On Thu, 2 Dec 2004 23:20:40 +0000, "Ian White, G3SEK"
wrote:


What does a typical "test jig" consist of Ian? That's one thing that's
always puzzled me about these kind of measurements; particularly at
UHF++.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul Burridge wrote:
On Thu, 2 Dec 2004 23:20:40 +0000, "Ian White, G3SEK"
wrote:


What does a typical "test jig" consist of Ian? That's one thing that's
always puzzled me about these kind of measurements; particularly at
UHF++.

One that minimizes unwanted or uncontrolled lead lengths. In general,
one that is based on solid lumps of metal and large, broad,
low-inductance conducting surfaces.

I had simply bent the resistor wires so that one end pushed into the
centre of the VNA's N socket, and the other wire was literally tied onto
the body of the socket. However, the measurement showed that most of the
small inductance could be accounted for by those two wires - which you'd
never leave as long as that in a practical layout.

To home in on the inductance of the resistor body itself, I'd have to
build a jig that allows the wire lengths to be reduced almost to zero.
Harold W4ZCB sent a picture of something he uses, which is just a brass
plate soldered to the back of an SMA connector. The Device Under Test is
then soldered directly between the centre pin and somewhere on the
plate.

But I'm afraid my only visit to the workshop last weekend was to dump
yet another cardboard box on the floor.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Tue, 7 Dec 2004 07:31:08 +0000, "Ian White, G3SEK"
wrote:

To home in on the inductance of the resistor body itself, I'd have to
build a jig that allows the wire lengths to be reduced almost to zero.
Harold W4ZCB sent a picture of something he uses, which is just a brass
plate soldered to the back of an SMA connector. The Device Under Test is
then soldered directly between the centre pin and somewhere on the
plate.

How about cutting the resistor's leads off completely and just
clamping it between say two 1" cube copper blocks? But then I guess
you would still have the problem of connecting it to the VNA. :-(
--

"What is now proved was once only imagin'd." - William Blake, 1793.

How about cutting the resistor's leads off completely and just
clamping it between say two 1" cube copper blocks? But then I guess
you would still have the problem of connecting it to the VNA. :-(
--
Would possibly work OK if your standards were fabricated in the same way.
When it makes a measureable difference whether you're measuring to the front
face of the SMA center pin and the back of that same pin, you can start to
develop an appreciation for the accuracy possible with the VNA. I haven't
measured the capacity between two copper faces one inch square and separated
by the length of a quarter watt resistor, but it would be appreciable. (and
measureable if I were really interested.)

W4ZCB

In article , Paul Burridge
writes:

On Tue, 7 Dec 2004 07:31:08 +0000, "Ian White, G3SEK"
wrote:

To home in on the inductance of the resistor body itself, I'd have to
build a jig that allows the wire lengths to be reduced almost to zero.
Harold W4ZCB sent a picture of something he uses, which is just a brass
plate soldered to the back of an SMA connector. The Device Under Test is
then soldered directly between the centre pin and somewhere on the
plate.

How about cutting the resistor's leads off completely and just
clamping it between say two 1" cube copper blocks? But then I guess
you would still have the problem of connecting it to the VNA. :-(

That is more trouble to set up than is needed.

In order to measure any inductance component, the only
requirement is to find the DIFFERENTIAL between a direct
short across the bridge/RLC-meter connections and the
device itself (in this case a resistor).

Neil Hecht's excellent little LC Meter II does this automatically
by the zeroing button that subtracts the shorting inductance
from the device measurement, done arithmetically in the internal
microcontroller's registers.

The inductance of a standard gauge round wire is fairly well
known and has been around for years in texts. One can compute
that inductance fairly accurately without using a bridge/meter and
then compare that to the leads of the device under test. The
difference between the shorting lead and the device leads would
then be the inductance of the device-under-test's body.

If a few nanoHenries are involved and affect the circuit the device
is to be used in, the frequency would be high enough that skin
effect would be operative. Skin effect is the curious phenomenon
where current flows in thinner and thinner volume spaces near the
surface of a conductor. The thickness of the bridge/meter
connections would only be advantageous at DC (no skin effect at
all, only "straight" volumetric conductor bulk resistance) or lower
frequencies (where the "skin" is quite deep in the conductor). An
approximation of the RF resistance in Ohms/Inch-length is:

R_rf = (2.61 x 10^-7 x Sqrt( frequency in Hz)) / (2 x (w + h))

for PCB foil and where w = trace width in inches, h = thickness
of the PCB foil in inches (about 0.002 times "ounce" rating of
foil thickness, give or take some).

