RF detecting question when tweaking to max or min very low RF voltages

[Robert Myers]

I am in the process of aligning a few devices.  Often, to optimize
things (way prior to any amplification) I have to measure very low
voltage level RF signals at various points on the circuit board.  When
at these points on the circuit board, and for various frequencies, I
usually either must maximize, or minimize, the voltage produced there
when certain other parts of the circuit are adjusted.

All I have is a Heath 309-C RF probe which I can feed into a ~11 M Ohm
input impedance meter (set to measure 'DC') to get some 'measured'
values.  Sometimes both the *theoretical* maximum RF value obtainable at
that point, and the minimum value (especially if that minimum value
should theoretically go to zero with careful adjustment) are well below
the so called knee of the detecting diode of the RF probe.

So are you still 'maximizing' anything (even if the 'measured value' is
way off -- which I accept) by making that 'indicated voltage' as big as
you can?  I.e., if you adjust something other parts in the circuit, and
the reading in question goes from 20 mV to say, 90 mV, is that still
valid in terms of *maximizing* the RF voltage that is at that point?
Have you still maximized it if you get a peak at 90 mV which is way
below the value of the knee voltage?  Or, is all this max-ing and
min-ing meaningless at this when you are in this area.  The same
question goes for minimizing.  Even if you are totally off in terms of
reading the actual voltages present, will tuning and adjusting things to
create a minimum measurement reading (say when down around 15 mV as your
'minimum') mean anything at all?  For example, suppose it was indicating
up around 120 mV on you DC meter via you RF probe, but by adjust some
R's, C's, and L's (somewhere), you could get the probe to end up showing
15 mV on the meter.  OK so you did that.  But have you done things
correctly?  I don't care about the voltage levels themselves, only that
they be max'd or min'd in *reality* at that point on the circuit --- but
is that an indicator of the reality of a max or min when you are so far
below the knee?

If you had a theoretical detector diode with a knee at 0 volts, lets
say, would you end up tweaking things (R, C, L, if that's what there
were to adjust) exactly to the same values anyway (only this time you
measurements would be realistic in an absolute sense)?  See what I'm
saying?  If this is true then I then non-zero knee of the real detector
diode wouldn't really matter if only max-ing or min-ing --- you'd still
do the same tweaking.  But I'm not sure about this.  Can someone
enlighten me?

How do you detect when you have hit zero RF volts with an RF detector
with a knee at 0.250 V, for example?  Just tweak things until it gets
lower and lower down to 10, 5, 2 mV or however smallest you can get
it???

Is the any way of 'jury rigging' the RF detector, with a battery, or
band gap stable reference, so that hitting say '2 volts' (on the DC
meter) would be where an actual zero RF voltage would be occurring?
Does anyone have a circuit for something like this that you could add to
a normal type RF detector?  Or as I said, if you are strictly tasked
with maximizing or minimizing RF values (even if they are well below the
knee) without caring about true values, would building such a circuit
get you anything that you wouldn't get anyway with just following the
normal 'bad' RF detector readings?

Thanks,
-- Rob

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