[R-390] Collins R 389 PTO

[Tim Shoppa]

W2EMN writes:
> I thought about a DDS replacement for my R-390A, (it would be nice to get  
> real 100 cycle readout accuracy) but the biggest stumbling block was the 
> lack of  a 10,000 count encoder.  The best available seems to be 1026, and so 
> that  throws you into dual encoders and a complex gear train.  But if you 
> could  get it going then the mechanical dial and calibration would all still  
> work.  The other issue is some way to remember the frequency setting with  
> the power off- probably not a big issue.  The complexities of a  
> croprocessor controlled DDS are beyond me, so I dropped the idea.  It  would be nice if 
> meone would tackle the problem and share their results.

There's no reason that the frequency step with a DDS has to be a nice round
number. With any of the modern implementations you will be driving the DDS through
a microcontroller which can do the math.

For those on the cheap, it is very common these days for people to homebrew encoders
using the components from a computer mouse.

(I remember a Simpsons episode where Homer is looking at the plans for some complex
nuclear doohickey and he remarks "Oh, that part you can find in any player piano...")

And all DDS's have spurs... the best synthesizers do use DDS's in the loop but with
tracking narrow-bandwidth PLL's to clean up the spurs. PLL's and DDS's can be put together intelligently
to make an excellent clean synthesizer, or they can be together incompetently and
result in incredible spurs or phase noise despite the high specs of the individual components.

Tim.