1. Home
  2. Hobby Electronics Basics
  3. Optimum RC values for crystal oscillator?

Optimum RC values for crystal oscillator?

I made a low frequency crystal oscillator using a couple CMOS inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large crystal package that measures about 3/4 by 3/8 inch from the 1970s.

The first inverter feeds the second, and the output of the second connects the crystal back to the input of the first inverter. The first inverter has a 100K resistor connected from input to output and there is a 470pF cap from the input to ground. Works well, but I'm wondering what the best RC values should be? I was thinking the reactance of the cap should be about equal to the resistor, but that works out to about 50pF which doesn't work. Lots of different combinations will work, such as 22K and 1000pF.

How does one determine the best values?

-Bill

Reply to
Bill Bowden
Loading thread data ...

The resistor controls loop gain. I would find the lowest value for which oscillation starts reliably and multiply it by 10.

The loading capacitance will affect frequency. Since you are using a crystal, presumably you care about frequency (if not, you could use a simple RC oscillator) so choose the capacitor which gets you closest to 31.500 KHz.

I was recently playing with miniature 32.768 KHz tuning-fork watch crystals, which require extremely low drive levels; but I presume your large 1970s crystal is not of this type.

Reply to
Andrew Holme

Thanks for the reply.

Yes, the little 32768 tuning fork watch crystals work well with little drive. I'm just playing around with this old "heavy duty" crystal, I really don't need to use. My understanding of crystals is they look like a high Q parallel LC circuit at resonance, and to excite them, they just need feedback at 180 degrees. But that doesn't seem to work, and some phase shift is needed for reliable operation, and so the addition of the RC components to shift the phase slightly so the feedback is not exactly 180 degrees.

Using 100K and 470pF yields a frequency of 31493 which is only a 7 cycle error out of 31500.

-Bill

Reply to
Bill Bowden 

--
Crystals can look like either series or parallel resonant circuits,
depending on how they\'re driven.

Here\'s a pretty good explanation:

http://download.intel.com/design/mcs51/applnots/23065901.pdf

JF
Reply to
John Fields

Yes, it looks like a series resonant circuit needs 360 degree feedback (two inverters) and the parallel circuit with 180 degree feedback needs only one inverter with some bias.

But, I broke my crystal when I swiped my hand across the table and the crystal went flying, hit the wall, and landed on the floor, and doesn't work anymore. So, I tried another crystal of larger size in the parallel configuration using just one inverter (parallel resonant) with good results. This thing is huge and measures 1.5 by 3/4 inch. It's 50KHz and works well using 47K and 470pF. I found I could adjust the duty cycle with the resistor value.

-Bill

Reply to
Bill Bowden

Hi

Maybe you can hep me. I tried a 32.768 with 8051 microcontroller. I used 11 pf capacitor and a 330k resistor in series and it worked well on the breadboard. But when I made a pcb and soldered all the parts exactly as was in the breadboard the crystal refused to oscillate.

What is missing?

Thanks for your time and attention Archer

Reply to
Ardent

Ardent wrote in news: snipped-for-privacy@4ax.com:

probably another 20-30 pf...

try something around 68 pf as a start.

----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==----

formatting link
The #1 Newsgroup Service in the World! >100,000 Newsgroups

---= - Total Privacy via Encryption =---

Reply to
me

Sorry for the delayed response, I was away.

Thanks for the suggestion, I will try it and let you know later

Thanks for you time

Sandy

Reply to
Ardent

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.