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Resistor measurements


 

I found this fascinating. I need to make an attenuator for my FT817 on
transmit as a 432 MHz
microwave IF radio. I have the appropriate carbon-film resistors. I
measured them on my NanoVNA to see what they measure at 432 MHz. This is
something I never could do before I had a NanoVNA. Two 200 ohm 1 watt
resistors measure 198 ohms on my DVM. The NanoVNA measures 198 ohms up to
about 500 kHz, and then the resistance starts to climb. At 432
MHz they both measure 225 ohms with a small amount of inductance (12 nH). I
measured a 68 ohm carbon-film resistor with similar results - it measures
67.5 ohms with the DVM and the NanoVNA up to about 500 kHz. At 432 MHz it
measures 86 ohms and a small inductance. I measured a 100 ohm carbon
composition resistor. It measured 102 ohms on the DVM and with the NanoVNA
up to about 1 MHz. The resistance barely changed on the NanoVNA above that.
At 432 MHz it measured 106 ohms.

I kind of figured the carbon film resistors weren't as accurate at VHF/UHF
frequencies compared to carbon composition resistors. This verified it.
Probably a skin effect problem.

73, Zack W9SZ


 

On 7/16/21 8:12 AM, Zack Widup wrote:
I found this fascinating. I need to make an attenuator for my FT817 on
transmit as a 432 MHz
microwave IF radio. I have the appropriate carbon-film resistors. I
measured them on my NanoVNA to see what they measure at 432 MHz. This is
something I never could do before I had a NanoVNA. Two 200 ohm 1 watt
resistors measure 198 ohms on my DVM. The NanoVNA measures 198 ohms up to
about 500 kHz, and then the resistance starts to climb. At 432
MHz they both measure 225 ohms with a small amount of inductance (12 nH). I
measured a 68 ohm carbon-film resistor with similar results - it measures
67.5 ohms with the DVM and the NanoVNA up to about 500 kHz. At 432 MHz it
measures 86 ohms and a small inductance. I measured a 100 ohm carbon
composition resistor. It measured 102 ohms on the DVM and with the NanoVNA
up to about 1 MHz. The resistance barely changed on the NanoVNA above that.
At 432 MHz it measured 106 ohms.
Yes, the NanoVNA opens up a whole new world of "hands on understanding" of things like parasitic L and C.? It's an amazing device for this.




I kind of figured the carbon film resistors weren't as accurate at VHF/UHF
frequencies compared to carbon composition resistors. This verified it.
Probably a skin effect problem.
And, thus, one begins diving into the rabbit hole <grin>


 

Not hard to imagine that a nichrome wire wound power resistor will have a lot of series inductance.

Metal film resistors are carbon ink coated ceramic forms. After firing they are trimmed to value with a barber-pole spiral trim cut either by laser or diamond cut wheel. The spiral trim gives them more series inductance. More inductance the greater the spiral trim length to get a given batch to resistance value so varied RF performance between same value part.

Chip resistors are made similarly but triming is usually inter-digital cuts from each side. Some are trimmed by a small sand blaster to one side of ink. Chip resistors are better for RF performance then leaded metal film resistors.

Best for RF performance are old carbon composition resistors with their core carbon brick. Hard to get and expensive these days.


 

Zack,

You also need a good test jig when making these measurements. What are you using for your tests?

Roger


 

Carbon film resistors are made by depositing a thin carbon film on a substrate, and then a laser cuts a spiral into the film to achieve the desired resistance. That spiral cut will make the resistor slightly inductive.

Carbon comp resistors, on the other hand, are typically non-inductive, however the leads will introduce some inductance at higher frequencies.

73 de Albert KK7XO


 

So a 50ohm metal film probably is going to be a horrible choice for
calibrating my NanoVNA on HF at the end of a coax run not terminated with a
coaxial connector (terminated with lugs to fasten to my vertical)

On Sat, Jul 17, 2021, 12:19 <roncraig1@...> wrote:

Not hard to imagine that a nichrome wire wound power resistor will have a
lot of series inductance.

Metal film resistors are carbon ink coated ceramic forms. After firing
they are trimmed to value with a barber-pole spiral trim cut either by
laser or diamond cut wheel. The spiral trim gives them more series
inductance. More inductance the greater the spiral trim length to get a
given batch to resistance value so varied RF performance between same value
part.

Chip resistors are made similarly but triming is usually inter-digital
cuts from each side. Some are trimmed by a small sand blaster to one side
of ink. Chip resistors are better for RF performance then leaded metal
film resistors.

Best for RF performance are old carbon composition resistors with their
core carbon brick. Hard to get and expensive these days.





