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Locking an oscillator with a 10Mhz reference source

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EI3HG

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Hi All,

I am building an VFO that will use a DDS chip for use in a homebrew Ham radio VHF transceiver project.

The DDS reference oscillator I plan to use is a standard 10 MHz crystal oscillator, but I would like the option of locking it to an external reference 10MHz source, such as that from a GPS.

I want to keep phase noise to an absolute minimum, and I also want there to be no major 'glitch' in the output of the 10 MHz DDS source if it loses external 10MHz source.

My question is whether I should switch between the internal 10MHz xtal oscillator and external 10MHz source, (using a simple pll chip to detect presence or absence of the ext source, or should i use a VXO type design, that is to say using a varicap and PLL circuit across the crystal just to pull it the few Hz needed to keep it synchronous with the external 10MHz source.

Any ideas, thoughts or other references gratefully received.

Andy EI3HG
 
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Andy

What are you using for a DDS?
What is the DDS reference frequency?
What is the output frequency of the DDS?

Your 10Mhz seems a bit low, if you are using something like an AD9850 or AD9851 I would have expected to see a DDS clock up around 120 or 180Mhz.

My next thought is, do you need GPS locked accuracy?
Why not just use a good OCXO?
That is what I use on my frequency counter, and when it has warmed up it stays within a few milli-hz at 5Mhz according to my homemade GBR frequency standard receiver.

Jim GM3ZMA
 
Hi Jim,

Thanks for the reply

I am using an AD9851

I could use an OXCO (and have tried this option), but I want the option to lock to a 10 MHz reference for some experiments I want to carry out regarding co channel interference and the effect of exactly synchronous signals versus un- synchronous. For this the frequency and phase of both tx's needs to be synchronized, 10MHz from a GPS receiver output is easy (and cheap) to do.

Andy EI3HG
 
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