I stumbled upon this document. Simple curiosity of choice of capacitance of the crystal master
See appendix B:
Turn the uBITX on and as soon as the Splash Screen (Banner) appears, press and hold the Encoder/Function button until “Alignment” appears. Immediately release the button. XTAL (6, 8 or 10pF) will appear along with "push to set". Using the encoder select the load capacitance of the master crystal on your Raduino. The default is usually 8 pF if it’s a factory Raduino card. If you don’t know the master crystal’s load capacitance, assume 8 pF and just press the encoder button to set.?



My questions:
This means that Arduino can have a different capacitance?
How do we find his?
In the software of KD8CEC is it stated somewhere?

Simply for curiosity
cdt

On May 17, 2021, at 3:29 PM, Jerry Gaffke via groups.io <jgaffke@...> wrote:

?

Best to leave it at 8pF, though it doesn't really matter.

Quartz crystals are cut for a parallel resonance that assumes some stray parallel capacitance
due to traces and device pins.? They typically assume around 18pF.

The Raduino board layout might add a couple pF, but not enough to get that up to 18pF.
For this reason the 25mhz reference oscillator is typically about 150khz too high.
This is easily compensated for by the calibration procedure, which adjusts the value of si5351bx_vcoa

Jerry, KE7ER

On Mon, May 17, 2021 at 09:57 AM, Gerard wrote:

I stumbled upon this document. Simple curiosity of choice of capacitance of the crystal master
See appendix B:
Turn the uBITX on and as soon as the Splash Screen (Banner) appears, press and hold the Encoder/Function button until “Alignment” appears. Immediately release the button. XTAL (6, 8 or 10pF) will appear along with "push to set". Using the encoder select the load capacitance of the master crystal on your Raduino. The default is usually 8 pF if it’s a factory Raduino card. If you don’t know the master crystal’s load capacitance, assume 8 pF and just press the encoder button to set.?



My questions:
This means that Arduino can have a different capacitance?
How do we find his?
In the software of KD8CEC is it stated somewhere?

Simply for curiosity
cdt

Thanks Jerry,

You beat me to the explanation. ?The OP mentioned a menu routine we (TSW) had in some of our early firmware but we removed it after we discovered it essentially made no difference. ?We just set it to a compromise 10pF and left it there. ?

?

Start up drift is so minimal it isn’t even worth spending time messing with it.

?

Jim, W0EB

TSW Project Coordinator

On May 17, 2021, at 18:34, Jerry Gaffke via groups.io <jgaffke@...> wrote:

?You're right, 10pf is the max value, not 8pf.
Probably best to set it to 10pf.

My understanding is that the load capacitance setting has nothing much to do with startup drift.
Only with the final parallel resonant frequency when the crystal is placed in a circuit.
These crystals are intended for a circuit with more parallel capacitance than we have,
so in our circuit it operates at about 25.150mhz instead of 25.000mhz.
But that's ok, it gets calibrated out.

Setting it for the minimum capacitance value of 5pF would cause it to operate at an even?
higher frequency.??

I would like to know if I am wrong about this startup drift thing.

Jerry, KE7ER



On Mon, May 17, 2021 at 02:54 PM, Jim Sheldon wrote:

Thanks Jerry,

You beat me to the explanation. ?The OP mentioned a menu routine we (TSW) had in some of our early firmware but we removed it after we discovered it essentially made no difference. ?We just set it to a compromise 10pF and left it there. ?

?

Start up drift is so minimal it isn’t even worth spending time messing with it.

?

Jim, W0EB

TSW Project Coordinator

My measurements have been more like 4 kHz off, 150 kHz off is a major major error.

On May 17, 2021, at 19:45, Jerry Gaffke via groups.io <jgaffke@...> wrote:

?Gordon,

You're right, thanks for the correction.
Like my mistake on the 8pf vs 10pf value, the 150khz was off the top of my head.
Who know's what else might be up there.

150khz/25mhz * 1million = 6000ppm
Maybe I was vaguely remembering 150ppm?
150ppm/1million * 25mhz = 3750hz

With proper capacitive loading, those quartz crystals are typically within 50ppm.
(ppm means "parts per million")

Jerry, KE7ER


On Mon, May 17, 2021 at 04:12 PM, Gordon Gibby wrote:

My measurements have been more like 4 kHz off, 150 kHz off is a major major error.

You're right, 10pf is the max value, not 8pf.
Probably best to set it to 10pf.

My understanding is that the load capacitance setting has nothing much to do with startup drift.
Only with the final parallel resonant frequency when the crystal is placed in a circuit.
These crystals are intended for a circuit with more parallel capacitance than we have,
so in our circuit it operates at about 25.150mhz instead of 25.000mhz.
But that's ok, it gets calibrated out.

Setting it for the minimum capacitance value of 5pF would cause it to operate at an even?
higher frequency.??

I would like to know if I am wrong about this startup drift thing.

Jerry, KE7ER



On Mon, May 17, 2021 at 02:54 PM, Jim Sheldon wrote:

Thanks Jerry,

You beat me to the explanation. ?The OP mentioned a menu routine we (TSW) had in some of our early firmware but we removed it after we discovered it essentially made no difference. ?We just set it to a compromise 10pF and left it there. ?

?

Start up drift is so minimal it isn’t even worth spending time messing with it.

?

Jim, W0EB

TSW Project Coordinator