[hpsdr] Fwd: Horton LO - further thoughts

[Jim Miller]

I'll admit right off that I'm new to ham radio but have done a lot of 
reading in the last year. Although I worked 30 years as an EE before 
retiring I never worked in RF so there's been a lot of fun learning.

The one thing I keep reading is that good phase noise performance in a good 
design is limited by the Q of the resonator. At HF nothing comes close to 
the Q of a crystal. For frequencies up to 500Mhz I'm guessing that it will 
be very hard to beat the phase noise of a frequency multiplied 100Mhz 
oscillator.

Second best is probably a well designed and accoustically isolated resonant 
cavity obviously at a much higher frequency considering practical sizes.

Although the DDS does have SFDR limitations at least the phase noise 
approaches that of a crystal oscillator.

I see mention of fine tuning resolution of the DDS as unnecessary yet it 
comes for free so I can't see it as a limitation. Small frequency changes 
can be useful for minimizing spurs and can be made invisible by software. 
Nothing says you have to use the fine stepping of the DDS for tuning. The 
SFDR is still a limitation.

I guess what I'm saying is that without a good Q at the heart of the 
oscillator I doubt we'll be satisified with phase noise results.

And achieving that maximum base Q may well require different means as the 
frequency goes beyond HF.

jim ab3cv



 1150564272.0