Are inductors "noisier" than capacitors?

Thread Starter

ApacheKid

Joined Jan 12, 2015
385
Sorry for such a badly phrased and imprecise question, but...

I have a trivial circuit:

1613092696998.png

The input is 17 KHz sine, I'm measuring the voltage across the capacitor and across the inductor, naturally one leads and one lags the input signal.

By manual adjustment, 17KHz is the frequency at which the capacitor voltage and inductor voltage are about equal on the scope (at about 650 mV PP) the input signal has amplitude of 4 V PP.

What I'm seeing is a bit odd, just not clear to me, the signal across the inductor has a slight evidence of noise, jitter, not much at all but visible, whereas the signal across the capacitor is rock steady.

The jitter is very small but obvious, I swapped probes to see if they were involved but no.

The circuit is on a breadboard and I've reseated the various components several times but cannot get the jitter to go.

As I say, its small, barely visible but one can see tiny irregularities as if there's a bad joint or something.

So could this be inherent, could it be something within the inductor itself?

OK Update, here's a pic:

1613093938907.png

I increased vertical sensitivity to "zoom in" and adjust trigger so I could see both signals on the screen. I then kept pressing Run/Stop and got the above snapshot.

The yellow is the inductor voltage (blue is capacitor) and based on the timebase setting it looks like the "noise" is actually bursts of approx 10 MHz, very tiny, like 10 mV PP or so.

This is a mystery - to me...

Update - one more thing I just noticed, if I reduce the input signal the noise gets worse! If I increase it reduces !!! making the input signal 10 V PP leads to both signals looking steady, no obvious difference anymore...
 
Last edited:

BobTPH

Joined Jun 5, 2013
3,308
Well, if you look at an old transistor radio, what is the antenna? It is an inductor! That may have something to do with it.

No surprise that increasing the signal decreases the noise, assuming you are looking at relative magnitude.

Bob
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
385
Well, if you look at an old transistor radio, what is the antenna? It is an inductor! That may have something to do with it.

No surprise that increasing the signal decreases the noise, assuming you are looking at relative magnitude.

Bob
Yes, its the inductor, it is picking up EM from nearby, if I unplug some small power adapters (like I have for several LED based desk lamps and so on) the noise diminishes...
 

SamR

Joined Mar 19, 2019
3,406
Have you played with the input filtering to see it can remove it? Low of course to get rid of the 60Hz and then High to get rid of cell phone noise.
 

DickCappels

Joined Aug 21, 2008
7,427
I was once interviewed by an engineer who swore he would never use an inductor in a video circuit because they pick up noise. Later I took a job at a company that used shielded inductors in their video amplifiers. The first company was never heard of again.
 

Ian0

Joined Aug 7, 2020
2,196
Your inductor-resistor ciruit is a high-pass filter with a -3dB frequency of 35kHz. If there is any high-frequency component in your signal it will accentuate it.
It is also an antenna as @BobTPH said.
If the noise appears at the same place in the waveform all the time, then it is distortion/instability from your signal generator. If it appears at random, then it is electromagnetic pickup.
If it just happens to be at the point that the signal crosses zero, then it is crossover distortion from the output amplifier in your signal generator. You need a better signal generator!
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
385
Your inductor-resistor ciruit is a high-pass filter with a -3dB frequency of 35kHz. If there is any high-frequency component in your signal it will accentuate it.
It is also an antenna as @BobTPH said.
If the noise appears at the same place in the waveform all the time, then it is distortion/instability from your signal generator. If it appears at random, then it is electromagnetic pickup.
If it just happens to be at the point that the signal crosses zero, then it is crossover distortion from the output amplifier in your signal generator. You need a better signal generator!
Well I'm glad you said what you did, I actually did some simple experiments and here's what transpired.

The noise does indeed appear at seemingly random points on the overall measured sine signal, the burst seen on the scope above is not always in some fixed region, it's unrelated to signal generator frequency it seems (but the duration of the burst and the apparent frequency of the noise signal seems to be fixed, but I see no evidence of a phase relationship to the signal generator source signal).

It does not seem to behave any different at the crossover point, seems uniform and random.

I then shut off the signal generator completely and the noise became a little higher, had extremely brief pulses occurring at 60 HZ and lesser amplitude noise that seemed to be like 5 to 10 MHz erratic sine shape.

Meanwhile the same measurement on the capacitor was clean, only see noise at very high sensitivity which is normal I think.

This is likely nothing at all to the professionals here, but although I'm not a novice this is the first time (since I was in electronics college in 1980) I've actually sat down and done stuff like this, also these modern digital scopes are truly astonishing in comparison to the analog one's we used at college in the late 70s.

You mentioned high pass filter, I need to refresh my memory on these as it sounds like something I should have already recognized!

This is actually all good, understanding these things is all part of the learning.

Thanks
 

Ian0

Joined Aug 7, 2020
2,196
Similarly, the resistor and capacitor form a low-pass filter, so you won't see any hf interference on the capacitor.
With the signal generator shut down, its output impedance will become higher, and interference will become more obvious.
At least it lets your signal generator off the hook!
 

MrChips

Joined Oct 2, 2009
23,519
You cannot compare the two components that way because you are examining different parts of the frequency spectrum.

The R-C circuit is a low-pass filter. It removes high frequencies.
1613143548624.png

The R-L circuit is a high-pass filter. It passes high frequencies.
1613143713779.png



If you want a fair comparison, use an L-R low-pass filter by interchanging the position of R and L.
1613143775634.png
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
385
You cannot compare the two components that way because you are examining different parts of the frequency spectrum.

The R-C circuit is a low-pass filter. It removes high frequencies.
View attachment 230221

The R-L circuit is a high-pass filter. It passes high frequencies.
View attachment 230222



If you want a fair comparison, use an L-R low-pass filter by interchanging the position of R and L.
View attachment 230223
That's interesting, I will explore that.

Of course all I was originally doing was wanting to observe the phase lead/lag of a signal when an inductor and capacitor were present, its been many years since I did these kinds of basic experiments. The noise just caught my eye, I was a bit concerned that this was something in the signal generator or even scope but it seems it wasn't.

The scope seems very good - certainly for my use - Siglent SDS 1204X-E and the signal generator is - Siglent SDG 1025.
 
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