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.