I've used these as a dummy load and they work
fine even at 440mhz..
http://www.ohmite.com/catalog/pdf/tah_tch_series.pdf
Mouser sells them..
http://www.mouser.com/catalog/620/415.pdf

Hope this answers your question..

-Pete

James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.


someone please help!

dr. x

On Mon, 29 Nov 2004 13:40:16 GMT, Pete D wrote:

I've used these as a dummy load and they work
fine even at 440mhz..
http://www.ohmite.com/catalog/pdf/tah_tch_series.pdf

I notice they say "very low inductance" but it would be nice to have a
figure!!!
--

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

"Reg Edwards" wrote in message
...
If lumps are not accurate enough then model it as a distributed
transmission
line, which it actually is, and calculate again.

If you don't know how to do these things then you are not qualified to
call
yourself an engineer which I'll admit is slightly off-topic.

Well, Reg, equivalent circuit modelling is not a common topic taught in
colleges, _especially_ in an undergraduate curriculum, and there are one
heck of a lot of EE's out there who work for companies that don't even
_have- the facilities (a network analyzer) to properly measure what their
resistor does... yet plenty of them are fine engineers.

Electrical engineering is quite broad these days. There are guys who sit
around designing communication systems who never touch soldering irons, and
I'm sure plenty of them would claim you're not qualified to be an engineer
because you can't derive some 'trivial' convolutional code off the top of
your head.

BTW, many SPICE simulators do a mediocre job of simulating lossy
transmission lines. Most people who are going to be using components at
frequencies where they care about distributed parasitics are probably
(hopefully) using frequency domain simulators anyway, but that too is an
area where today's undergraduate curriculum tends to be somewhere beween
weak and non-existant. (Using simulators other than SPICE... e.g., harmonic
balancers, periodic steady staters, linear frequency sims, etc.)

---Joel

Duncan Munro wrote in message .. .

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I wonder, though, if the AADE meter is not getting confused by the
resistance of the resistor. The fact that the measured inductance is
just about proportional to the resistance might be evidence for that.

73,
Mike, KK6GM

Mike Silva wrote:
Duncan Munro wrote in message
. ..

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I wonder, though, if the AADE meter is not getting confused by the
resistance of the resistor. The fact that the measured inductance is
just about proportional to the resistance might be evidence for that.

Duncan has kindly sent a couple of samples, with duplicates that have
had the paint scraped off. I just arrived home from a few days away, so
haven't had time to measure them yet.

Each one is only an open spiral of about two turns along the whole
length of the 3W resistor body, so you can see immediately that there's
no way the inductance can actually be more than a few hundred nH.

This actual, physical inductance is in *series* with the resistance.
What seems to be happening is that the AADE meter displays the
resistance and reactance in their equivalent parallel form, which is a
function of the measurement frequency (which varies, but is understood
to be in the order of a few MHz).

Guessing a frequency and then doing the parallel - series
transformation on 22 ohms in parallel with 4.5uH produces results in the
right ballpark: R is still around 22 ohms but the *series* inductance is
100-200nH.

I will try to measure the resistors tomorrow.


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

On 30 Nov 2004 12:37:57 -0800, Mike Silva wrote:

Duncan Munro wrote in message .. .

The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the
aade.com meter. Both values (if the readings are correct) would represent
a high ratio of X to R at HF frequencies...

I wonder, though, if the AADE meter is not getting confused by the
resistance of the resistor. The fact that the measured inductance is
just about proportional to the resistance might be evidence for that.

Mike, I think you've hit the nail on the head. To be fair to AADE, they
warn that the 'Q' of the inductor has to be reasonable to get a sensible
measurement.

The kind of measurement frequencies we are talking about are in the order
of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is
19.8 ohms, not a million miles from the 22 ohms of the resistor itself -
this is not what I would call a reasonable 'Q' value. Fair play to AADE,
it's designed to measure the inductance of inductors, not other components
;-)

--
Duncan Munro
http://www.duncanamps.com/

On Tue, 30 Nov 2004 21:04:28 +0000, Ian White, G3SEK wrote:

Guessing a frequency and then doing the parallel - series
transformation on 22 ohms in parallel with 4.5uH produces results in the
right ballpark: R is still around 22 ohms but the *series* inductance is
100-200nH.

