On Tue, 21 Dec 2004 21:16:03 GMT, (Robert
Lay) wrote:

It has been suggested that a
virtue of the T pad would give a clean 6 dB loss instead of some
"not-so-nice" loss, like 5.7 dB. Well, that's not a problem because
the L-Pad can also be designed for exactly 6 dB. In fact, it can be
designed to provide ANY loss you want, so long as it's AT LEAST 5.7
dB. You see, the 5.7 dB L-Pad happens to be the MINIMUM loss design
for that particular mismatch.

Hi Bob,

The difference between 5.7 and 6dB is immaterial if neither is
calibrated. As for the desire for a nominal 6dB pad, that too is
hardly of great merit for quality measurements with a good Spectrum
Analyzer.

Just like directional coupler design, precision applications focus on
directivity and sacrifice round numbers in coupling to achieve better
separation of ports. A 6dB attenuator will isolate your precision
gear from the unknown better than a 3dB attenuator, but not as well as
a 10 or 20dB attenuator.

Now, as to the term isolation. It has a variety of meanings which in
this case means that your measurement is less perturbed by the literal
unknowns of your proverbial unknown being measured. That is, in your
attempt to find a value (the proverbial unknown) your accuracy can be
upset by variables whose magnitude can affect that accurate
determination. Large attenuators obviously de-sense the
instrumentation, but if you have sufficient dynamic range, then that
is not a debit, but actually an asset. Hence de-sense or isolation is
benign. When this large attenuator is placed on the source, it
reduces the load's influence to pull or mismatch there too.

This says nothing of actual mismatches, it simply presents what is
called swamping. That is, you introduced known and controlled losses
to buffer the measurement. Later you can subtract out the losses to
find your proverbial unknown.

I already alluded to the virtue of using attenuators to increase the
power tolerance to the input of a Spectrum Analyzer, aside from this,
the only practical use of attenuators is to introduce controlled loss
to isolate the unknowns' influence.

To answer "Why Match?" returns us to isolation. Once this is
achieved, the measurement can be trusted to be faithful in proportion
to that isolation. With this example of a simple 50 to 75 Ohm
conversion, that measurement's faithful accuracy is fairly good. As
for it being a ~6dB attenuator, by placing it into another test with
an unknown, it will offer mixed results - a T or PI configuration at a
higher attenuation would be far more flexible, and faithful.

73's
Richard Clark, KB7QHC

In message , Richard Clark
writes
On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson
wrote:

However, this is not the best design for a
matching network, they are usually three resistors in either a PI or T
configuration.
Several manufacturers of precision matching pads might disagree!

Hi Ian,

I seriously doubt it, but you are free to offer examples.

73's
Richard Clark, KB7QHC



http://www.maxim-ic.com/appnotes.cfm...mber/972/ln/en
http://www.maxim-ic.com/appnotes.cfm...te_number/3250
http://www.maxim-ic.com/appnotes.cfm...mber/972/ln/en
http://www.testmart.com/estore/produ...Fsearch%2Fspec.
cfm~~MICCOM~~AGILEN~~11852B~~%20~~%20|1.html
http://www.g4fgq.regp.btinternet.co.uk/padmatch.pas
http://www.g4fgq.regp.btinternet.co.uk/padmatch.pas
http://used-line.com/b2544p1pr0-Used-pads.htm
http://www.minicircuits.com/dg03-159.pdf#search='minimum%20loss%20pad'
+ many, many more!
Ian.

--

On Tue, 21 Dec 2004 22:51:30 +0000, Ian Jackson
wrote:

In message , Richard Clark
writes
On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson
wrote:

However, this is not the best design for a
matching network, they are usually three resistors in either a PI or T
configuration.
Several manufacturers of precision matching pads might disagree!

Hi Ian,

I seriously doubt it, but you are free to offer examples.

73's
Richard Clark, KB7QHC
+ many, many more!
Ian.

Hi Ian,

Perhaps you can share from those many, many more, those which
disagree?

73's
Richard Clark, KB7QHC

In message , Richard Clark
writes
On Tue, 21 Dec 2004 22:51:30 +0000, Ian Jackson
wrote:

In message , Richard Clark
writes
On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson
wrote:

However, this is not the best design for a
matching network, they are usually three resistors in either a PI or T
configuration.
Several manufacturers of precision matching pads might disagree!

Hi Ian,

I seriously doubt it, but you are free to offer examples.

73's
Richard Clark, KB7QHC
+ many, many more!
Ian.

Hi Ian,

Perhaps you can share from those many, many more, those which
disagree?

