[R-390] R389 PTO substitution

[Perry Sandeen]

Gents,

Wrote: There are several DDS (Direct Digital Synth) VCO kits running around, mostly based on the AD9850 or AD9851, for a hundred dollars or so.  With a range of 0-40+ MHz, it would be ideal.  The issue is the encoding. The PTO  covers 1.06 MHz in 10.6 turns, or 100 kHz in one turn.  To get 10 Hz resolution, you would need to have an encoder capable of a resolution of 10,000 per revolution.<snip>

Not necessarily.   Several DDS designs I’ve read about use a high frequency and divide it down.  Also the literature for the one from N3ZI  (progance.com) indicates it’s programmable. In QST several years back was a VCO microprocessor controlled unit for the Drake 4B IIRC.  It divided down a mini-circuit’s 100 Mhz VCO. 

Wrote: Unfortunately, you actually need an 11 turn pot to account for the overrun in the tuning (10.6 turns end to end).  

With both designs I saw each used an optical encoder

Wrote: Next, you will have to calibrate it for non-linearity, easily enough done through a table and interpolation.  And finally, add a thermistor to temperature compensate the 20ppm temperature drift of the potentiometer.  

No drift with an optical encoder and the N3ZI kit can be purchased with a LCD readout which is probably much more accurate than the OEM dial.

Wrote: The problem here is that the cost would be more than a whole R-389/R-390/R-390A on the market.

Well, a dead receiver is probably worth almost nothing to anybody who’d want one.  And the market for such a radio isn’t overwhelmingly large.  Even if one had to spend say $300 (which I think is wildly more than needed) to have one that required custom step programming, in the end one would have a radio that would be of far more value than the dead chassis plus the synthesizer investment.

My opinion is that while difficult, a DDS replacement wouldn’t be prohibitive.

Would some one with DDS experience please chime in?

Regards, Perrier