There is an additional complication in that there is another inductor in
the box itself of 680uH, LX (or should I say RX) is in series with that.
It's late now, but I will try and work out what's going on tomorrow night.

I will try to measure the resistors tomorrow.

If you get the opportunity, it would be much appreciated.

--
Duncan Munro
http://www.duncanamps.com/

Duncan Munro wrote:

The kind of measurement frequencies we are talking about are in the order
of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is
19.8 ohms, not a million miles from the 22 ohms of the resistor itself -
this is not what I would call a reasonable 'Q' value. Fair play to AADE,
it's designed to measure the inductance of inductors, not other components
;-)

If the reactance is much lower than the resistance, it's generally
inconsequential in a practical application. I think that's almost always
the case for carbon film resistors, and I suspect it's nearly always the
case for metal film resistors.

Probably, if the Q is so low as to make measurement difficult, it's
probably low enough that the X isn't important in a practical application.

Roy Lewallen, W7EL

On Wed, 01 Dec 2004 16:35:36 -0800, Roy Lewallen wrote:

Duncan Munro wrote:

The kind of measurement frequencies we are talking about are in the order
of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is
19.8 ohms, not a million miles from the 22 ohms of the resistor itself -
this is not what I would call a reasonable 'Q' value. Fair play to AADE,
it's designed to measure the inductance of inductors, not other components
;-)

If the reactance is much lower than the resistance, it's generally
inconsequential in a practical application. I think that's almost always
the case for carbon film resistors, and I suspect it's nearly always the
case for metal film resistors.

Probably, if the Q is so low as to make measurement difficult, it's
probably low enough that the X isn't important in a practical application.

(adds) ..... at the test frequency

budgie wrote:

On Wed, 01 Dec 2004 16:35:36 -0800, Roy Lewallen wrote:


Duncan Munro wrote:

The kind of measurement frequencies we are talking about are in the order
of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is
19.8 ohms, not a million miles from the 22 ohms of the resistor itself -
this is not what I would call a reasonable 'Q' value. Fair play to AADE,
it's designed to measure the inductance of inductors, not other components
;-)

If the reactance is much lower than the resistance, it's generally
inconsequential in a practical application. I think that's almost always
the case for carbon film resistors, and I suspect it's nearly always the
case for metal film resistors.

Probably, if the Q is so low as to make measurement difficult, it's
probably low enough that the X isn't important in a practical application.


(adds) ..... at the test frequency

Sure. Any statement about a frequency-dependent property like X or Q
applies only at the frequency at which the component has that particular
X or Q.

As you raise the frequency, the X of course increases while the R stays
relatively constant. But other effects like shunt C and the physical
length of the part eventually start coming into play, making the
simplistic model of a series RL inadequate. My general experience has
been that I can ignore the inductance of leaded carbon film resistors up
to a frequency where the leads and component length become a problem,
and I need to go to chip components. I've never seen significant
reactance from the trim cuts on a thick film chip resistor -- the shunt
C across the narrow cuts pretty much makes them invisible.(*) I suspect
that carbon film resistors likewise have a narrow cut. But I don't have
much experience with metal film resistors. I assume the base material
has less resistivity, so is probably cut into thinner strips with more
"turns" and more spacing between "turns". So there might be a
combination of R and frequency where the reactance is objectionable,
below the frequency where you need to abandon leaded parts. I'm watching
this thread with interest for any good measurement results. I could try
making some measurements up to 1.3 GHz with my 8505A network analyzer,
but I wouldn't trust the results. I think the measurements probably
would have to be done on a system with good, computer-directed
calibration, a good set of calibration standards, a decent and
well-characterized test fixture, and an operator who's very familiar
with the many traps you can fall into when making subtle measurements
like these -- and I have none of the above.

(*) I've used thick film resistors at frequencies up to 20 GHz or so, in
very sensitive time-domain applications. In those applications, I
modeled nearly every component as a transmission line or a pi or tee
approximation to a line, with the R in one or two lumps. Those models
agreed quite well with actual results.

Roy Lewallen, W7EL