cheap on ebay these days.

John

 

[Jasen Betts]

On 2010-12-11, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

No. Given a 50 ohm generator driving a 50 ohm cable, open on the far
end, the end of the coax looks like a 50 ohm source. The transmission
line doesn't look like an attenuator unless it's lossy, which effect
will be negligable for a few feet of RG58 at reasonable frequencies.

not really. the unterminated end of the coax looks like a mirror,
asif the coax continued for the same length to an identical source
producing an identical signal.

if the end is short-circuited at the end it looks like a mirror (as
above) but with the other source producing an exact inverted signal.

Any reflections will be absorbed by the generator impedance. If you
sweep the gen frequency, the scope will see a voltage that looks just
like a 1st order RC lowpass response, flat to about 200 MHz and then
rolling off at -6 dB per octave. If there are any standing waves, they
are somewhere inside the cable, totally invisible from outside; they
won't cause frequency peaks or dips at the scope.

yeah, the reflections will be in-phase with the source signal at the
unterminated end, so there they will always add constructively.

A good signal generator should be a 50 ohm source at all frequencies.

If it's not a good one the reflections may be partially reflected from
it and add constructively or destructively to the source signal.

--
⚂⚃ 100% natural

 

[John Fields]

On Sun, 12 Dec 2010 18:27:50 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 19:23:27 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 10:16:50 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 05:05:05 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 12:04:34 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


"John Fields"


** I set up the same conditions as the OP described - ie an RF generator,
1.5 meters of RG58 co-ax and a 60MHz scope with 1 Mohm //20pF input.

AS EXPECTED the input impedance of the cable dips sharply at the quarter
wave frequency, 22MHz for my set up - or 28 MHz if the cable is left
completely unterminated. This was determined by monitoring the signal at the
output connector of the RF gen with a 10:1 probe.

However, at the scope end of the cable there is little sign of any dipping
or peaking.

So, the cable acts like a quarter wave stub all right, but the unterminated
end has a fairly consistent signal level since peaking and dipping occur
simultaneously at opposite ends.

Sue as hell looks like standing waves at play to me.



.... Phil

---
I agree.

Run an AC analysis of:

news:5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and take a look at the inputs of the transmission lines for your
Himalayas!


But not at the scope. One normally doesn't open up a coax and probe
points internally.

---
Depends.

Ever heard of a slotted line?
---

"Standing waves" do not explain the OP's issue. A 50 ohm generator,
driving a 50 ohm coax, into a capacitive load, is flat at the load, up
to the -3 dB corner, roughly 200 MHz for a typical scope Cin. Then it
rolls off nicely at 6 dB/octave. There are no frequency peaks or dips
at the load.

---

From the 200MHz corner it rolls off at 3db per octave until it gets to
400MHz, and _then_ it rolls off at 6dB per octave after that.

It rolls off at all possible slopes from udB/octave far below the
corner to almost 6 dB/octave far above. I was describing the
asymptotic bahavior, which is the way people talk in this business.


---

Below the corner frequency, the scope sees the same voltage as the
unloaded generator would put out at its connector, at all frequencies,
no matter how long the cable is.

---
Well, being picky, and since the lowpass isn't a brickwall filter, not
really.

The RC forms a voltage divider, so as the reactance of the cap changes
with frequency, so also does the voltage into the scope even if the
output amplitude from the generator remains constant.

of course the change would be very small, far from the corner
frequency, but the output voltage from the generator and the output
from the divider still wouldn't be the same.
---

If you add a 50 ohm feedthru terminator at the scope, the amplitude
drops in half and the corner frequency doubles, independent of cable
length. Still no whoopy-doos.

---
Driving the line through, and terminating it with its characteristic
impedance makes it transparent, except for delay and attenuation
losses with regard to its length, so there's no surprise there.

I was somewhat taken aback by the change in the terminated VS
unterminated corner frequency until I realized that the current
divided between the cap and the load resistor.

Thanks for that.
---


(I have described the asymptotic behavior, which is usual in this
business. Don't get preachy about a dB or so near the corner
frequency.)

John

---
Your admonition falls on deaf ears and since, if it pleases me to do
so, why shouldn't I?

Because it makes you look ignorant.

---
You take almost every opportunity you can, including lying and
cheating, to try to make me look ignorant and now you're seemingly
concerned about it?

Piss off, you pompous, patronizing windbag.

Plonk.
---
JF

 

[John Larkin]

On Mon, 13 Dec 2010 08:02:50 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 18:27:50 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 19:23:27 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 10:16:50 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 12 Dec 2010 05:05:05 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 12 Dec 2010 12:04:34 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


"John Fields"


** I set up the same conditions as the OP described - ie an RF generator,
1.5 meters of RG58 co-ax and a 60MHz scope with 1 Mohm //20pF input.

AS EXPECTED the input impedance of the cable dips sharply at the quarter
wave frequency, 22MHz for my set up - or 28 MHz if the cable is left
completely unterminated. This was determined by monitoring the signal at the
output connector of the RF gen with a 10:1 probe.

However, at the scope end of the cable there is little sign of any dipping
or peaking.

So, the cable acts like a quarter wave stub all right, but the unterminated
end has a fairly consistent signal level since peaking and dipping occur
simultaneously at opposite ends.

Sue as hell looks like standing waves at play to me.



.... Phil

---
I agree.

Run an AC analysis of:

news:5k49g6dkto6kak9k86v8fp5135kvqpi25q@4ax.com

and take a look at the inputs of the transmission lines for your
Himalayas!


But not at the scope. One normally doesn't open up a coax and probe
points internally.

---
Depends.

Ever heard of a slotted line?
---

"Standing waves" do not explain the OP's issue. A 50 ohm generator,
driving a 50 ohm coax, into a capacitive load, is flat at the load, up
to the -3 dB corner, roughly 200 MHz for a typical scope Cin. Then it
rolls off nicely at 6 dB/octave. There are no frequency peaks or dips
at the load.

---

From the 200MHz corner it rolls off at 3db per octave until it gets to
400MHz, and _then_ it rolls off at 6dB per octave after that.

It rolls off at all possible slopes from udB/octave far below the
corner to almost 6 dB/octave far above. I was describing the
asymptotic bahavior, which is the way people talk in this business.


---

Below the corner frequency, the scope sees the same voltage as the
unloaded generator would put out at its connector, at all frequencies,
no matter how long the cable is.

---
Well, being picky, and since the lowpass isn't a brickwall filter, not
really.

The RC forms a voltage divider, so as the reactance of the cap changes
with frequency, so also does the voltage into the scope even if the
output amplitude from the generator remains constant.

of course the change would be very small, far from the corner
frequency, but the output voltage from the generator and the output
from the divider still wouldn't be the same.
---

If you add a 50 ohm feedthru terminator at the scope, the amplitude
drops in half and the corner frequency doubles, independent of cable
length. Still no whoopy-doos.

---
Driving the line through, and terminating it with its characteristic
impedance makes it transparent, except for delay and attenuation
losses with regard to its length, so there's no surprise there.

I was somewhat taken aback by the change in the terminated VS
unterminated corner frequency until I realized that the current
divided between the cap and the load resistor.

Thanks for that.
---


(I have described the asymptotic behavior, which is usual in this
business. Don't get preachy about a dB or so near the corner
frequency.)

John

---
Your admonition falls on deaf ears and since, if it pleases me to do
so, why shouldn't I?

Because it makes you look ignorant.

---
You take almost every opportunity you can, including lying and
cheating, to try to make me look ignorant and now you're seemingly
concerned about it?

You do it to yourself.

John