[moved] How to design a crystall oscilator circuit using CMOS 40106 DESIGN

[thandana]

Hi,

I need some help in designing this crystal oscillator circuit. I have attached the diagram. I want it to oscillate around 1MHZ.
How do i calculate the capacitor and resistor values values?
Can you please share the formulas

 

[betwixt]

Re: How to design a crystall oscilator circuit using CMOS 40106 DESIGN

The capacitor values come from the crystal manufacturers data sheet, if you use 33pF or 39pF it should work fine, the values are not particularly critical.
Likewise, use the resistor values for 5MHz, R1 will stay the same regardless of the frequency, it is just to provide some DC feedback for the inverter, R2 governs the feedback (drive) level to the crystal but using 5K should be fine to keep the level within a safe range.

Brian.

 

[KlausST]

Re: How to design a crystall oscilator circuit using CMOS 40106 DESIGN

Hi,

I want it to oscillate around 1MHZ.

The frequency is determined by the XTAL. If you use a 1MHz one, than you won´t be able to 0.99MHz or 1.01MHz.

I agree with betwixt:
1M, 39pF, 5k should work.

Read the XTAL datasheet and read the application notes provided by the XTAL manufacturer. They are for free.

Klaus

* btw: what supply voltage? Why no ready to buy oscillator?

 

[thandana]

Re: How to design a crystall oscilator circuit using CMOS 40106 DESIGN

Hi Klaus

I want to use an 8MHz crystal.
Would i still get it to oscillate at fo 1Mhz with provided values?

 

[KlausST]

Re: How to design a crystall oscilator circuit using CMOS 40106 DESIGN

Hi,

All information is already given:
--> read the XTAL datasheet (most reliable information)
--> use the recommended values (general recommendation. And your 8MHz isn´t far away from 10MHz in the table)

Klaus

 

[schmitt trigger]

The schematic has one major error: it is incorrectly showing a schmitt trigger inverter, whereas an unbuffered inverter is required for the crystal oscillator.

 

[FvM]

I agree about unbuffered inverter. Other configuration like buffered inverter or ST inverter are likely to cause unstable operation or possibly oscillation on overtones.

The Epson Oscillation Circuit Design Guide is often quoted in this regard, various sources on the internet, e.g. **broken link removed**

It's doesn't give values for 1 MHz but explains the design method. In addition, the total load capacitance is basically specified in the crystal data sheet, you keep it to achieve the nominal crystal frequency.

 

[Aussie Susan]

Re: How to design a crystall oscilator circuit using CMOS 40106 DESIGN

Hi Klaus
I want to use an 8MHz crystal.
Would i still get it to oscillate at fo 1Mhz with provided values?

Short answer: no
Longer answer: not without adding a whole lot more circuitry.
The underlying issue is that a crystal is designed to oscillate at a given frequency. Depending on how it is constructed, it can be possible to drive a crystal in 'overtone' mode (e.g. an 8MHz crystal can be driven at 24MHz) but these are higher frequencies than the fundamental for the crystal.
You can also 'pull' a crystal a little bit but only very small amounts. (*small* here means a few ppm. As to why you can do this, look at the equivalent circuit for a crystal and consider what adding a bit of parallel capacitance would do.) Crystals have a very high 'Q' (look up 'Q Factor' if you don't know what this is) and so the bandwidth (needed for oscillation) is very narrow.
What you can do is to make an 8MHz oscillator and put a 'divide by 8' circuit after it - that is what I mean by adding more circuitry.
Susan