[R-390] Collins R 389 PTO

[2002tii]

Bob wrote:

>While the digital VFO ideas surely have merit,  it seems to me that 
>a simpler approach would be a heterodyne analog approach, mixing the 
>output of a fixed crystal oscillator with a a variable one to get 
>the desired tuning range.  The variable could be another PTO or a 
>conventional oscillator, of course the tricky part would be to get 
>the mechanics to match up so the digital readout would be 
>accurate.  Or something like the AADE digital dial would be an option.

Simpler, perhaps (although the scheme I suggested yesterday is pretty 
simple for a digital-savvy designer -- it all depends on what tools 
one is comfortable with), but the temperature and vibrational 
stability of any analog VFO -- not to mention the linearity -- is 
going to be orders of magnitude worse than a DVFO running off a TCXO, 
even if you put all of the design effort into the analog VFO that 
Collins did originally.

Note that it's not just the digital readout -- all of the tracking 
filters need to synchronize with the PTO (or replacement VFO), so the 
VFO needs to be 50 turns and linear.

Anyway, as I've said, none of this is easier than fixing the original 
VFO in my opinion -- at least all the design work has been done, all 
you have to do is make it work again.

Best regards,

Don