pushing and pulling crystal's frequency
Basher
Posts: 25
I remember reading a post that said it was possible to adjust the frequency of the xtal crystal in either direction by using a variable inductor or a variable capacitor, but I've not been able to find it again.
What sort of adjustment would be possible with this kind of circuit?
Could a 14Mhz crystal be pulled up to 14.1876Mhz and still remain stable?
Is it a try it and see sort of thing or are there calculations? (I know there are always calculations. What I mean is are there calculations I would be able to make sense of. No calculus or hard sums
)
Would it be possible to set up a circuit to switch the inductor/capacitor in and out, either via a propeller pin or a jumper/dip switch on the PCB.
I've tried searching the web but just keep turning up stuff on PLL's
What sort of adjustment would be possible with this kind of circuit?
Could a 14Mhz crystal be pulled up to 14.1876Mhz and still remain stable?
Is it a try it and see sort of thing or are there calculations? (I know there are always calculations. What I mean is are there calculations I would be able to make sense of. No calculus or hard sums
)Would it be possible to set up a circuit to switch the inductor/capacitor in and out, either via a propeller pin or a jumper/dip switch on the PCB.
I've tried searching the web but just keep turning up stuff on PLL's
Comments
No, that is a whopping 1.34% movement, which is way too far for a crystal.
Tens of ppm, or in some cases towards 100ppm is all you can practically expect from a high Q crystal.
Ceramic resonators can pull further, but even there 1.34% is a long way.
It is mostly a try it and see, and you can derive figures of ppm/pF, which will be different on each side of the crystal amplifier.
If you want your own frequency, Search Digikey for '504 oscillator'. and look for SI5351A-B-GT
- both allow you to generate almost any frequency (to some ppb error)
I was fooled by the decimal point, anything to the right is insignificant
The SI5351 looks interesting. Am I right in thinking that if I used something like this to clock the propeller at 14Mhz I wouldn't be able to use the propeller's xtal x8 setting to multiply the system clk?
The Prop data says thi s
The PLL internally multiplies the XIN pin frequency by 16. ... The PLL’s internal frequency must be kept within 64 MHz to 128 MHz – this translates to an XIN frequency range of 4 MHz to 8 MHz.
So that fixed x16, locks the Crystal in to be within 4..8MHz, to stay within spec.
The Prop does have a DC to 80 MHz Input mode, and there, you can clock directly from the SI5351A-B-GT
(or even try overclock up to 160MHz )