On 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote:

Hi All,

I was reviewing a 75 to 50 ohm resistive matching network using two resistors,
the insertion lost was 5.7 db.

If we have a 100Vrms source with 50 ohm source impedance and it is driving a
matched 50 ohm load then the load takes 1A and the power in the load is 50
watts.

If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is
48 watts. (I*I*R) == (0.8)*(0.8)*75.

I guess I must be not be taking something in account, but 2 watts does not
equal 5.7 db.

I know there must be a good reason to put the matching pad in line for the
sprectrum analyizer but I don't under why.

Thanks,

de KJ4UO

Dear KJ4UO,

I have fundamental objections to responding to respondants, so I will
continue to address this thread directly. 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.

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

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

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 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote:

Hi All,

I was reviewing a 75 to 50 ohm resistive matching network using two resistors,
the insertion lost was 5.7 db.

If we have a 100Vrms source with 50 ohm source impedance and it is driving a
matched 50 ohm load then the load takes 1A and the power in the load is 50
watts.

If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is
48 watts. (I*I*R) == (0.8)*(0.8)*75.

I guess I must be not be taking something in account, but 2 watts does not
equal 5.7 db.

I know there must be a good reason to put the matching pad in line for the
sprectrum analyizer but I don't under why.

Thanks,

de KJ4UO

Dear KJ4UO,

First, I must apologize for not noticing that your posting DOES
mention that the application is a Spectrum Analyzer. That being the
case, I can be much more enlightening about WHY we want a flat line
(no standing waves).

A spectrum analyzer is of use primarily in showing WHAT frequencies
are present in the specturm AND what their relative levels are. Unless
the line is flat, the relative levels will be distorted or inaccurate.
This comes about due to the fact that a mismatched line becomes an
impedance transformer that is very frequency dependant. That is, some
frequencies will be artificially enhanced and others artificially
subdued due to the varying impedance impedance seen by the source.
Conversely, when the source sees a perfect match across its acceptance
spectrum, only the built-in non-linearities will degrade the accuracy.

In short, the line must be properly matched in order that the spectrum
analyzer give the accuracy that it is capable of.

BTW, it should be understood that the matching L-Pad, in order to be
effective, must be physically located at the Spectrum Analyzer end of
the line - not at the source end of the line.

Interestingly enough, hams do not usually take this into account when
tuning an antenna system, because they are only interested in a flat
line for that short little piece of coax between the tuner and the
transmitter. They don't normally do anything about the horrible SWR
that exists on the feedline - Hi!

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