[TenTec] Corsair II DDS VFO
wb8yqj at yahoo.com
Tue Mar 22 08:53:29 PDT 2011
Just to add to what Steve has properly suggested, the most practical problem with the PTO precision is not with the display, rather it is with the slop within the worm gear, pistol, grease and spring.
Even with the smoothest PTO if you are trying for (example 14.250), there is some backlash and so you end up wherever the pistol settles with the worm gear. The most subtle twist of the main tuning knob and that spring is only building up pressure inside, a little more twist and then it pushes the pistol too far, past your intended target. Repeat coming back from 14.250.1 ;-)
Ignoring drift, you could never get to 14.250 precisely to start with, and if you did, you would drift away from it anytime you transmitted, the finals heating up the PTO coil when you transmit and then it cools when you are off the key.
The N4YG DDS is not affected by PA heat so your drift is limited to the Corsair base frequency crystal drift plus some negligable amount from the DDS board reference.
In practical terms for CW and SSB it's a night and day improvement.
For repeatability, I will tune to (example 14.249.9 just changing to 14.250.0) with the fine precision of the DDS then a very small rotation in "fine" mode using the main tuning knob to push the display over to 14.250 solid.
While it is true that repeatability is ultimately determined by the accuracy of that counter board and some drift in the band crystal, it's still a world better than before and quite acceptable for CW/SSB net buddies who otherwise would have howled uncontrollably about having to adjust their RIT control for you - ONLY you - end EVERY time it is your turn to transmit!!! ;-)
Steve's design improvements have been incorporated into the new N4YG boards with the added benefit of the removal of a daughterboard circuit.
The DDS modification still uses the Corsair's "native" frequency readout
circuitry, so setting accuracy is no better or worse than with the PTO.
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