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Design >> Analog Design >> Oscillator Design
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Message started by RuRob on Aug 23^rd, 2010, 12:24pm

Title: Oscillator Design
Post by RuRob on Aug 23^rd, 2010, 12:24pm

Hello, I started with the oscillator design, I need an 20MHz oscillator with low jitter, I selected the Pierce Topology, and i'm going to start with the the design and simulation phase.

I have a problem with de simulation phase, because i don't know how i'm going to start this phase, when i'm going to implement the oscillator schematic, i need the circuit equivalent of the crystal, i searched for a Crystal that worked in fundamental mode, with this characteristic, but i can't find the circuit equivalent of the crystal in the datasheets. I don't know if this values are important in the simulation phase.

Anyone have experience with the oscillator design?

Title: Re: Oscillator Design
Post by uu@uk on Aug 23^rd, 2010, 1:59pm

Hi,

since you are concerned about the jitter, I think you need to figure out what is the Q of your crytal quartz, which depends on the equivalent circuit of the crustal. It will affect the phase noise hence the jitter.

I am not quite sure the phase simulation you mentioned. Do you mean phase noise simulation.

Title: Re: Oscillator Design
Post by RuRob on Aug 23^rd, 2010, 3:37pm

Hello, Thanks for the answer.

When i'm writing "simulation phase"... i'm writing about that i'm doing the simulations using spectreRF, now... i know about de Q of the crystal... but the problem is that i can't find the circuit equivalent of the crystal... i need a crystal with 20MHz of frequency and a capacitance load of 20 pF. Do you know some web, where I can find the circuit equivalent for a crystal with this specifications?

Thanks!

Title: Re: Oscillator Design
Post by uu@uk on Aug 23^rd, 2010, 5:26pm

The attachment discusses an equivalent circuit of crystal, which might be helpful. 20 pF load cap crystal is rarely seen to me. Maybe you can try some crystal manufacurer like citizen or TXC. good luck.

Title: Re: Oscillator Design
Post by RuRob on Aug 23^rd, 2010, 5:40pm

Thanks for your help!

I will continue looking for a crystal with these characteristics.

=)

Title: Re: Oscillator Design
Post by loose-electron on Aug 26^th, 2010, 4:59pm

Crystal has a Q of 10E5 or 10E6 - very high.

You need to derive an equivalent crystal model (RLC)
from the equivalent frequency response of the Crystal.
This is a network problem - the equations are out there.

Then it gets interesting - The Q is so high for a crystal
you need to "lower the Q" in order to make a simulation that will
run properly!

Also, simulation times are really long to get the device running and up to amplitude. A useful trick here is to put an initial condion of current on the inductor in the crystal model.

should get you started.

Title: Re: Oscillator Design
Post by RuRob on Aug 26^th, 2010, 5:25pm

Hello.

I started with the initial condition and... the design is running ok... now i'm going to work with a 12MHz crystal.

Thanks!