Posted by
Don Wilhelm-4 on
Oct 19, 2007; 4:28pm
URL: http://elecraft.85.s1.nabble.com/KAT2-Can-t-Get-Bridge-Null-Adjustment-C55-Below-012-mV-tp454913p454914.html
Michael,
Although an absolute null would indeed be zero, the 10 to 12 range is
about as low as you can get - the K2 display is reading in millivolts
and there is some noise picked up and coupled into the op amp input
which limits the low end of the reading capability. The same problem is
true when making any low level measurements.
Try to fine the 'edges' of the indicated low point and set the capacitor
halfway between.
The operation of the KAT2 will not be affected much at all - the result
may be a slight error in the SWR indication. Actually the null point is
more dependent on the dummy load used during the process. If you want
the greatest accuracy, use a dummy load that is known to be a true 50
ohm non-reactive load at the frequency of operation - I use a Caddock
MP9100-50 1% thick film resistor mounted on a heat sink with very short
leads to the back side of a BNC connector. This makes a very good 100
watt dummy load usable up to 500 MHz. I rate that resistor as one of
the best under $10 purchases I have made (the heat sink was free - from
a discarded computer MPU cooler).
73,
Don W3FPR
Michael Linden wrote:
> I'm performing the Bridge Null Adjustment (C55) on my KAT2, but can't
> get it below 012. It is clear when adjusting C55 that I _AM_ centered
> in the middle of a "dip". However, I can only get the value to go down
> to 012. Per the manual, I've checked T1's windings and their continuity
> and have checked the continuity and arrangement of the cable wires (J7
> and J8) -- I don't see any problems.
>
> Is a minimum value of 012 "good enough", or do I need to look more
> closely at the SWR bridge components?
>
>
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