Posted by
Ron D'Eau Claire-2 on
Apr 25, 2006; 6:19pm
URL: http://elecraft.85.s1.nabble.com/Need-help-for-sick-K1-tp389102p389118.html
In addition to the inevitable drift in tunable oscillators, there is the
accuracy ("resolution" is a more common term) of the Digital-to-Analog
Converters (DACs) used in the K2 logic.
During CAL PLL and CAL FIL, the DACs must 'read' the analog tuning voltages
used to set the oscillators on frequency and convert it into a digital value
for storage. The DACs do this. They don't read the voltage to an infinite
number of places, so it gets 'rounded off'. The part that's rounded off is
lost and the value stored isn't exact.
When tuning around, the logic reads the tuning voltage data needed to put
the K2 on whatever frequency you've chosen from memory (they were stored
there during CAL PLL and CAL FIL) and runs the digital value through the
DACs again to produce the analog tuning voltages required by the varicaps in
the oscillators. Once again, the analog voltage produced by the DAC's is not
precise.
That's where most of the about 30 Hz ambiguity comes in. And it's why two
different K2's can vary in frequency by 30 Hz or so even if their
oscillators were absolutely accurate and drift free which, of course, they
are not.
Many folks new to the K2 don't realize that what they see on the frequency
display is NOT the actual frequency. It's the "requested" frequency chosen
by either punching in a frequency on the keypad or turning the dial. It does
not show any errors caused by oscillator drift or inaccuracy in the DACs.
The "plus" side of using tunable oscillators under logic control is that we
can easily set the filter bandpass positions using CAL FIL and the extremely
smooth tuning of the local oscillator with a very low level of phase noise.
The latter is the result of using not making the phase-locked loop step
constantly in very small frequency steps as you tune. The K2 PLL jumps in
fairly large steps. In between the large steps, the PLL Reference Oscillator
frequency is tuned to provide virtually continuous analog-style tuning.
Ron AC7AC
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