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
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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