FREQUENCY RESPONSE OF RESISTORS.

Program METALFLM.exe 41 Kbytes.

This program models small low-wattage wire-ended resistors including
spiralled metal-film resistors. The model's equivalent circuit is a
chain of T and L networks. Values of lumped inductance and stray
capacitance are estimated by the program from the component's physical
dimensions.

The component with it's terminating leads is treated as a lumped L and
C non-uniform transmission line. The program computes input impedance
from 10 KHz up to 5 GHz with the other end grounded. Fast and slow
frequency sweeps are available to observe maxima and minima of Zin.

Max and min Zin are approximately related to the physical length of
the component, including connecting leads, but occur at frequencies
far higher than the usual and useful working frequency range.

The reflection cefficient and standing-wave-ratio are calculated with
reference to Zo, Zo being the DC value of the resistor. The usefulness
of the component as a dummy load or for other purposes at any
frequency can then be ascertained.

The effectiveness versus frequency of solid carbon-rod resistors and
multi-turn UHF chokes, which are of similar wire-end construction, can
also be determined. Chokes can be wound over the bodies of high-value
wire-ended resistors, the parallel resistance value perhaps having
some desirable effect on the ultimate response.

Another use for resistors having a good high frequency response, or at
least a known frequency response, is for ratio arms and switched
resistors in impedance bridges and other measuring instruments.

Download METALFLM in a few seconds and run immediately.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Hi Reg,

Thanks for making more of your handiwork available to the group!

Any idea how the model predictions track with real-world measure-
ments on various kinds of resistors?

Many years back in our GTE (now Verizon) RF development lab,
we found that we could make much better impromptu RF terminations
using big 2W carbon resistors than with the smaller sizes. It seems
the internal carbon rod was closer to the surrounding BNC connector,
thus producing less of an impedance bump. These impromptu loads
worked fine to beyond 500 MHz if the "outside" lead was just bent
over and soldered to the connector shell. But if a metal disk was used
between the lead and shell, the upper end was extended to 1+ GHz.
(Shouldn't try to push a BNC beyond that anyway!)

Ed

"Reg Edwards" wrote in message
...
FREQUENCY RESPONSE OF RESISTORS.

Program METALFLM.exe 41 Kbytes.

This program models small low-wattage wire-ended resistors including
spiralled metal-film resistors. The model's equivalent circuit is a
chain of T and L networks. Values of lumped inductance and stray
capacitance are estimated by the program from the component's physical
dimensions.

The component with it's terminating leads is treated as a lumped L and
C non-uniform transmission line. The program computes input impedance
from 10 KHz up to 5 GHz with the other end grounded. Fast and slow
frequency sweeps are available to observe maxima and minima of Zin.

Max and min Zin are approximately related to the physical length of
the component, including connecting leads, but occur at frequencies
far higher than the usual and useful working frequency range.

The reflection cefficient and standing-wave-ratio are calculated with
reference to Zo, Zo being the DC value of the resistor. The usefulness
of the component as a dummy load or for other purposes at any
frequency can then be ascertained.

The effectiveness versus frequency of solid carbon-rod resistors and
multi-turn UHF chokes, which are of similar wire-end construction, can
also be determined. Chokes can be wound over the bodies of high-value
wire-ended resistors, the parallel resistance value perhaps having
some desirable effect on the ultimate response.

Another use for resistors having a good high frequency response, or at
least a known frequency response, is for ratio arms and switched
resistors in impedance bridges and other measuring instruments.

Download METALFLM in a few seconds and run immediately.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

Hi Ed.

Regarding your big 2-watt carbon resistors at 500 MHz -

what values of reflection coefficient or SWR were experienced when you
say they "worked fine". Can you remember? Or did you have some other
way of assessing "fine" performance?

The DC value has to be allowed a resonably large tolerance about its
nominal value and the HF performance cannot be any better than that.
----
Reg, G4FGQ

Hi Reg,

Our definition of "worked fine" was that the measured SWR was
consistent with the rated DC resistance and tolerance. As I recall,
we were using 51 Ohm 5% resistors. Typical SWR was under
1.1 up to 100 MHz, and 1.2 to 1.3 at 500 MHz for the bent-lead
(lesser performing) version.

These loads were just for handy lab use, and were not products or
calibrated test equipment, of course.

Ed

"Reg Edwards" wrote in message
...
Hi Ed.

Regarding your big 2-watt carbon resistors at 500 MHz -

what values of reflection coefficient or SWR were experienced when you
say they "worked fine". Can you remember? Or did you have some other
way of assessing "fine" performance?

The DC value has to be allowed a resonably large tolerance about its
nominal value and the HF performance cannot be any better than that.
----
Reg, G4FGQ

Thanks Ed for the information. I need time to think about it.

At first sight your data falls in the same ball park as that generated
by program METALFLM below 500 MHz. Ball-park accuracy is all that
anybody can expect under the circumstances.

It is a fact that program METALFLM is the first of many programs I
have ever produced without the support of personal measurement
experience. I have very little practical experience of frequency
response of resistors above above 30 MHz and not very much below that.

But I DO have confidence in my assessment of L and C values as
calculated from physical resistor dimensions. Also I have confidence
in my ability to model equivalent circuits of distributed L, C and R
components.

In the end, all that's needed. are the limits of reflection
coefficients versus frequency for ordinary wire-end resistors mounted
on circuit boards.

And of course, rubbish in = rubbish out.

Cecil, is there an IEEE definition of "Ball Park Accuracy" ? I'm on
South African red. Gone off Californian white.
----
Reg, G4FGQ

By the way, the program has brought to light the fact that
board-mounted resistors in the range 100 to 400 ohms have a slightly
better frequency response than those around 50 ohms. Therefore,
wideband 50-ohm dummy loads are best made from a nunber of higher
value resistors connected in parallel. Avoid series connections. Which
is what most people do anyway.
----
RJE