[Elecraft] Calibration of BFO - FWD from KI6WX

[Ron D' Eau Claire]

John, KI6WX, wrote:

> The PLL reference that controls the VFO frequency is tuned by a
> 12 bit DAC,
> which has about 3000 steps that are useful.  For the worst case
> tuning on 10
> meters, this will yield a frequency resolution of about 8 Hz.  It is much
> better on the lower bands, getting down to about 2 Hz on 80
> meters.

I looked up what Elecraft had to say about the resolution. According to the
K2 Manual, Theory of Operation, "Signal Flow" section (Page 102 in my Rev.
C, April 19, 2000 manual) it says, "... 10 Hz increments are provided by a
12-bit DAC driving an 11 MHz VCXO (PLL reference oscillator)."

Since this oscillator is operating over the same frequency range regardless
of the band in use, so I don't see how the band selected affects the
resolution. I would expect 10 Hz on any band.

> The best way to avoid offset frequencies is to use the same filter for
> transmit and receive, or to set the BFO for a different receive filter to
> the same BFO frequency as the transmit filter.  If you do the latter, be
> sure to reset both the transmit and receive BFO's at the same time so the
> firmware knows they are at the same frequency.

I think we are talking "Apples and Oranges" here. I was talking about the
fact that commonly there will be a 10 Hz step between two different i-f
filters as one steps through them during receive - not about a difference in
the receive passband center frequency and the transmit frequency.

You make a good point about the stability of the oscillators also causing a
shift in the BFO frequencies as one steps through the filters during
receive.

Ron AC7AC
K2 # 1289