signal generator output

Guest
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
 
<entropy4269@yahoo.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?
Click to expand...

** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.


..... Phil
 
Thanks, Hmmmmm, is there a gereral formula I could use to predict the
voltage loss givin the scope impedence ect??

On Fri, 10 Dec 2010 13:28:24 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

entropy4269@yahoo.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?


** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.


.... Phil
Click to expand...
 
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
Click to expand...
Since the TDS210 scope does not have 50 ohm input coupling (As many scopes
do) the voltage will be double what is expected. 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.
 
On 12/10/2010 05:10 AM, jfisher864@comcast.net wrote:
Thanks, Hmmmmm, is there a gereral formula I could use to predict the
voltage loss givin the scope impedence ect??

On Fri, 10 Dec 2010 13:28:24 +1100, "Phil Allison"<phil_a@tpg.com.au
wrote:


entropy4269@yahoo.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?


** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.


.... Phil
Click to expand...
For a generator described by Vg and Rg (with Rg=Zo as in your case), the
transfer function VL/Vg is H=ZL/(Zo+ZL)exp(-theta).
In the lossless case (for short cable runs) theta is j*w*l/Vp, with l
the cable length [m] and Vp=2E8 [m/s]. In your case ZL=inf and abs(H)=1.

But, the generator usually displays the rms voltage to a matched load.
This means that you get twice the voltage for on open circuit (as in
your case) and the peak to peak voltage is sqrt(2) times the rms
voltage. This means an amplitude 2*1.41 times the displayed value.

Pere
 
On Fri, 10 Dec 2010 13:28:24 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

entropy4269@yahoo.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?


** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.
Click to expand...
If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source. 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.

John
 
My mistake, the scopes specs say the input impedence dc coupled is 1
M ohn. the 50 ohn terminator shouls still work??
jk
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. 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.
Click to expand...
 
On Fri, 10 Dec 2010 16:46:39 -0600, jfisher864@comcast.net wrote:

My mistake, the scopes specs say the input impedence dc coupled is 1
M ohn. the 50 ohn terminator shouls still work??
Click to expand...
---
Yes, and please bottom post.

---
JF
 
"John Larkin is a mental case"

Phil Allison

** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.



If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source. 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.
Click to expand...
** Total insanity.



..... Phil
 
On Fri, 10 Dec 2010 09:05:41 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 10 Dec 2010 13:28:24 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


entropy4269@yahoo.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?


** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.



If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source.
Click to expand...
---
For an arbitrary frequency and an arbitrary length of cable, that's
only true if the source impedance, the transmission line impedance,
and the load impedance are all equal and resistive, with the
transmission line looking like an attenuator.
---

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.
Click to expand...
---
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.

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.

---
JF
 
"John Fields"
John Larkin

If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source.

---
For an arbitrary frequency and an arbitrary length of cable, that's
only true if the source impedance, the transmission line impedance,
and the load impedance are all equal and resistive, with the
transmission line looking like an attenuator.
---
Click to expand...
** The fuckwit error JL makes is in considering the scope input to be a
source.

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.



..... Phil
 
On Fri, 10 Dec 2010 18:32:22 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 10 Dec 2010 09:05:41 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 10 Dec 2010 13:28:24 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


entropy4269@yahoo.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?


** The scope is not 50 ohms input impedance.

You are seeing the effects of standing waves in the RG58 cable.

Use a BNC T adaptor and a 50 ohms dummy load at the scope end.



If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source.

---
For an arbitrary frequency and an arbitrary length of cable, that's
only true if the source impedance, the transmission line impedance,
and the load impedance are all equal and resistive, with the
transmission line looking like an attenuator.
Click to expand...
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.


---

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.

---
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.
Click to expand...
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.

LT Spice has a tranny model, so you can try 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.
Click to expand...
A good signal generator should be a 50 ohm source at all frequencies.

John
 
On Sat, 11 Dec 2010 11:39:59 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Fields"
John Larkin

If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source.

---
For an arbitrary frequency and an arbitrary length of cable, that's
only true if the source impedance, the transmission line impedance,
and the load impedance are all equal and resistive, with the
transmission line looking like an attenuator.
---

** The fuckwit error JL makes is in considering the scope input to be a
source.
Click to expand...
Crazy. I'm considering the scope input to look like a 15 pF cap. If
it's different, the 3 dB rolloff point just moves.

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.
Click to expand...
Not at all; it will be flat up the the 3dB rolloff point, about 200
MHz in my example. Look up "source termination."

Try this: connect a fast 50 ohm pulse generator, through a length of
RG58, to a hi-z scope. A square pulse in will make a square pulse on
the scope, no ringing or anything. For long cables with a fast
generator/scope, you may see a little rising-edge slowdown from cable
losses.

Try it. This ain't audio!

John
 
"John Larkin is INSANE "

** The fuckwit error JL makes is in considering the scope input to be a
source.

Crazy. I'm considering the scope input to look like a 15 pF cap.
Click to expand...
" If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source. "

** Makes no sense at all.


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.


Not at all;
Click to expand...

** Of course it will - you fucking NUT CASE !

Never heard of a " quarter wave stub " ??




..... Phil
 
On Sat, 11 Dec 2010 11:39:59 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Fields"
John Larkin

If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source.

---
For an arbitrary frequency and an arbitrary length of cable, that's
only true if the source impedance, the transmission line impedance,
and the load impedance are all equal and resistive, with the
transmission line looking like an attenuator.
---

** The fuckwit error JL makes is in considering the scope input to be a
source.
Click to expand...
---
I disagree.

He states that the end of the cable feeding the load is the source,
which would make the scope input the load.
---

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.
Click to expand...
---
Indeed.

Nice call. :)

---
JF
 
On Fri, 10 Dec 2010 19:23:54 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 11 Dec 2010 11:39:59 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:


"John Fields"
John Larkin

If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source.

---
For an arbitrary frequency and an arbitrary length of cable, that's
only true if the source impedance, the transmission line impedance,
and the load impedance are all equal and resistive, with the
transmission line looking like an attenuator.
---

** The fuckwit error JL makes is in considering the scope input to be a
source.

---
I disagree.

He states that the end of the cable feeding the load is the source,
which would make the scope input the load.
---

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.

---
Indeed.

Nice call. :)
Click to expand...

Wrong. Try it.

John
 
On Sat, 11 Dec 2010 12:16:14 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Larkin is INSANE "


** The fuckwit error JL makes is in considering the scope input to be a
source.

Crazy. I'm considering the scope input to look like a 15 pF cap.

" If the generator is a 50 ohm source, the end of the coax looks like a
50 ohm source. "

** Makes no sense at all.


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.


Not at all;


** Of course it will - you fucking NUT CASE !

Never heard of a " quarter wave stub " ??




.... Phil
Click to expand...
Good grief, you don't understand transmission lines. Too much audio, I
suppose.

Try it. 50 ohm generator, 50 ohm transmission line, capacitive load.

Real life or in Spice.

John
 
"John Larkin is INSANE "


** The cunt needs a bullet in his head.



.... Phil
 
"John Larkin is a TOTALLY INSANE autistic pig.


** Fraid he has been that way since birth.





..... Phil
 
On Sat, 11 Dec 2010 12:45:43 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Larkin is INSANE "


** The cunt needs a bullet in his head.



... Phil
Click to expand...
Chicken! Try it.

John
 
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