Measuring 50Ohm load terminator across frequency?

[Dummy]

How do I measure the 50Ohm load terminator performance across
frequency, let's say at VHF range?
One option is to use network analyzer.
Maybe multimeter? But that is only in DC mode, not across certain band
of frequency. So what's the best and simplest way to do the
measurement without using network analyzer? Please shed some light.

 

[John Larkin]

How do I measure the 50Ohm load terminator performance across
frequency, let's say at VHF range?
One option is to use network analyzer.
Maybe multimeter? But that is only in DC mode, not across certain band
of frequency. So what's the best and simplest way to do the
measurement without using network analyzer? Please shed some light.

The two classic ways would be to use a network analyzer (frequency
domain) or TDR (time domain). You could also measure VSWR over a
frequency range, which is really scalar network analysis, I guess.

You can do TDR with a pulse generator and a scope, but they have to be
pretty fast to get the equivalent of VHF frequency results.

John

 

[Tweetldee]

You could do it with a sweep generator loosely coupled to the termination.
Then hang a detector across it and plot the result on a scope in XY mode.
Hook the sweep ramp to the X channel, the detector output to the Y channel.
Classic Freq response hookup.

--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Never take a laxative and a sleeping pill at the same time!!

 

[John Woodgate]

How do I measure the 50Ohm load terminator performance across
frequency, let's say at VHF range?
One option is to use network analyzer.
Maybe multimeter? But that is only in DC mode, not across certain band
of frequency. So what's the best and simplest way to do the
measurement without using network analyzer? Please shed some light.

You need a source of signals that delivers the same voltage at all
relevant frequencies and a voltmeter for the relevant frequency range.
You apply signals at known frequencies and measure the voltage across
the 50 ohm. It should not change with frequency.

For the signal source, you can use a signal generator or a sweep
generator. For the voltmeter you can use an actual r.f. voltmeter, a
fast-enough scope, the 'other half' of the sweep generator or a spectrum
analyser.

But 50 ohm terminations, even quite crude ones (such as two 100 ohm
resistors in parallel inside a BNC connector), work quite well. It isn't
normally necessary to query their performance.

 

[Tim Shoppa]

How do I measure the 50Ohm load terminator performance across
frequency, let's say at VHF range?
One option is to use network analyzer.
Maybe multimeter? But that is only in DC mode, not across certain band
of frequency. So what's the best and simplest way to do the
measurement without using network analyzer? Please shed some light.

*If* the terminator had any resonances in the VHF region, you could
measure them with a grid dip meter and (by adding additional L or C)
estimate the L and C components.

Problem with this, is that I cannot imagine a terminator so lousy that
it would have a resonance in the VHF region. I've seen terminators fail
open, but a multimeter will tell you that. What makes you suspect that your
terminator is not worthy of VHF? (This isn't a Cantenna or something
like that, is it? I assumed that it was a commercial terminator made out
of a chip resistor...)

Tim.

 

[Paul Burridge]

You can do TDR with a pulse generator and a scope, but they have to be
pretty fast to get the equivalent of VHF frequency results.

How does this method work, then?

 

[Jim Weir]

Isn't usenet wonderful. Five answers, one from a person who knows what they are
talking about and four from people who have never done a vswr sweep in their
lives.

Use a sweep generator and reflection (vswr) bridge. The cheapo and crude guts
of a network analyzer.

Jim



[email protected] (Dummy)
shared these priceless pearls of wisdom:

->How do I measure the 50Ohm load terminator performance across
->frequency, let's say at VHF range?
->One option is to use network analyzer.
->Maybe multimeter? But that is only in DC mode, not across certain band
->of frequency. So what's the best and simplest way to do the
->measurement without using network analyzer? Please shed some light.

 

[John Woodgate]

Isn't usenet wonderful. Five answers, one from a person who knows what
they are talking about and four from people who have never done a vswr
sweep in their lives.

Use a sweep generator and reflection (vswr) bridge. The cheapo and
crude guts of a network analyzer.

It's a question of assessing the OP's level of knowledge and likely
access to equipment. Your advice is good, IF the OP has a VSWR bridge.
If not, you could have posted random characters.

 

[GPG]

Two methods come to mind. 1. noise bridge, see amateur radio
pblications, cheap.