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On Sat, Jul 17, 2021 at 02:39 PM, Matthew Chambers NR0Q wrote:


So a 50ohm metal film probably is going to be a horrible choice for
calibrating my NanoVNA on HF at the end of a coax run not terminated with a
coaxial connector (terminated with lugs to fasten to my vertical)

For HF frequencies the effect of resistor lead inductance will have a very small affect on calibration.

Roger


 

A long time ago I spent too much time going around just getting the part.
They cost more back then when money wasn't worth a lot.
Recently I needed an attenuator and went to Amazon. They seem to have a
nice assortment, probably not good from DC to daylight, but in your choice
of connectors.
How many contacts would have missed giving someone else your grid square?
I don't sell for Amazon or like their politics, but I like to play radio.
Good luck on your project.


On Sat, Jul 17, 2021, 6:37 PM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:

On Sat, Jul 17, 2021 at 02:39 PM, Matthew Chambers NR0Q wrote:


So a 50ohm metal film probably is going to be a horrible choice for
calibrating my NanoVNA on HF at the end of a coax run not terminated
with a
coaxial connector (terminated with lugs to fasten to my vertical)

For HF frequencies the effect of resistor lead inductance will have a very
small affect on calibration.

Roger






 

On 7/17/21 2:38 PM, Matthew Chambers NR0Q via groups.io wrote:
So a 50ohm metal film probably is going to be a horrible choice for
calibrating my NanoVNA on HF at the end of a coax run not terminated with a
coaxial connector (terminated with lugs to fasten to my vertical)
Maybe, maybe not - 50 ohms is a low resistance. They cost pennies, if you have one, hook it up directly to the NanoVNA and see.? You could solder it to a SMA jack and use a barrel to connect it.

Chip resistors are almost all low inductance - and even cheaper (although you probably need to buy 100 of them). Harder to solder a wire onto.



On Sat, Jul 17, 2021, 12:19 <roncraig1@...> wrote:

Not hard to imagine that a nichrome wire wound power resistor will have a
lot of series inductance.

Metal film resistors are carbon ink coated ceramic forms. After firing
they are trimmed to value with a barber-pole spiral trim cut either by
laser or diamond cut wheel. The spiral trim gives them more series
inductance. More inductance the greater the spiral trim length to get a
given batch to resistance value so varied RF performance between same value
part.

Chip resistors are made similarly but triming is usually inter-digital
cuts from each side. Some are trimmed by a small sand blaster to one side
of ink. Chip resistors are better for RF performance then leaded metal
film resistors.

Best for RF performance are old carbon composition resistors with their
core carbon brick. Hard to get and expensive these days.






 

Try measuring higher resistance carbon resistors at RF. The shunt C of somewhere under 1 pF becomes the issue as frequency is increased. My first investigation into this was with the ancient HP(Boonton Radio Corp design) 250B RX meter back years ago (maybe in the '80's). The lead inductance is swamped out by this shunt C. The 250RX meter could indicate the Cp down to tenths of a picofarad and the Rp up to 100 k. I still have the old beast and use it and an old GR-821. I was using them to investigate the dielectric properties of PVC at RF when there was a discussion about how lossy it is at RF (not what I found). I didn't see the value of chucking a piece of the stuff into a microwave oven at 2.45 GHz when we were concerned <30 MHz. I wound up figuring out how to determine the dielectric constant along with the loss tangent of the PVC and for other materials.


 

High value resistors will become capacitive at high frequencies, while low value ones will become inductive.

Both the series inductance and the parallel capacitance of a resistor will of course have an increased effect as the frequency rises, but depending on the resistor's value, and its type/construction/size, one or the other will become problematic first, as you raise the frequency.

So for each type and physical size of resistor, there is a certain range of values that tend to remain resistive to the highest frequencies, while those above degrade into capacitors and those below into inductors. The higher the frequency of operation, the narrower the usable resistance range becomes.

RF circuits are usually designed so that only resistors inside this sweet range are used in RF-carrying paths.

With the kind of quarter watt carbon film resistors I used for many years, and in the range of frequencies I use in my ham activities, typically values from about 15 to 200 ohm work best. Using carbon film resistors in the tens of kiloohm range at RF is futile, and so is the use of resistors around 1? and below.

SMDs are much better at RF than leaded resistors, and the smaller they are, the better. This extends the range of resistance values that can be used at a given frequency. If the resistor is so small that you can't see it, let alone handle it, then it's probably pretty good at UHF! Say, 0201 size.

Thin film SMD resistors are better in this regard (but also less common and more expensive) than the usual thick film resistors.

Wired metal film and metal oxide resistors are made in the same way as wired carbon film ones: Thin resistive layer applied to ceramic rod, spiral-cut to value. Only the resistive material changes. But I suspect that metal film ones, given the lower specific resistance of their material, may get more turns cut into them, and thus become slightly more inductive. At least my measurements with the NanoVNA tend to show slightly higher inductance for metal film resistors, compared to carbon film or metal oxide film.