Reference: Nicholas Gray, Staff Applications Engineer,
National Semiconductor, "Design Idea" insert in electronics
trades for December, 2004. See also http://edge.national.com

However, when push comes to shove on measuring things, a
built-in inductance might be as much as 10 nanoHenries. At
100 MHz that inductance is equal to +j 6.283 Ohms. A precision
resistor of 50 Ohms at DC would have an equivalent series
magnitude of 50.39 Ohms, an increase over DC of only 0.79%.
The end result to a circuit using that device wouldn't amount to
very much change.

If that inductance were (somehow) ten times higher to 100 nHy,
then it would have +j 62.83 Ohms and the series magnitude would
(of that 50 Ohm resistor at DC) be 80.30 Ohms. That WILL have
some noticeable effect on a circuit at 100 MHz.

The upshot of all this is that one considers the frequency(s) of
operation first, then measures a part second to find out if the
resulting part inductance will have any effect on things. In order
to measure the part, the characteristics of the measuring instrument
ought to be known so as to get a reasonably accurate measurement.
Comparing the part to a wide conductor across the instrument
connections as the reference inductance, the part is then measured
with the wide conductor inductance subtracted from the mesaured
series inductance.

Some LC bridges measure admittance rather than impedance. If
that's the case, then the complex admittance (an equivalent R &
C in parallel) must be inverted to find the complex impedance
which is a series R and L. A grid-dipper sort of measurement
just won't yield any meaningful result for either Z or Y.

It's not at all my intent to make up more work for you to do. What I was
mostly interested in was just how much the spiral construction of the
resistor adds to its inductance -- is it or isn't it significant, or is
the inductance mostly due to the component size and the leads? And it
still seems to me that you could just measure a wrapped or painted
resistor in the same fixture as you did the intact resistor. The
difference in measured inductances should be the contribution of the
spiral element.

Although it would be interesting to see how small the inductance of a
leaded resistor could possibly be, it's probably of more practical use
to know the inductance of a leaded resistor with some realistic length
of leads attached, which is I believe what you've already done. Anyone
needing less inductance than that would be wise to abandon leaded
resistors and go to SMD.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:

Paul Burridge wrote:

On Thu, 2 Dec 2004 23:20:40 +0000, "Ian White, G3SEK"
wrote:


What does a typical "test jig" consist of Ian? That's one thing that's
always puzzled me about these kind of measurements; particularly at
UHF++.


One that minimizes unwanted or uncontrolled lead lengths. In general,
one that is based on solid lumps of metal and large, broad,
low-inductance conducting surfaces.

I had simply bent the resistor wires so that one end pushed into the
centre of the VNA's N socket, and the other wire was literally tied onto
the body of the socket. However, the measurement showed that most of the
small inductance could be accounted for by those two wires - which you'd
never leave as long as that in a practical layout.

To home in on the inductance of the resistor body itself, I'd have to
build a jig that allows the wire lengths to be reduced almost to zero.
Harold W4ZCB sent a picture of something he uses, which is just a brass
plate soldered to the back of an SMA connector. The Device Under Test is
then soldered directly between the centre pin and somewhere on the plate.

But I'm afraid my only visit to the workshop last weekend was to dump
yet another cardboard box on the floor.

Len wrote:

In order to measure any inductance component, the only
requirement is to find the DIFFERENTIAL between a direct
short across the bridge/RLC-meter connections and the
device itself (in this case a resistor).

Neil Hecht's excellent little LC Meter II does this automatically
by the zeroing button that subtracts the shorting inductance
from the device measurement, done arithmetically in the internal
microcontroller's registers.

I completely agree that all impedance measuring devices should be
"zeroed" in this way. But the problem with the resistor wires that we're
discussing here is *additional* to that.

We want to know the inductance of the metal-film resistor body, with the
wires cut very short as they would be for any application where low
inductance is important. However, for convenience, my first measurements
used almost the full length of the resistor wires to connect to the N
socket of the VNA.

It turned out that the total measured inductance is comparable to what
you'd find from the wires alone, so the body inductance is very small
(which is entirely consistent with the physical construction).

The suggestion had been to determine the inductance of the resistor body
by first repeating that original measurement, then applying conductive
paint to short out the resistor body, and then measuring again. The body
inductance would then be the difference between those two measurements.

Unfortunately that would be a poorly designed experiment, because the
resistor wires were bent around into a floppy loop whose size and shape
- and therefore inductance - is not very well controlled. The very small
inductance of the resistor body could easily become lost in variations
caused by small accidental movements of the wires. It is also an
experiment that cannot be repeated, because of the conductive paint.

If I'd found time at the weekend, I would have made up a little plate
like W4ZCB described. The N2PK VNA uses a three-step calibration with
open, short and 50R standard loads. For this jig, I'd have had to start
with the open-circuit connector spill, followed by a solder-blob short,
and finally by the best solderable 50R load I could make (probably two
100R chip resistors in parallel). Then I'd have cut short the wires of
the test resistor, and soldered that in place on the plate for the
actual measurement.