73's
Richard Clark, KB7QHC

The point I was trying to make was, if the L-pad was 'not the best
design for a matching network', why are there so many about?
It gives the minimum loss for a match in both directions (with the 43and
86 ohm resistors). What works better?
Ian.
--

On Tue, 21 Dec 2004 21:54:30 GMT, Richard Clark
wrote:

On Tue, 21 Dec 2004 21:16:03 GMT, (Robert
Lay) wrote:

It has been suggested that a
virtue of the T pad would give a clean 6 dB loss instead of some
"not-so-nice" loss, like 5.7 dB. Well, that's not a problem because
the L-Pad can also be designed for exactly 6 dB. In fact, it can be
designed to provide ANY loss you want, so long as it's AT LEAST 5.7
dB. You see, the 5.7 dB L-Pad happens to be the MINIMUM loss design
for that particular mismatch.

Hi Bob,

The difference between 5.7 and 6dB is immaterial if neither is
calibrated. As for the desire for a nominal 6dB pad, that too is
hardly of great merit for quality measurements with a good Spectrum
Analyzer.

Just like directional coupler design, precision applications focus on
directivity and sacrifice round numbers in coupling to achieve better
separation of ports. A 6dB attenuator will isolate your precision
gear from the unknown better than a 3dB attenuator, but not as well as
a 10 or 20dB attenuator.

Now, as to the term isolation. It has a variety of meanings which in
this case means that your measurement is less perturbed by the literal
unknowns of your proverbial unknown being measured. That is, in your
attempt to find a value (the proverbial unknown) your accuracy can be
upset by variables whose magnitude can affect that accurate
determination. Large attenuators obviously de-sense the
instrumentation, but if you have sufficient dynamic range, then that
is not a debit, but actually an asset. Hence de-sense or isolation is
benign. When this large attenuator is placed on the source, it
reduces the load's influence to pull or mismatch there too.

This says nothing of actual mismatches, it simply presents what is
called swamping. That is, you introduced known and controlled losses
to buffer the measurement. Later you can subtract out the losses to
find your proverbial unknown.

I already alluded to the virtue of using attenuators to increase the
power tolerance to the input of a Spectrum Analyzer, aside from this,
the only practical use of attenuators is to introduce controlled loss
to isolate the unknowns' influence.

To answer "Why Match?" returns us to isolation. Once this is
achieved, the measurement can be trusted to be faithful in proportion
to that isolation. With this example of a simple 50 to 75 Ohm
conversion, that measurement's faithful accuracy is fairly good. As
for it being a ~6dB attenuator, by placing it into another test with
an unknown, it will offer mixed results - a T or PI configuration at a
higher attenuation would be far more flexible, and faithful.

73's
Richard Clark, KB7QHC

Dear Richard,

OK - you're not getting any argument from me.

Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk

On Wed, 22 Dec 2004 00:19:12 +0000, Ian Jackson
wrote:

The point I was trying to make was, if the L-pad was 'not the best
design for a matching network', why are there so many about?
It gives the minimum loss for a match in both directions (with the 43and
86 ohm resistors). What works better?
Ian.

Hi Ian,

The topic is Why Match? and the context is with bench gear,
specifically a Spectrum Analyzer. It is pleasing that no sardonic
quips as to this device's suitability for antenna matching has sallied
forth. Attenuators serve a limited purpose - Isolation. They serve
this well when they don't introduce uncontrolled error. They also
serve as range extenders as do directional couplers, often with
attenuators paired with them.

In the case of using them with couplers, they insure port isolation by
insuring port loading. They buffer any deviance from an expected 50
Ohm load that may be presented by monitoring gear attached through
them to that port. This is by and large the context of the mini
circuits links you offered. The extent of this "insurance" can be
observed by computing how much load is presented to the affected
instrument when the attenuator is left open, or shorted. T or PI
configurations show a higher tolerance. An L Pad is a special case
(less general form) of either T or PI, being that one of the three
elements is replaced with either a short or an open.

73's
Richard Clark, KB7QHC

On Wed, 22 Dec 2004 00:19:12 +0000, Ian Jackson
wrote:
What works better?

Hi Ian,

In following up the links that you provided, there is a superior
reference found at the same site:
http://www.minicircuits.com/appnote/an70001.pdf
which in turn leads to a treasure trove on the topic that supports my
favorite discussion on Mismatch Uncertainty. By following the links,
they offer you articles and software to compute the accumulation of
errors (and loss) found in mismatched sources looking at mismatched
loads.

A notable quote:
"A fixed attenuator can help to lower the VSWR of cascaded
(connected) components by providing isolation between the
impedances, effectively masking the impedance mismatches."

For both mismatched source and load, one handy shortcut offered is
that the system suffers a SWR that is not the aggregation of the two,
but the multiple of the two. This is not particularly significant for
a source SWR of 2 seeing a load SWR of 2 (same result of 4 for either
addition or multiplication), but above this value and loss begins to
climb dramatically.

73's
Richard Clark, KB7QHC