Posted by
wayne burdick on
Oct 31, 2011; 6:59pm
URL: http://elecraft.85.s1.nabble.com/New-K3EXREF-results-tp6944433p6949378.html
On Oct 30, 2011, at 8:01 AM, Matt Zilmer wrote:
> Hi Bob,
>
> I know for certain the the actual VFO shift is not reflected in the
> REF*CAL value. Otherwise, we'd all be seeing massive frequency error
> on RX and TX. The reference frequency is used in the DDS to derive
> the injection freq, so the error should be reduced substantially by
> that alone. The ref freq is also divided down by 6 to get the 8.23MHz
> 2nd LO injection freq. If Wayne's design followed others (Motorola
> for one), the shift in LO and first IF would buck (cancel) each other
> to some extent.
That's correct. We don't need REF CAL to be exactly 49,380,000 Hz
because we take the actual frequency of the reference into account in
firmware when setting up the synthesizer. The 2nd LO is compensated at
the same time, at 1/6th the rate (because of the /6 relationship
between that and the 1st LO).
If there's a K3EXREF installed, its microcontroller measures the
frequency of the 49 MHz reference down to 1 Hz using the 10 MHz
external reference as a timebase. This data is sent to the K3's main
MCU, which then uses it to "capture" the normal REF CAL value.
One advantage of this method (frequency-locked loop) is that no matter
how noisy the external reference is--possibly due to being distributed
around the shack--it can't have any effect on the radio's RX or TX
noise floor. It is used only as a counting gate.
73,
Wayne
N6KR
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