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 Post subject: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 2:35 pm 
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I have some NP0 capacitors do they have a black marking on top? I was using one NP0 cap in my FM TX and frequency was very stable. Now I replaced them with silver mica and I am seeing some frequency drift. Why is that so?

I connected both NP0 and silver mica cap to my DE-5000 LCR meter and heated both the caps with soldering iron. The NP0 capacitance started decreasing as soon as I applied the heat but silver mica's value was as it is and there was zero change.

Does that mean silver mica caps are better than NP0 for use in high frequency oscillators? I am new to VFOs. Can somebody with more experience enlighten me?

I still think I saw less drift with the NP0 cap.

Thanks
Joy


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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 2:56 pm 
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Here's an article with temperature coefficients of various types of ceramic capacitors:
https://forum.digikey.com/t/understandi ... cients/727

For silver-mica, I found 50ppm/K

In tuned circuits, caps with a significant temperature coefficient are sometimes used to compensate for some other effect.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 4:39 pm 
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Inductors have positive temperature coefficients. So in LC oscillators, you need to use capacitors with matching negative temperature coefficients to compensate. Using mica caps is not necessarily a good thing.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 4:44 pm 
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Unless built to be so, a silver-mica does not do temperature compensation. If it is marked, it will either be in print or color dots what the ppm is.

That said shotgun with a generic silver mica may get a radio that drifts all over. Not just the frequency going away but in the RF a loss of sensitivity during warm up or poor sensitivity until it does warm up...

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 4:54 pm 
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bb.odin wrote:
Inductors have positive temperature coefficients..

Air-core?---what would cause that?

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 5:03 pm 
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pixellany wrote:
bb.odin wrote:
Inductors have positive temperature coefficients..

Air-core?---what would cause that?
Minute expansion/contraction of the winding and the metal used in the windings.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 8:44 pm 
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Simply the change in geometry due to the tempco of the copper wire?

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 05, 2020 11:44 pm 
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Dare4444 wrote:
I have some NP0 capacitors do they have a black marking on top? I was using one NP0 cap in my FM TX and frequency was very stable. Now I replaced them with silver mica and I am seeing some frequency drift. Why is that so?

....

Thanks
Joy


Yes, Black tipped ceramic disc caps are NP0 or C0G capacitors. The other two common colors are Orange (N150) and Purple /Violet (N750) :
Attachment:
Tempco Colors.gif
Tempco Colors.gif [ 119.23 KiB | Viewed 1319 times ]


EDIT: LINK: https://archive.org/details/MullardBook ... /page/n131

US manufacturers, for the most part, often used a single letter code to denote Tempco where space was limited. With the tubular ceramics and their all-color-code scheme, you saw the Tempco color coded - same with some military spec ceramic caps. Now with the plethora of EIAJ and Euro capacitors out there, the code is more prevalent than say, pre-1990 or so.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Mon 06, 2020 1:03 am 
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pixellany wrote:
Simply the change in geometry due to the tempco of the copper wire?

Exactly. Take the case of a solenoid. Its inductance is proportional to its area and inversely proportional to its length. The copper wire expands with high temperature causing the area to increase and thus the inductance. To a lesser extent the wire spacing also increases causing a decrease in inductance.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Mon 06, 2020 4:32 am 
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Frequency was drifting all over with silver mica in my oscillator. It's weird. Then I replaced them with NP0 the ones with black markings and drift was 3KHz over a 9 hour period and the board wasn't even protected from air currents as it lay open. NP0s are the best.


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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Wed 08, 2020 4:42 am 
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Thank you everyone for all the valueble info.
I've saved the replies.


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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Wed 08, 2020 8:02 am 
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Greetings to Dare4444 and the Forum:

Dare4444 wrote:
NP0s are the best.


They may be best in your particular circuit, but that does not mean they are the best in all temperature / frequency sensitive applications. As bb.odin pointed out, some circuits require negative temperature compensation and some positive.... and some require both, such that two or more capacitors with different temperature coeffecients are employed in combinations so that the curves cross each other.

As a general rule, it is best to replace capacitors in such circuits as VFO's with the same value and temperature coefficient as the originals. Sadly, in some cases (some Collins PTO's for example) that is not possible, as the special caps employed are no longer made.

Regards,

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Fri 10, 2020 5:16 pm 
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...
C0G/NP0 ceramic temperature coefficient of capacitance can be up to plus or minus 30 parts per million per degree C.

