[Elecraft] [K2] Frequency Counter Gate Time Short

[Don Wilhelm]

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.)
>