"Richard Clark" wrote in message
...
On Thu, 16 Sep 2004 21:26:48 -0400, "Tam/WB2TT"
wrote:

To make a long story short, the resistor error will be about 20% where the
Megahertz x MegaOhms = 1.

And not so curiously Trc = 1 MOhm · 1 pF = 10^-6

I think the curve ignores C, and is based on skin effect only. There is no
explanation for the data.


F = 1 / T
F = 1 MHz
perhaps the product rule should be:
Megahertz x MegaOhms x picoFarads = 1

Go to http://www.xicon-passive.com/resistor.html and click on CC. There is
also info on resistor performance vs frequency in the W6SAI book. He shows
curves for 5 different carbon resistors vs frequency without identifying the
resistor values. As a gross average, they show about 50% error at 15 MHz.

The 20% error is, of course, simply the rolloff response at the RC
inflection point described by 1/Trc.


The curve goes from 0 - 100. I arbitrarily picked 20 % as being a point
where there is apreciable error.

Tam/WB2TT

Let's see if anybody
shoots this down.

Hi All,

I think chipping at the clay feet of saints is more appropriate
metaphor.

What is the saint? The RF response of the resistor. It should be
suspect right out the gate. Being suspect, you employ the
conventional techniques already evidenced even by the cheapest Power
Meter builder (MJF) by swamping the stray capacitance with series
capacitors (paralleling the resistors). One capacitor is either
variable, or further paralleled with a trimmer. The saint is also the
unspecified requirement: is this divider BEFORE OR AFTER the detector?

If before, and thus subject to RF, the simple RC compensated divider
has served for eons. If after, and thus subject to only DC - who
cares? The one clay foot of the discussion.

The other clay foot of the discussion is that for placement before OR
after the detector, ALL ratios are post-hoc determinations (in other
words, design with variable components fully expecting you WILL be
wrong).

So true. I notice the series C in the Kenwood meter is variable.

Further, ALL descriptions to this point have been of
normalized levels. With the RC compensated divider, you are throwing
the knee if rolloff into lower frequencies so that ALL frequencies of
interest reside on the same slope. Hence the common "calibration"
procedure has you adjust the resistors for the low frequency readout,
and the capacitors at the high frequency readout. This "calibration"
is simply distributing the error so that it doesn't accumulate
outrageously.

The greater challenge is how do you know how much power you are
setting your meter to read? Compounding errors are common in RF.

73's
Richard Clark, KB7QHC