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Pulling a Ceramic Resonator

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jpanhalt

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Chip = 16F1829
Ceramic resonator: muRata (source: DigiKey), 8 MHz
PCB = soldered breadboard FR4

Problem: In HS mode (datasheet specifies that mode for 4 MHz to 20 MHz), frequency is 0.3% high. Still within spec, but would like to pull it lower. Rock stable for frequency. In XT mode, frequency is better, but not as stable.

Attached an EZ micrograbber to one leg (OSC1/CLKIN) and frequency got better. The datasheet shows some external resistors:
upload_2017-8-23_11-10-30.png

Space is very limited, so any modifications take more time, even just swapping resonators. I am wondering whether it is being over-driven. Question: Can RP and/or Rs be used to pull the frequency down?

Other suggestions?

John

BTW: Curious bit in the errata: The OSTS status bit remains clear when using 4XPLL through version 4. Problem was easily discovered. I guess I wasn't the first to find out.
 

JimB

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I would expect C1 and C2 to have an effect on the frequency, the same as they would with a quartz crystal.
Increase the values to lower the frequency.
JimB
 

jpanhalt

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Yesterday, I tried a cap labeled 4 pF and it pulled it way too slow. Based on your comment, I remembered that I had some smaller ones. It looks like about 2.2 pF does it. And, I can probably get it squeezed in.

Thanks for jogging my memory.

John

Edit: 3.3 pF (label) got me to 0.1% high after soldering. Of course, with a temporary lead to ground, it was right on. ;)
 
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JimB

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Why not use a small variable capacitor (trimmer).
JimB
 

jpanhalt

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For a final version, I might put a little trimmer there or use my SMD's. It is really easy to put those things in parallel. This is just to get my lightning detector antenna tuned up. For now, I can live with 0.1% as it is very steady compared to using the internal oscillator on the PIC.

John
 

BobW

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I'm surprised that the capacitors are so small. I typically use 22 pF caps on the resonator. I guess I'll have to have another look at the spec sheet.
 

jpanhalt

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The Ceralock line (muRata) of resonators I use have built in capacitors. They are Type CSTLS, and their datasheet gives a value of about 47 pF for the included capacitors.
 

BobW

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I wasn't familiar with those ones. Nice that they have the capacitors built in. For something like frequency measurement where the clock frequency is critical, I always use a crystal. It may actually be cheaper since you save the cost of a trimmer capacitor. I see that Digikey has some ridiculously cheap 8 MHz crystals. I didn't check the specs though.
 

jpanhalt

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Hi Bob,
My first choice was, of course, the capacitors. Unfortunately, I had forgotten that I had some leaded capacitors less than "4 pF" from another project. So, the crux of my question was whether those resistors shown in the Microchip schematic could be be used to pull it slightly lower.

Then, I remembered that I had some mica capacitors down to 1.0 ± 0.5 pF and ceramics a bit higher . They are relatively huge but solved the problem. In another thread, it is asked what is on our "to order" list. I have added some 3.9 pF ceramic, COG capacitors, as my choices were 3.3 pF and 4.7 pF. The latter was too high. My capacitor meter is a DE5000 LCR, which is generally quite satisfactory, but fails in sorting such low value capacitances.

My main concern was linearity about the target frequency. From 436 kHz to 564 kHz, the response is absolutely linear (given 0.1 kHz resolution), so I am not particularly worried about that small offset now.

Right now, I am on a bit of a tangent to get a faster 32-bit x 24-or 32-bit division routine.

John
 
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