Inductor for low frquency filter 1

[rfonboard]

I am trying to build a 5KHz passive elliptic LPF for harmonic supression. The problem was that the 1.2mH axial high current inductor caused a severe harmonic distortion. The frequecy response was exactly as it supposed to be designed. But the filter introduced higher harmonics rather than to suppress them.

Do you guys know a better inductor capable for this filter?

 

[nbucska]

what is your signal like and what do you need ? <nbucska@pcperipherals.com>

 

[rfonboard]

The filter is designed to filter out the harmonics of 5KHz, which are 10KHz and 15KHz. Since the harmonics of 5KHz signal from the synthesized generator are -80 dBc relative to the fundamental, through this filter, the harmonics should be -110 dBc, which should noe be observed by using a spectrum analyzer.

But the problem was through the passive filter the harmonics were &quot;amplified&quot; to just -60dBc. I am pretty sure that the inductor caused the distortion.

I want to find a 1.2mH inductor for this filter not to induce the harmonic distortion. I hope you guys have some experience with this problem.

 

[nbucska]

why not use active filter ? The inductor can pick up noise
from ext. magnetic field, too. What is your waveform or
spectrum and amplitude ?

<nbucska@pcperipherals.com>

 

[rfonboard]

I have tried all kinds of active filter, like Chebychev, Multi-feedback, GIC, GIC elliptic, etc. Even the distortion performances of some OP AMP can be -120 dB in data sheet, once built in filter, the harmonic distortion can not be better than -80 dBc.

Active device normally be more distortable than passive device. That's why I went to use passive filters.

 

[melone]

Try using 2 inductors in series. When one &quot;breaks down&quot; (with regards to frequency suppression), the other will still block.

Just a thought (I don't know too much about inductor technology).

 

[rfonboard]

The filter is designed to filter out the harmonics of 5KHz, which are 10KHz and 15KHz. Since the harmonics of 5KHz signal from the synthesized generator are -80 dBc relative to the fundamental, through this filter, the harmonics should be -110 dBc, which should noe be observed by using a spectrum analyzer.

But the problem was through the passive filter the harmonics were &quot;amplified&quot; to just -60dBc. I am pretty sure that the inductor caused the distortion.

I want to find a 1.2mH inductor for this filter not to induce the harmonic distortion. I hope you guys have some experience with this problem.

 

[melone]

Why did you repost the same message?

 

[rfonboard]

Sorry! I thought my post weren't updated.

 

[nbucska]

It is not easy to get 110 dB attenuation. You can have distortion due to saturation or noise or simply your
output circuit may pick up the noise or radiation
generated by your input circuit. You may need shielding
or more careful layout.
<nbucska@pcperipherals.com>

 

[rfonboard]

You are right. Shielding will help for 110 dB attenuation. But at this frequency it should be very easy to achieve 90 to 100 dB attenuation. It did not gete close to it and the harmonics out of the filter were much higher than the input. Talking with the inductor manufacturer, I know the inductor is not in saturation problemsince the current is 50 times smaller than the saturation current.

 

[BrianG]

Hi there, If I have understood this thread correctly this is effectively an a &quot;audio&quot; filter. You state that 110dB attenuation should be no problem to achieve at 5kHz, but you use the dBc weighting, i.e. referenced relative to the carrier. What is the actual level of the fundamental signal?
From my experience designing modem filters I think you may find that even at 5kHz you will require very careful layout - and measurement technique - to achieve a real 110dB attentuation. Have you actually measured the noise spectrum from the filter, because it is very easy to pick up noise at those sort of expected low levels from other test equipment, e.g. PC c.r.t. screens, which can add to the apparent output voltage from the filter.

 

[rfonboard]

Right now, the problem is the harmonic distortion. With 1V pk-pk fundamental signal, the harmonics are way way higher than the noise floor. They suppose to be less than the noise floor.

 

[nbucska]

I gave BRIANG a star for being right : it is not a trivial
task to get -110 dB.

I measured at 1 MHz. -60dB between two 1/4 W resistors
at 6&quot;.

You can try to divide the 1.2 mH into two and arrange them
antiparallel, or rotate it to minimum. Try shielding, better grounding scheme, etc.
<nbucska@pcperipherals.com>

 

[rfonboard]

I discussed this problem with an enginner from Coilcraft. I thought this distortion might still be caused by the saturation distortion of the inductor. It seemed that the ferrite core used for inductor would not work well at such a low frequency.

I am going to try the inductor built with either air core or iron core, which are used for audio frequency.

 

[nbucska]

Before you build a 1.2 mH aircore inductor, perhaps it
would help us to know, what you are trying to do and why ?

My experience is that if something is very difficult, often
it is an indication that we picked a very long shortcut. <nbucska@pcperipherals.com>

 

[kevinkr]

You didn't mention what type of inductor you are using, from the symptoms described it sounds like you are using an iron-cored or ferrite inductor, and that the core is saturating.. I would recommend you use a large air core inductor and if you are handling significant power a speaker cross-over inductor would be helpful.. Solen amongst others makes an excellent and inexpensive line of air core inductors.. Check out their website at

http://www.solen.ca

- they sell direct.. Note I have no affiliation with them in any way.

 

[rfonboard]

Tnanks! I did try the air core coil used for speaker crossover. The distortion was still bad. Finally I got the idea that the distortion was caused by the 1 uF ceramic chip capacitors used in this filter. It seems that at such a low frequency the dielectric material of the caps has a high voltage coefficient. After changing them to some mylar caps (I think the polypropylene cap will be even better), the filter did achieve -110 dBc harmonic distortion. I may try to use the polypropylene caps in my active filter circuits to see if I could have the same distortion performance.

I appreciate your help very much.

 

[kevinkr]

Hi,
Interesting, I have had lots of problems with ceramic caps and distortion in audio circuits.. Note Y5V dielectric is about the worst and X7R isn't much better, anything above 0.1uF should be regarded with suspicion in a ceramic. Unfortunately ceramics are quite microphonic too, and this correlates well to the distortion problem as well. Tantalums are better in terms of distortion and generally non microphonic, still not my first choice. Polypropylene are quite good, however just like large ceramics they can be microphonic, so if this is a problem there are special relatively non-microphonic types available.. (Tighter wrap I believe.)