CDE gives up to +70 ppm as the lowest tempco for their cataloged radial lead dipped micas:
Attachment:
CDE_RADIAL_MICAS.jpg
CDE_RADIAL_MICAS.jpg [ 93.96 KiB | Viewed 1081 times ]

CDE shows up to 0.075% mica dissipation factor for 1000 pF or less:
Attachment:
HIGH_REL_CDE_MICA_DF.jpg
HIGH_REL_CDE_MICA_DF.jpg [ 103 KiB | Viewed 1081 times ]


KEMET gives maximum dissipation factor for C0G/NP0 radial and axial lead ceramic as 0.15% maximum for all capaciitances:
http://www.kemet.com/Lists/FileStore/Ce ... %20X7R.pdf
Attachment:
Kemet_C0G_DF.jpg
Kemet_C0G_DF.jpg [ 34.25 KiB | Viewed 1081 times ]

Temperature compensation of L-C resonant circuit by selected tempco capacitor as portion of total capacitance, ref. Charles D. Perrine, "An Answer to the E.C.O. Problem", QST, Sept. 1939:
Attachment:
TEMPERATURE_COMPENSATION_CAPACITOR_01.jpg
TEMPERATURE_COMPENSATION_CAPACITOR_01.jpg [ 188.31 KiB | Viewed 1081 times ]

Attachment:
TEMP_COMP_CAPACITOR__02.jpg
TEMP_COMP_CAPACITOR__02.jpg [ 53.17 KiB | Viewed 1081 times ]

-----------
WB5HDF


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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Fri 10, 2020 5:52 pm 
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It gets even more complicated if a powdered iron core is used to increase inductance. They can greatly influence
thermal influences.

One trick was to use a small differential variable capacitor with a NPO and a cap with a Negative temp coefficent
on either side. This was used to balance out the drift in the inductor by varying the amount of compensation.
Adjusting the differential cap did not affect the frequency, just the drift correction.

One problem is that the compensating capacitor must follow the thermal changes on the other frequency
determining elements. Usually a small value negative compensation cap is used to trim out drift.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Fri 10, 2020 9:55 pm 
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Does etching the coil on PCB eliminates expansion of copper? I've seen several FM transmitter circuits on Harry Lythall's website http://www.sm0vpo.com/ and they have the inductor etched on PCB itself. He says it greatly reduces frequency drift. And if capacitors are so sensitive to temperature changes, shouldn't an oscillator contain as few capacitors as possible?


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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Sun 12, 2020 11:41 am 
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Dare4444 wrote:
Does etching the coil on PCB eliminates expansion of copper?

No, it does not. Coils etched on PCB usually have a spiral shape. FR4, the material commonly used for PCB, has a CTE of about 15 ppm/degree C. The area and thus the inductance of the coil increases by the same amount. However the turn spacing also increases leading to an inductance reduction. By carefully designing a geometry for an etched coil, and taking into account that the equivalent inductance varies with frequencies, the CTE can be reduced to a few ppm but not zero.

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 Post subject: Re: Silver Mica vs. NP0 I am confused
PostPosted: Jan Tue 21, 2020 2:51 pm 
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bb.odin wrote:
Dare4444 wrote:
Does etching the coil on PCB eliminates expansion of copper?

No, it does not. Coils etched on PCB usually have a spiral shape. FR4, the material commonly used for PCB, has a CTE of about 15 ppm/degree C. The area and thus the inductance of the coil increases by the same amount. However the turn spacing also increases leading to an inductance reduction. By carefully designing a geometry for an etched coil, and taking into account that the equivalent inductance varies with frequencies, the CTE can be reduced to a few ppm but not zero.


This is the FM oscillator in my circuit.

https://ibb.co/987cZCp

When I used all NP0 capacitors then stability was poor. Then I used NP0 for only C1 and C2 while C3 and C4 are normal ceramic disks and Stability is excellent now. The capacitance of C4 changed a lot when I heated it with a soldering iron. C4 was connected to a LCR meter for testing. So the frequency in a VFO depends a lot on the combination of capacitors used.

The coil is very tightly secured with superglue. Touching the coil and warming it with my fingers only shifts the frequency upwards by 3-4KHz or so.

So all NP0 capacitors might not be a good idea for a VFO. The right combination has to be chosen by trial and error for maximum stability.

It's an art.

Replacing C4 with high quality parallel NP0 capacitors caused the oscillator to drift while using cheap ceramic capacitor for C4 is giving good stability. Weird and counterintuitive.


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