[Elecraft] Tr: K3 Ref connector

[Brendan Minish]

On Sat, 2009-05-16 at 08:03 +0100, David Pratt wrote:
> The connector is referred to on page 17 of the latest K3 Owner's Manual 
> and my K3 rear panel is labelled "REF."
> 
> In consequence, I have bought a Racal-Dana 9474 Rubidium Frequency 
> Standard which is now poised to plug in as soon as the KREF3-EXT is 
> available, hopefully quite soon.

Couple of thoughts here 

Unlike the Racal RA3702 I have here; the K3 is incapable of phase
accuracy (RF to audio), or indeed exact frequency accuracy (multiple DDS
LO conversions with ensuing decimation errors, varying offsets for
roofing filter positioning etc)     

As a result phase locking or replacing the internal REF with a highly
stable and accurate external source is not likely to be very useful. The
(better than) 1ppm ref carefully set up and used in a stable temperature
environment is probably approaching the limits of the K3's architecture
with regard to frequency setting accuracy. (although an external ref may
further improve frequency stability. ) 

The K3 is not alone here, my Icom IC-7800 was often up to 0.25 Hz or so
off frequency when driven with a external GPS disciplined Rubidium
reference source, I suspect that the main cause of this was DDS
Decimation errors but it was also an issue between main and sub making
true diversity a non-runner on the 7800 despite having so called
'matching' main and sub receivers. 

Lastly be careful with Rubidium as it's phase noise performance is far
poorer than a good Xtal Reference and this can be a problem at times as
it may degrade the strong signal performance of a High Dynamic range
receiver. 

I am a time nut and do make on occasion some interesting measurement on
LF & HF with my RA3702 using my external 10MHz ref but I do not think I
will be adding this external locking feature to the K3 when it is
released.

-- 
73
Brendan EI6IZ