[Elecraft] Filter Calibrate Question
Masleid, Michael A.
[email protected]
Fri Dec 12 15:35:00 2003
Hi John,
>Once again the mysteries of filter calibration escape me.
Much of the mystery is explained on page 104 of the manual.
The rest of the mystery may have to do with CAL FCTR.
The K2 controls the PLL, BFO, and filter bandwidth using D/A converters,
but it doesn't actually "know" the frequencies. You have to teach it.
First, you do CAL FCTR to calibrate a frequency counter.
Then you do CAL PLL, and the K2 "learns" learns how to set the D/A
converter to get a particular frequency out of the PLL reference.
Then you do CAL FIL. You get to hear how each filter setting sounds.
You also adjust the BFO control voltage (frequency) so that signals
show up in the filter the way you want.
The K2 knows about the voltage.
It has to measure the resulting frequency using CAL FIL.
It won't do that unless you've made a change.
Once it knows what voltage produces what frequency, the K2 can compute
what settings it will use for the PLL, VCO, and BFO so that what you
hear matches the dial calibration.
>So, how does changing filters result in a change of frequency?
It doesn't. But a new value for the BFO frequency is measured, which
will cause the calculations for the PLL and VFO to change. That changes
the frequency if the calculations are off a bit.
The real reason everything has to move around is that the center =
frequency
for the crystal filter changes when you change the bandwidth, so the
K2 needs new values for the BFO, PLL and VCO to keep the singal inside
of the IF filter. =20
>How does one fix this other than resetting the BFO frequencies
>to where they had been?
The K2 will re-measure the BFO frequency if you make any change to
the filter or BFO setting - even changing the value away and back to
the same setting.
>Why can't I get all the filters to receive signals on exactly the
>same frequency?
If you let a lot of time (or temperature change) go by, you have
to do CAL FCTR again before you do CAL FIL.
Even if you have just done CAL FCTR, the step size for the D/A =
converters
doesn't allow super fine adjustment of the VFO, so calibration can be =
off
by a few Hertz.
You could get lucky. My guess is that some BFO settings line up better
than others for any given K2. I would try adjusting the "bad" filter
BFO setting one tick up or down, and see if the K2 can get a better
reading on that. The exact setting for the BFO is important for =
centering
signals in the narrow filters, but you have more options for the BFO on =
the
wider filters.
So, changing the BFO frequency in CAL FIL is not supposed to change
the receive frequency. Once the K2 measures your new BFO frequency, it
will twiddle the PLL and VFO so that (hopefully) only the filter
center frequency moves. =20
I hope this helps.
73 de Michael, AB9GV