2. SWEEP GEN---50R---LENGTH OF CABLE--TERM
l
l
SCOPE-<-DETECTOR--<--l

Scope and gen use same sweep, germanium diode in detector gives best
results.
Length of cable causes refected phase relative to gen to change
giving a ripple display on scope, ripple frequency dependent on cable
length.
Higher amlitude, poorer match. Used this (Kay multisweep) to check
antenna
nstallation. Quite useful at VHF with moderate cable runs.

 

[John Larkin]

How does this method work, then?

A 'real' TDR does it all for you. But you can do this:

Fast 50-ohm pulse gen.....tee connector into scope.....coax to DUT.

Program the pulser to make a clean positive pulse, many times longer
than expected cable delays. Observe the waveform. You'll see on the
leading edge...

stuff
----------------/////
|
|
----------

where 'stuff' is the echo from the terminator, about 2*Tc after the
main step, where Tc is prop delay from tee to DUT. Disconnect DUT to
get oriented; that should do...

---------------------- 2v
|
|
---------------- v
|
|
---------- 0


The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

John

 

[Paul Burridge]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]

The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

 

[Keith R. Williams]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]

The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

Time Domain Reflectometer (or reflectometry). Think of a TDR as a
network analyzer that has had an inverse Fourier transform. ;-) TDRs
are a rather useful piece of equipment.

 

[Paul Burridge]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]

The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

Time Domain Reflectometer (or reflectometry). Think of a TDR as a
network analyzer that has had an inverse Fourier transform. ;-) TDRs
are a rather useful piece of equipment.

Well I was going to buy a VNA at some point soon. Does that obviate
the need for a TDR or is it advisable to have a TDR in addition,
anyway? I mean, is there something the TDR can do that a VNA can't?

 

[John Larkin]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]
The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

Time Domain Reflectometer (or reflectometry). Think of a TDR as a
network analyzer that has had an inverse Fourier transform. ;-) TDRs
are a rather useful piece of equipment.

Well I was going to buy a VNA at some point soon. Does that obviate
the need for a TDR or is it advisable to have a TDR in addition,
anyway? I mean, is there something the TDR can do that a VNA can't?

If you plan to do RF (ie, sine waves, low amplitudes, Smith charts)
the VNA is much better. TDR data is harder to use for RF design, and
the step amplitudes are large so will mess up small-signal stuff like
semiconductors and amplifiers. TDR is great for analyzing PC boards
and transmission lines, and for poking around switching-type (time
domain) circuits.

TDR can directly plot, say, a graph of trace impedance versus distance
on a multilayer board. A dual-channel TDR can also be used as a very
fast pulse generator-oscilloscope combination.

John

 

[Ross Mac]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]
The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

Time Domain Reflectometer (or reflectometry). Think of a TDR as a
network analyzer that has had an inverse Fourier transform. ;-) TDRs
are a rather useful piece of equipment.

Well I was going to buy a VNA at some point soon. Does that obviate
the need for a TDR or is it advisable to have a TDR in addition,
anyway? I mean, is there something the TDR can do that a VNA can't?

If you plan to do RF (ie, sine waves, low amplitudes, Smith charts)
the VNA is much better. TDR data is harder to use for RF design, and
the step amplitudes are large so will mess up small-signal stuff like
semiconductors and amplifiers. TDR is great for analyzing PC boards
and transmission lines, and for poking around switching-type (time
domain) circuits.

TDR can directly plot, say, a graph of trace impedance versus distance
on a multilayer board. A dual-channel TDR can also be used as a very
fast pulse generator-oscilloscope combination.

John

Polar Instruments makes a good TDR. We used to use that device to measure
impedance for high speed telecom boards up to 52 layers....Damm those boards
are heavy!!
........Ross

 

[John Larkin]

Polar Instruments makes a good TDR. We used to use that device to measure
impedance for high speed telecom boards up to 52 layers....Damm those boards
are heavy!!
.......Ross

Polar is interesting: when Tek pulled out of the Channel Islands (a
silly venture, overall) they let the locals buy some of the guts of
their TDR products and set up their own business. They only go to
about 200 ps, so I'm guessing they use the innards of the little cable
TV handheld TDRs, which are the slowest stuff Tek ever made. Last time
I talked to Polar people, a couple years ago maybe, they were still
buying boards from Tek. They also have some nice looking field-solver
software, but it's expensive.

