[K2] Frequency Counter Gate Time Short

[David Woolley (E.L)]

Whilst trying to optimise the VCO calibration on my K2, I've come to the
conclusion that effective frequency counter gate time is slightly short
(more than would be accounted for by simply rounding down).  The
consequence is that one actually wants the 4MHz oscillator slightly low
of 4MHz (equivalent to about 20Hz low at 7MHz.

I wasn't using the standard calibration method (which seems to rely on
the frequency counter and lots of swapping of its probe[A], but rather I
trimmed the "4MHz" oscillator to create the same offset and beat against
the spot tone as a 41m broadcast carrier[B} when looking at the 7MHz
birdie, which should only be about 2% away in frequency.  (I assume
these birdies are the result of the microprocessors instruction clock,
divided from the 4MHz.)

What I found was that, after PLL calibration, the birdie seems to end up
at about 6.99998 Mhz.  Checking the other instances of the birdie tended
to confirm that it scaled with frequency in the expected way.  In the
end I got to better than 5Hz at 7.1MHz by then retuning the oscillator
down by its measured error at 15MHz (I probably should have gone for 28).

In doing this, I also wondered why:

- CAL PLL isn't run at 28MHz, to get greater sensitivity to the
frequency (I suspect this is historical, because the basic setup is for
7MHz and one used to have to calibrate per band);

- Why isn't a longer gate time used for critical measurements, e.g. when
storing CAL FIL data, and when one has decided one needs a new control
point in the CAL PLL curve.  It wouldn't matter if the microcontroller
counter overflowed, as long as one knew the approximate frequency.

[A] Has anyone found a good way of strain relieving the KAT2 RF cable,
as I'm worried that it will break through flexing when removing the
cover, at the point where it is soldered to the board.  I'm wary of
using adhesives, because they might make it more difficult to repair if
it does break.

[B] Most of these produce consistent beat tones, and I assume that they
are not all off by the same amount.  I also think I found a weak WWV at
15MHz, last night, which was beating with the birdie before I introduced
the deliberate error.


--
David Woolley
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[Don Wilhelm-4]

David,

The procedure which relies on tuning to a broadcast signal of known
frequency and uses the internal counter (along with some swapping of the
counter probe between TP1 and TP2) eliminates any slight error in the
'exactness' of the 4 MHz reference - it is automatically equalized out
of the equation.

Yes, I do believe your observation is correct that the reference must be
set a tiny bit lower than 4 MHz, but I have not actually measured it lately.

Another way to accurately set the 4 MHz reference is to use a known
accurate external frequency counter and measure the frequency at TP1
with both the external counter and the internal counter - adjust C22
until the internal counter displayed frequency matches that of the
external counter.  If the external counter is accurate to 1 part in
10^-8, then the 4 MHz counter can be relied on for measurement to 1 part
in 10^-7 - or 10 Hz at 10 MHz (that is using the 'rule of thumb that the
calibration instrument should be 10 times more accurate than the device
being calibrated).  Not just any counter in the hamshack is sufficiently
accurate.

73,
Don W3FPR

David Woolley wrote:

> Whilst trying to optimise the VCO calibration on my K2, I've come to the
> conclusion that effective frequency counter gate time is slightly short
> (more than would be accounted for by simply rounding down).  The
> consequence is that one actually wants the 4MHz oscillator slightly low
> of 4MHz (equivalent to about 20Hz low at 7MHz.
>
> I wasn't using the standard calibration method (which seems to rely on
> the frequency counter and lots of swapping of its probe[A], but rather I
> trimmed the "4MHz" oscillator to create the same offset and beat against
> the spot tone as a 41m broadcast carrier[B} when looking at the 7MHz
> birdie, which should only be about 2% away in frequency.  (I assume
> these birdies are the result of the microprocessors instruction clock,
> divided from the 4MHz.)
>

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