[time-nuts] oscillators

Jim Lux jimlux at earthlink.net
Fri Aug 31 20:55:18 EDT 2012


On 8/31/12 7:06 AM, Bob Camp wrote:
> Hi
>
> An SC is going to have it's temperature curve centered up around 95C
> or so. If it's been cut as an OCXO crystal the turns will be up there
> as well. By the time it gets to room temp, the delta F / delta T is
> moving mighty fast. Think in terms of multiple ppm/C. A typical cell
> phone TCXO crystal is in the sub 0.1 ppm/C range in the vicinity of
> room temp. In addition, SC's are pretty hard to pull. For a normal
> TCXO (no DDS) something > 40 ppm of range would be needed. By the
> time you get the wide tune stuff in the circuit, the phase noise
> isn't going to be anything special.
>
> Bob
>

We're not pulling the crystal.. we just let it sit.
What we do is change an NCO in the software radio implementation. 
Typically, you have a coarse tune with a PLL in steps of 500kHz-1MHz, 
and then you do the fine tune in software with a digital 
mixer/downconverter and an NCO (which also is part of the carrier 
tracking loop).






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