Are Metallized Resistors Noisy?

Thread Starter

SamR

Joined Mar 19, 2019
5,020
In AF and RF design metallized capacitors are placed foil side to ground to shield and eliminate noise problems. But what about metallized resistors? Are they a noise source in AF and RF designs? Should I be using carbon film or composition resistors instead?

Also, are 1% resistor really needed in most AF and RF designs unless they are being used in an oscillator or filter design application? When is the greater expense of higher specification resistors justified?
 

Ian0

Joined Aug 7, 2020
9,523
Metal films resistors are the best, the usual surface mount resistors are Ruthenium Oxide, which is not completely linear and does produce slightly more noise than the maths suggests vn=√(ktfR) but your amplifier would have to be damn' good before anyone would notice.
Metal oxide are better than carbon film, and carbon composition are only really suitable for snubber networks and other applications that have high pulse power.
Doug Self has a good chapter in his book - I can scan it for you if you don't have a copy.
 

nsaspook

Joined Aug 27, 2009
12,840
In AF and RF design metallized capacitors are placed foil side to ground to shield and eliminate noise problems. But what about metallized resistors? Are they a noise source in AF and RF designs? Should I be using carbon film or composition resistors instead?

Also, are 1% resistor really needed in most AF and RF designs unless they are being used in an oscillator or filter design application? When is the greater expense of higher specification resistors justified?
Metal film resistors are my choice for high resolution DAQ systems. Yes, they in general, have lower noise than most.
The front-end for a typical dose processing input for a faraday with pA current resolution uses resistors like these.

https://www.mouser.com/datasheet/2/414/IRCIS00639_1-2564659.pdf
https://www.vishay.com/en/applicati...diophile_gradecomponents/resistors/low_noise/
 

Thread Starter

SamR

Joined Mar 19, 2019
5,020
Interesting but a bit counter intuitive to me. It would seem like any metals hanging in an AF/RF environment would act as an antenna (especially wire wound resistors acting like inductors) which is why effort is made to put the foil side of capacitor to the ground side. And, yeah, all metal resistors are ultimately grounded. Doug Self talks a lot about reducing resistance due to its Johnson Effect and using Op Amps due to their quieting effect (even with their resistor feedback).
 

Thread Starter

SamR

Joined Mar 19, 2019
5,020
Doug Self has a good chapter in his book
Which book? I have his Self on Audio and Audio Power Amplifier Design Handbook. I've got another technical book somewhere on noise in electronics but can't find it...

Aha, it's in his Small Signal Audio Design book! Need to give it a good look-through!
1697482593908.png
 
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Ian0

Joined Aug 7, 2020
9,523
Which book? I have his Self on Audio and Audio Power Amplifier Design Handbook. I've got another technical book somewhere on noise in electronics but can't find it...
I think it's his small signal book. I'd have to look - that's why I didn't just post it! Give me half an hour. . . .
 

Thread Starter

SamR

Joined Mar 19, 2019
5,020
Small signal audio design, Focal Press, Third Ed, Douglas Self
Definitely added to my To-Do list, great book! And I like his writing style and humor!
 

Ian0

Joined Aug 7, 2020
9,523
Small signal audio design, Focal Press, Third Ed, Douglas Self
Definitely added to my To-Do list, great book! And I like his writing style and humor!
I was there at the beginning. My copy is a first edition. I think there's some good new chapters in the third edition.
 

Thread Starter

SamR

Joined Mar 19, 2019
5,020
I went ahead and read the first 2 chapters since I found it so interesting. His concepts of using multiple low-cost components to produce better quality components cheaper than off the shelf low tolerance components through statistical analysis were very interesting reading. As well as such gems as...

1697487469432.png
Excellent book! I had never heard "Gerswhin's Law" (who was himself a conductor) postulated before! Or that any professor who postulated such "heresies" as Ohm's Law "was unworthy to teach science."
 
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Thread Starter

SamR

Joined Mar 19, 2019
5,020
Aha! I found it. "Electronic Noise and Interfering Signals Principles and Applications"
by Gabriel Vasilescu
As dry and uninteresting as its title... Nothing new here other than a lot of mathematical derivations and technical descriptions without any real-world examples and explanations.

1697488994760.png
1697489562646.png
 

Ian0

Joined Aug 7, 2020
9,523
Nobody really understands 1/f noise. It always ends up with a mysterious constant that can't be defined mathematically or measured.
 

shortbus

Joined Sep 30, 2009
10,044
foil side of capacitor to the ground side
I'm kind of dumb, so could you explain how to do that? It's my understanding that metalized caps have "foil" internally on both leads? If not what makes the internal connection on the other lead?
 

Thread Starter

SamR

Joined Mar 19, 2019
5,020
The metallic cans are marked but only some of the old "wax tube" capacitors (and those like it of that era) had a bar marked on the outer foil end. Same for some of today's poly and ceramic capacitors. You can determine this by hooking one up to the oscilloscope and then flip it over to the opposite connections. The scope will show the noise being picked up by the capacitor but you will notice that the noise is not as high for one way. That is when the outer foil side lead is connected to the probe ground. Which is why you always want to connect the capacitors "foil side" lead to ground and not toward the signal. Otherwise, noise will be introduced into the signal. Yes, both ends connect to foil, but one end is connected to the "Outer Foil" which shields the inner foil from noise.
 

joeyd999

Joined Jun 6, 2011
5,182
Nobody really understands 1/f noise. It always ends up with a mysterious constant that can't be defined mathematically or measured.
I understand it: the longer you observe, the more things change. This holds from milliseconds to eons.

In nature -- in the long term -- nothing's constant.
 

WBahn

Joined Mar 31, 2012
29,883
Nobody really understands 1/f noise. It always ends up with a mysterious constant that can't be defined mathematically or measured.
I think this is true in the general case, as in coming up with a theory that applies universally to all situations that are described by pick noise. But I'm pretty sure that the mechanisms that drive it in specific systems are pretty well understood.
 
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