[Elecraft] K2 Resistor Tolerance

Sat May 17 12:35:01 2003

Good questions, Bill...

The reason for suggesting that you check the resistors with a DMM is to
ensure you aren't mis-reading the colors. It's easy to do in a lot of =
cases.
The idea is to avoid your putting a 100k ohm resistor in where a 10k ohm
resistor belongs.=20

The accuracy of the K2 power readout at 5 watts is 10% in any case, =
which is
typical of this type of power meter. That's because 10% is still very
accurate in terms of the needs of a communications system. A change of =
10%
is less than 0.5 dB. When you consider that, under ideal laboratory
conditions, 1 dB is the smallest change in a sound level that a Human is
capable of detecting by ear, and that in most radio communications it is
considered that the smallest change a Human can detect by ear because of
QRN, QSB, etc., is 3 dB, measuring within 0.5 dB is very accurate =
indeed.=20

The potential error in the K2 power measuring system is more dependent =
upon
the characteristics of the diode used than anything else. The recovered =
d-c
the diode provides does vary with frequency slightly, even across the HF
range. That particular diode type was chosen over for its fairly flat
response, but it's not THAT flat, nor are the diodes all exactly the =
same.=20

Even the popular OHR wattmeter (http://www.ohr.com/wattmeter.htm) isn't =
any
better than that. It's spec'd at 5% of full scale, but "full scale" is =
10
watts. So the accuracy at 5 watts is also 10%.=20

IF you really want a more accurate power measurement system, you'll =
probably
need to go to a much more expensive system such as the Bird instruments. =
But
they will cost you a large fraction of what you paid for the K2 just to
measure the power output a little bit more accurately and allow you to =
make
adjustment not detectable in normal operation.=20

Ron AC7AC

-----Original Message-----
From: [email protected] =
[mailto:[email protected]]
On Behalf Of Bill Henderson
Hi

Thanks to all who took the trouble to answer..

I am still stuck though. Why would the manual tell you to check the 1%
resistors with an ohmeter if a resitance outwith the tolerance wasn't
mportant.

Bob AG5Q says that it effects the accuracy of the power readout. Since I
will be using the K2 for QRP this is important to me.

I'll stick the resistor on the board but will probaly change it later.

Thanks for your help.