They really should buy my TDR deconvolution software.

John

 

[Ross Mac]

Polar is interesting: when Tek pulled out of the Channel Islands (a
silly venture, overall) they let the locals buy some of the guts of
their TDR products and set up their own business. They only go to
about 200 ps, so I'm guessing they use the innards of the little cable
TV handheld TDRs, which are the slowest stuff Tek ever made. Last time
I talked to Polar people, a couple years ago maybe, they were still
buying boards from Tek. They also have some nice looking field-solver
software, but it's expensive.

They really should buy my TDR deconvolution software.

John

I have heard that the head is a Tek design...I am not sure if they modified
it or it is the same one or not. The Polar TDR was the unit of choice for
the high speed PCB builders and did replace the older Tektronics TDR scopes.
It really boiled down to making sure the board manufacturers and the buyers
were using the same test equipment so parts wouldn't bounce. Matching test
equipment with the customer was usually my solution to these quality type
test problems....I was actually an automation guy so they would come to me
because the board business is usually pretty thin on electronic
experience....most of their people are process guys....
Have a great week John.....Ross

 

[Keith R. Williams]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]
The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

Time Domain Reflectometer (or reflectometry). Think of a TDR as a
network analyzer that has had an inverse Fourier transform. ;-) TDRs
are a rather useful piece of equipment.

Well I was going to buy a VNA at some point soon. Does that obviate
the need for a TDR or is it advisable to have a TDR in addition,
anyway? I mean, is there something the TDR can do that a VNA can't?

If you plan to do RF (ie, sine waves, low amplitudes, Smith charts)
the VNA is much better. TDR data is harder to use for RF design, and
the step amplitudes are large so will mess up small-signal stuff like
semiconductors and amplifiers. TDR is great for analyzing PC boards
and transmission lines, and for poking around switching-type (time
domain) circuits.

Absolutely! The theory of devices differs from application (I
hope no one took my flippant comments about the similarities at
face value). Network Analysis is in the domain of "small-
signals".

TDR can directly plot, say, a graph of trace impedance versus distance
on a multilayer board. A dual-channel TDR can also be used as a very
fast pulse generator-oscilloscope combination.

Hmm, a dual channel TDR? Is that anything like a VNA doing S12
in the time domain? ;-) Sorry!

 

[John Larkin]

Hmm, a dual channel TDR? Is that anything like a VNA doing S12
in the time domain? ;-) Sorry!

Absolutely. It's called TDT, Time Domain Transmission measurement. If
you put a fast step into port1 of a 2-port box, and scope what comes
out of port2, that's the time domain equivalent to S21. The port-1
TDR, which you can do simultaneously, is (almost) just a Fourier
transform away from S11.

A Tek SD-24 (DC-20 GHz) sampling head does dual-channel TDR and is
cool for stuff like this; just turn on the TDR step of one channel and
use the other channel in pure sampling mode. If you attenuate the
step, you can use this to test the time-domain response of wideband RF
amplifiers and things like that.

John

 

[Ian Buckner]

On Tue, 11 Nov 2003 19:53:29 -0800, John Larkin


[snip]
The 'stuff' is perfectly flat for a perfect 50 ohm termination, spikes
up if the DUT is inductive, spikes down if capacitive, rings if
resonant. There is, in theory, enough info in the reflection to fully
characterize the impedance.

Both the pulse gen and the scope need to have ~~ 1 ns response to
resolve components close to 300 MHz. I have a Tek 11801 that has about
20 ps TDR resolution, and it's great for characterizing the impedance
of PCB traces.

I think there are some pretty good google-able TDR tutorials.

Interesting. So what does "TDR" stand for? time-domain something,
presumably?

Time Domain Reflectometer (or reflectometry). Think of a TDR as a
network analyzer that has had an inverse Fourier transform. ;-) TDRs
are a rather useful piece of equipment.

Well I was going to buy a VNA at some point soon. Does that obviate
the need for a TDR or is it advisable to have a TDR in addition,
anyway? I mean, is there something the TDR can do that a VNA can't?

Winston Churchill

All the VNA's I am familiar with have internal transforms available
either as an option or standard, so you can get the TDR as well.

Regards
Ian