But unfortunately the whole weekend timed ou


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Len wrote:

All noted and good, Ian. The thread had become (in my estimation)
one of those "intellectual exercises" that don't have much real
practical significance. :-)

"Small enough for VHF" is about as much as we need to know.

Agilent (what HP test and measurement
division became after Big Business of computer product take-
over of HP)

Or as someone said to me, "We're the world's #1 test equipment
manufacturer, and they just gave our trademark away to the world's #3
computer manufacturer."


I have some of the original
free App Notes from HP (back when HP was HP...:-) and the going
gets tough through all that four-port math. If someone else has
gone the same route and put it all into some software/firmware to
let the casual hobbyist go at it easily, then good all around.

Yes, indeed someone has!


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On 09 Dec 2004 19:12:18 GMT, (Avery Fineman)
wrote:

Of course the resistor lead contribute some inductance. In fact,
most of it. However that inductance is directly calculable based
on old, available data. What remains is the resistor body itself.
A very quick approximation of that body would be to get a scrap
of kitchen aluminum foil and wrap it tightly around the body with
the overlap tight around the leads to make contact. [won't make
much difference because the body, being larger in diameter than
the leads, will have much less inductance than those leads]

Sorry, Len, I stand to be corrected (no doubt) but surely this way of
shorting the ends together is going to make matters much worse? Aren't
you going to end up with a significant amount of capacitance between
your outer foil and the inner spiral of resistive film? Isn't that
going to just throw another complex variable into the mix and probably
completely change the resistors SRF?
--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Tue, 07 Dec 2004 21:45:43 -0800, Roy Lewallen
wrote:

It's not at all my intent to make up more work for you to do. What I was
mostly interested in was just how much the spiral construction of the
resistor adds to its inductance -- is it or isn't it significant, or is
the inductance mostly due to the component size and the leads? And it
still seems to me that you could just measure a wrapped or painted
resistor in the same fixture as you did the intact resistor. The
difference in measured inductances should be the contribution of the
spiral element.

Has anyone carried out any tests to compare conventional spiral
elements with the 'doubling-back spiral' types that claim to be
"non-inductive"? I just wonder if that doubling-back of the resistive
element to 'eliminate' inductance is as effective as it's made out to
be.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

In article , Paul Burridge
writes:

On 09 Dec 2004 19:12:18 GMT, (Avery Fineman)
wrote:

Of course the resistor lead contribute some inductance. In fact,
most of it. However that inductance is directly calculable based
on old, available data. What remains is the resistor body itself.
A very quick approximation of that body would be to get a scrap
of kitchen aluminum foil and wrap it tightly around the body with
the overlap tight around the leads to make contact. [won't make
much difference because the body, being larger in diameter than
the leads, will have much less inductance than those leads]

Sorry, Len, I stand to be corrected (no doubt) but surely this way of
shorting the ends together is going to make matters much worse? Aren't
you going to end up with a significant amount of capacitance between
your outer foil and the inner spiral of resistive film? Isn't that
going to just throw another complex variable into the mix and probably
completely change the resistors SRF?

Paul, don't kick yourself after reading this, but shorting out the body
with foil will put a conductor in contact with BOTH ends of the
resistor body. :-)

I mentioned that only in passing since it isn't necessary to do in order
to find out anything significant.

Finding a "self resonant frequency" involves doing several
measurements of the total R, C, and L of the device, finding the
complex R and X at each, then plotting that (a Smith Chart will do
it nicely) to see the skew shape of the curve as compared to a
perfect resistor (a single point on a Smith Chart). You will have to
work out the SRF yourself based on that information; that is going
to vary with each specified R value, film type, and the kind of laser
trimming (or whatever) is done to get the DC/low-frequency R value
precise through a film spiral or gouging or whatever. Not needed.

If you just measure the device with a bridge/instrument yielding the
complex impedance or admittance, you just apply that to the
circuit taking the device and be done with it. You will have to allow
for some adjustment in the circuit itself to compensate for the
device characteristics (whatever they come out to be).

An analytical model of the resistor is an R component in series
with an L component (due to any spiral of film, if any, plus the
length/diameter of the resistor body), but that has fringing capacity
between the ends of the leads inserted into the body...that capacity
being in parallel to the series R-L connection. You can estimate
that and do a paper exercise to see the effect for jollies...or just
skip it, use the device measurements to base the overall model as
it applies to the circuit and go on with the project.

The effect, if any, is going to be minimal with 1/4 Watt or smaller
resistors at VHF on up to low UHF bands. Nothing to worry about
provided the leads themselves follow the usual "short as possible"
rule.

