[Elecraft] All PLL reference oscillator questions answered

[Wayne Burdick]

Here are the answers to several recent questions concerning the PLL reference
oscillator modification:

- The "d" seen on the LCD during CAL PLL indicates storage of one data point.
What the firmware is doing is sweeping the reference oscillator through its full
range using the 12-bit DAC, and storing the DAC value associated with each 100
Hz point. (When you actually tune the VFO later, these DAC values are recalled;
pairs of them are used to interpolate down to 10 Hz steps.)

- The K2 needs to be set to 40 meters in order to do CAL PLL, but the exact
frequency doesn't matter.

- A PLL reference oscillator range of 11 kHz is plenty. The present K2 manual,
on page 57, specifies a minimum range of 9.8 kHz.

- If a strong signal is present in the I.F. passband when the VFO is tuned
across a 5 kHz synthesizer boundary, a brief audible artifact may be heard. This
is not related to any reference oscillator modification; it's always been there.
We tried experiment with muting the receiver during these shifts, but this
results in a much more noticeable artifact (silence), so we left it as is.
Hopefully, noone will be greatly inconvenienced by it. Technical details:
Suppose the tuning rate is set to 10 Hz. On 5 kHz boundaries, the PLL reference
oscillator will shifted by +/- 4.99 kHz, and at the same time the VCO divider is
shifted by +/- 5 kHz. The net difference is only 10 Hz, but the counterbalancing
shifts put a short impulse on the error voltage, which in turn shifts the VFO
frequency some greater distance for a few milliseconds. (Other rigs that use
DAC-controlled fine steps do the same thing; in the case of the Icom IC730, for
example, it happens on 1 kHz boundaries.) The 5 kHz boundaries do not correspond
to 0.00 and 5.00 kHz points on the dial. They will occur at a different point on
each band, and take into account the BFO frequency and (in CW mode) the receive offset.

73,
Wayne
N6KR

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