looking for a suitable vco

SteGeb 19 years ago

Hi,

In my project, I drive an oscillating device (oscillator) with an VCO reference signal. In order to lock the oscillator's frequency, a PLL is employed. To keep the device oscillating the input frequency needs to be hit at least as precise as 0.005 Hz.

The problem, though, when the oscillator is changed, its center frequency changes, too. It can be in the range between 11.75kHz-12kHz.

Because of other clocks that need to be synchronized and generated, the VCO's output frequency must be in the range 13.865-14.16 MHz. Actually, a VCXO would fit my needs best to tune the frequency as accurate as 5.9 Hz at 14 MHz (0.005 Hz at 12kHz). However, if I pick a VCXO for say

14.1 MHz that is +/-100ppm pullable, it may work with some oscillators, but not with all possible.

So, I need a VCO whose center freqency is coarse adjustable in a relatively wide band (13.8-14.2 MHz) and it should have a fine frequency tuning resolution using the input voltage.

Has anyone an idea what VCO could fit for this?

Thanks,

Stefan

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bill.sloman 19 years ago

Have a look at a digital phase locked loop controlling an Analog Devices direct digital synthesis chip. The original was the AD9850

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but the range is now huge. The cheapest, slowest part - the AD9833 - is probably too slow, with a clock frequency of 25MHz, to produce a respectable looking 14MHz clock. Something with a clock frequency closer to 100MHz would make life a lot easier, but you'd still have to be pretty careful with the design of the anti-aliasing filter. Happily, the Aanlog Devices range goes up to 1GHz, albeit at a similarly astronomical price.

Bill Sloman, Nijmegen (but in Sydney at the moment)

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SteGeb 19 years ago

Hi,

a dds seems to be a good choice. But i need to program the dds's output frequency with an microcontroller which means, that I need to transfer parts of the PLL into software, right?

Thanks,

-Stefan

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maxfoo 19 years ago

You could use the DDS as a reference frequency for a PLL/VCO, or just use a DDS alone. try the analog devices 98xx series they have very good phase noise specs.

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