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Design >> Analog Design >> 3rd overtone xtal modeling
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Message started by ali kotb on Mar 5^th, 2012, 5:17pm

Title: 3rd overtone xtal modeling
Post by ali kotb on Mar 5^th, 2012, 5:17pm

hi guys
iam trying to make a 3rd overtone xtal osc.
fundamental (40MHz) , 3rd overtone (120MHz)
i need a resonant amplifier to be in feedback loop with the xtal osc. to suppress the fundamental
the design should be coil less cuz it has to be integrated on chip (L would be huge for resonance at 120MHz)
any advices for such resonance amplifier

i was thinking of something like this to be in the fb loop of xtal osc.
but i couldn't get to suppress the fundamental
what do u think ?

Title: Re: 3rd overtone xtal modeling
Post by wave on Mar 6^th, 2012, 2:45pm

First, what is your gain element of the oscillator look like - is this a Pierce configuration?

Looks like you drew a goofy amp - is that a Neg R configuration to (attempt to) knock out the fundamental 40M ??

For Crystal 3rd OT, there are a couple classic choices.
1) Use an external L/C trap, or
2) a very small R in feedback for the Pierce configuration.

(Intuition says a large R is appropriate, but there is some very tricky analysis that has a small R knock out the 1OT. It's MIL spec even, if I recall.)

If this doesn't make sense yet, read some app notes. They is a lot of piecemeal material out there on crystals, so after reading 20 or so different ones, re-read this. Seriously.

Wave
:)

Title: Re: 3rd overtone xtal modeling
Post by sushanth on Mar 6^th, 2012, 9:36pm

Hi
Is the reason for having the specific values for C1 & R5 is to filter out the fundamantal & couple only 3rd order overtone?

Title: Re: 3rd overtone xtal modeling
Post by loose-electron on Mar 7^th, 2012, 4:53am

since the crystal is outside the chip
you should put the tuned circuit outside the chip

LC tuned BPF centered at the desired frequency will work.