One can go nuts on the intellectual paper exercise and about all
it is good for is mumbling-bragging over glasses at the local pub. :-)

On 11 Dec 2004 19:39:18 GMT, (Avery Fineman)
wrote:

In article , Paul Burridge
writes:

Has anyone carried out any tests to compare conventional spiral
elements with the 'doubling-back spiral' types that claim to be
"non-inductive"? I just wonder if that doubling-back of the resistive
element to 'eliminate' inductance is as effective as it's made out to
be.

Yes, Paul, it is effective...

[comprehensive explanation snipped]

Thanks, Len. That's about as in depth as it gets, I reckon!
Ian swears by his VNA to do these kind of measurements, but you strike
me as the kind of guy who might know of an alternative way using more
conventional test gear... care to share?

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

In article , Paul Burridge
writes:

[comprehensive explanation snipped]

Thanks, Len. That's about as in depth as it gets, I reckon!
Ian swears by his VNA to do these kind of measurements, but you strike
me as the kind of guy who might know of an alternative way using more
conventional test gear... care to share?

The Vector Network Analyzers yield the complex-quantity Z or Y
directly...which is a wonderful thing for really detailed design and
checking to see if a part/device/etc. will fit an application.

A very few "RF Bridges" might do that but their upper frequency
limits are rather limited for venturing into the high end of HF and
certainly not VHF and up.

For the hobbyist lacking a VNA, I would suggest the neat little
LC-Meter from AADE. I have one and find it extremely handy
for winding lots of coils (particularly toroids) without firing up the
VHF Q-Meter now rather on its last legs and needing calibration.

I don't worry about the "intellectual exercise" sort of things, such
as "self-resonant frequency" of a resistor. That is mainly up to the
lead length rather than any spiral of resistive film in the resistor
body. The circuit to receive such a resistor has to be calculated
to take an _estimate_ of the residual/parasitic L or C with, if the
need is critical, some sort of trimmer to "tune out" that L or C.

Case in point at HF: Bandswitched coils for an old multi-band
"communications receiver" took care of tolerance variations in
those coils either by using adjustable powdered iron cores or by
individual trimmer capacitors (usually compression micas). Some
of the same method can be applied to higher frequencies, such as
a rotary ceramic or piston trimmer cap or turns-spreading/
-compressing of a toroid or solenoidal core coil to adjust the L.
The point I'm trying to make is to prepare ahead of time.

That preparation is anathema to some who just like to grab parts
and stuff them together based on old rules of thumb or imprecise
nomographs, then "hope for the best." One can get away with
that at lower HF most of the time but that gets tougher going up
in frequency. Analysis models will take care of finding a spread of
part tolerances and what to do about trimming them out. That's
all paper work and - unfortunately - shunned by many. One excuse
is "I haven't got the time!" However, bad preparation will have the
TIME spent in trying to adjust something, taking comparative
measurements, etc., is usually takes more time than doing the
initial paper work on analyzing the circuit.

As to measurement methods with different bridges, there's lots of
texts available to cover that, most of them old ("old" being at least
30 years back, heh heh). General Radio wrote up the circuit details
on their old RF bridges in fine form, sometimes including that in
their owner's manuals. Problem is, that sort of thing takes even
more skull work to make sure of accuracy...plus planning on how
to calibrate it once it is built. [ain't no good if it reads wrong or has
improper calibration]

Some of the simpler bridges are the "substitution" variety such as
the extremely portable RX Noise Bridge (noise excitation for the RF
source makes it portable, it works fine with a known frequency
source too). One arm has a calibrated R and C in parallel (the
"known" side) while the other arm has the unknown with half-value
of the calibrated C duplicated across the unknown. It basically
reads admittance when balanced but that can be recalculated to
impedance with a handy scientific calculator such as the HP 32 SII
or the newer 33 model (each does a single-key complex inversion).
The simplicity is what makes it good; symmetry in physical
construction is easy to achieve, that increasing the accuracy of
the little instrument. The known arm of the bridge can be calibrated
with some of the newer digital capacitance meters that go down to
picoFarads of resolution, the R part of the known arm calibrated with
an accurate DVOM. A dual carbon/metal film potentiometer wired in
parallel will reduce the internal distributed inductance of the varying
position of the wiper arm.

One problem of those simple bridges is that one spends a LOT of
time taking readings and then converting them to useful number
quantities. A VNA using a microprocessor can do all that math
for you and at many frequencies in an eyeblink. Reduction of
TIME spent in getting data is very powerful with the VNA. An
extension of that would be a Smith Chart presentation on-line
(as the fancier HP analyzers had/have) for direct visualization of
what the heck is going on inside something. Hobbyists are
always short of time. :-(

On 12 Dec 2004 17:47:00 GMT, (Avery Fineman)
wrote:

[**snip**}

Thanks again, Len! I'm going to need a while to digest that lot!

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.