And lastly, series inductance and parallel capacitance are not the only problems at RF. Change of resistance value happens too, as correctly stated by Zack at the beginning of this thread.

I really enjoy using the NanoVNA now to test every part that goes into my circuits. This has removed a lot of surprises and head-scratching. When I started building RF stuff while still at school, I didn't even suspect anything about all these real-world effects. As a result, my RF circuits often had some strange behavior which I didn't expect nor couldn't explain. The NanoVNA came out about 40 years too late for me... And I think that many people share this thought!


 

On Fri, Jul 16, 2021 at 10:13 AM, Zack Widup wrote:


I kind of figured the carbon film resistors weren't as accurate at VHF/UHF
frequencies compared to carbon composition resistors. This verified it.
Probably a skin effect problem.
There is possibly even more important effect - nanoVNE does NOT measure impedance as an absolute value - it compares the current one with what you have declared as 50+j0 at the time of calibration. If what you used for calibration is not an ideal 50ohm, all the measures after that are invalid, or at least have inherited some "shift" in value.

If you used some of high quality (usually means expensive :)) 50ohm terminators, you are much closer to getting reliable results :)


 

On Sun, 18 Jul 2021 at 16:43, Manfred Mornhinweg <manfred@...> wrote:

High value resistors will become capacitive at high frequencies, while low
value ones will become inductive.

Both the series inductance and the parallel capacitance of a resistor will
of course have an increased effect as the frequency rises, but depending on
the resistor's value, and its type/construction/size, one or the other will
become problematic first, as you raise the frequency.

So for each type and physical size of resistor, there is a certain range
of values that tend to remain resistive to the highest frequencies, while
those above degrade into capacitors and those below into inductors. The
higher the frequency of operation, the narrower the usable resistance range
becomes.

RF circuits are usually designed so that only resistors inside this sweet
range are used in RF-carrying paths.

With the kind of quarter watt carbon film resistors I used for many years,
and in the range of frequencies I use in my ham activities, typically
values from about 15 to 200 ohm work best. Using carbon film resistors in
the tens of kiloohm range at RF is futile, and so is the use of resistors
around 1? and below.

SMDs are much better at RF than leaded resistors, and the smaller they
are, the better. This extends the range of resistance values that can be
used at a given frequency. If the resistor is so small that you can't see
it, let alone handle it, then it's probably pretty good at UHF! Say, 0201
size.

Thin film SMD resistors are better in this regard (but also less common
and more expensive) than the usual thick film resistors.

Wired metal film and metal oxide resistors are made in the same way as
wired carbon film ones: Thin resistive layer applied to ceramic rod,
spiral-cut to value. Only the resistive material changes. But I suspect
that metal film ones, given the lower specific resistance of their
material, may get more turns cut into them, and thus become slightly more
inductive. At least my measurements with the NanoVNA tend to show slightly
higher inductance for metal film resistors, compared to carbon film or
metal oxide film.

And lastly, series inductance and parallel capacitance are not the only
problems at RF. Change of resistance value happens too, as correctly stated
by Zack at the beginning of this thread.

I really enjoy using the NanoVNA now to test every part that goes into my
circuits. This has removed a lot of surprises and head-scratching. When I
started building RF stuff while still at school, I didn't even suspect
anything about all these real-world effects. As a result, my RF circuits
often had some strange behavior which I didn't expect nor couldn't explain.
The NanoVNA came out about 40 years too late for me... And I think that
many people share this thought!






 

An interesting discussion about lead inductance and shunt capacitance of carbon composition, carbon film and high wattage carbon resistors in this article by Bob Botos of HP.



Roger


 

A side note here. For a geezer whose background extends back into the
tube era, er, valve era, the expression "carbon composition" brings to
mind the matter of stability.

Some users on this list may be attempting precise, repeatable,
measurements with jigs. Those users should be aware that carbon
composition resistors, in the past, could not be trusted to hold their
value. They demonstrated a very distinct drift toward a higher value.
The drift could be rapid. That change in value occurred not just in the
hard service seen in tube circuits but while brand new and sitting in a
parts drawer. In our ignorance, we used to conjecture that the material,
compressed during manufacture, was expanding minutely over time and so
causing a drift to higher resistance.

If relaxation of the compressed material is really the issue, then it
would be reasonable that there would be a change in reactance, too.

John,
at radio station VE7AOV
+++++


On 2021-07-18 9:09 a.m., Roger Need via groups.io wrote:
An interesting discussion about lead inductance and shunt capacitance of carbon composition, carbon film and high wattage carbon resistors in this article by Bob Botos of HP.



Roger
...
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