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Re: Problem with common leads in dual channel scopings?
Original poster: davep <davep-at-quik-dot-com>
>This brings me to a question of scope operation: can
>anyone simply say why grounding is necessary?
Whole books have been writ on practice in
this area. Detailed Answer depends on
intricacies of EUT (Equipment under Test)
grounding, and, sometimes, consideration
of personnel safety: eg: keeping the test
equipment from being at 'strange voltages'.
These voltages can result from nonobvious
sources: eg if an apparently LV source
(an automotive alternator, say, with
floating field supply) Might float 'way up'
due to _capaciticavely_ coupled (stray
capacity) FROM the load, if it is driving
a load which generates HV.
Such stray HV can do strange things to the operator,
or drive the scope amps (or metering amps)
crazy. Such amps, used outside their ratings,
WILL give strange readings.
>These scope leads were only monitoring voltages from an
>alternator stator being under 12 volts or so, so the
>voltage ranging precautions of the scope were
>available, BUT it might be true that since it is
>monitoring a circuit of higher amperage delivery,
>might any special precautions be necessary?
Always.
Especially if driving a HV-generating load.
(true Tale from an old Engineer:
They were doing studies on ignitions on auto engines.
They designed an elaborate filtering/scaling
system for their probes.
They found that the (well known here) '12" of wire'
worked as well. (this was found by accident: the
Official Scaling arrangement came undone, readings
continued unabated due to stray fields.) It takes
time, experience, and understanding to get the
Real Indication sorted from the artifacts.
>In fact what strangely occurs with these primaries, what
>happens is that when a certain volume of amperage rise
What is a 'volume of amperage rise'?
>occurs by increasing the alternator input via DC variac
What is a 'dc variac'?
>to field, all of a sudden the primary being monitiored
>simply stops resonating. Could it be possible that the
>internal impedance of the scope might act differently when
>observing a 480 hz signal?
Unlikely.
Impedance (ignoring gross internal breakdown) is
fixed more or less by passive components. The freq
range will be in the manual, at a guess DC to a few
hundred MHz.
Probably the 'resonance is stopping' for other
reasons.
>Doesnt sound too likely, more like a problem involving the
>probe connections I thought...
>In any case a solution to these monitorings was found,
>by making the dual channel leads connected to the
>voltage rise of the circuit, WITHOUT any of those
>probe leads having a common connection point.
Are the probes being used 'differential mode' that is
two probes for one signal, with one scope input
inverted, and the 'sum' mode selected?
If So, for Well mannered signals, it is usual to
tie the scope 'tip' 'grounds' together, but not
specifically connect the 'tip grounds' to anything.
else. (for 480 Hz, the 'pigtail' connection
is fine. For Seriously Higher Freqs, getting
'to the tip' can be critical.
>In the past I think I was able to past this obstacle just by
>using two different scopes for each probe lead, and of
>course centering each sweep line, then visually
>comparing each scope sweep; which would be necessary
>to circumvent this short problem when observing only a
>single phase of series resonance. But because here we
>have 180 phased dual series resonances, instead we
>have the opportunity to measure just the voltage rise
>between the inductors in series resonance, and then
>those probe leads are not having a common joining pt
>from those on the outside of the circuit.
>When doing this the shortcomings of the method were
>quickly shown. At the lowest input voltages, a
>ordinary digital voltage meter records 8 volts across
>the primaries voltage rises, given the minimal 2 volts
>made in operation of spin. One channel of the scope
>can show that 2 volt signal from the alternator. But
>however once the second channel midpoint voltage of
>scoping is added, the measured voltages from both
>meter and scope drop down to ~6 volts. When the
>primaries are tuned for more effective resonance, the
>drop upon scope monitoring drops the action 50 %.
How is it known that the indication drops 'on the
scope monitoring'?
>so for this case example anyways, it is definite to
>conclude that the action of scope monitoring the
>primaries resonance, acts to significantly drop the
>possible amount of resonance obtainable given a fixed
>stator voltage input. This scope has a 30 pf internal
>capacity.
Calcite the impedance of 30 pf at 480 Hz.
It is large.
is 30 pF the scope or the _probe_ impedance?
Is a probe being used?
What cap resonates the load?
If stray capacity/self capacity, then yes, 30 pF
MAY be significant.
The wire from 'coil' to scope has capacitance, in
addition to that of the cope amp, per se.
>I have seen that at higher input frequencies, the internal
>impedance of a meter can influence what is recorded. At
>some 20,000 hz from solid state neon transformer; a convential
>needle AC voltage meter will record a different result for each
>voltage scale, and these values vary widely. Can
>anyone make a comment on the problems of the internal
>impedance of the measuring instrument affecting what
>is being monitored?
Of Course.
(hint:
I have a text book 1.5" thick on the intricacies of
measurement technique. And that is an older one.)
Impedance of the meter may vary, independently the
_response_ of a meter to frequencies can vary. Full
specs on the meter will discuss these.
>I have also noticed that when observing inductive high
>frequency signals near the 5 volt range,
Meter or Scope?
>that when we make the change to 10x on the probe this can
>also dramatically change the frequency
_frequency_?
Again, what is the resonating cap on the
(480 Hz?) specimen under observation?
If the coil is self resonating, the change from
1x to 10x on the probe (?) may well be
significant. Also, (back to top) the capacitance
represented at the probe tip will change _radically_
depending on length and placement of ground lead.
>that is observed by the scope, so the reasoning
>becomes that less internal capacity made by the 10x
>probe change, and it then hinders the observed
>vibration less, since the recorded frequency in those
>cases is increased.
'hinders'?
Increasing capacitance lowers freq. decreasing it
raises freq. (All of which is subject to:
what the resonating cap on the coil under test IS?
If a few nF, then changing 30 pF is unmeasurable.
if the coils is 'self resonating' the change of
a few pF may change the 'self resonance' radically.
>The changes in one sample of
>spirals tested for resonant frequency were readings of
>550,000 hz to over 800,000 hz, so surely this is an
>issue to be contended with, if the measuring
>instrument can deliver so different an answer
>depending on which range we choose, then the scoping
>of course loses quite a bit of relevance????
Rule 1:
The measuring device ALWAYS affects the observed
result, may change the original source operation.
The user has to understand by how much and why.
If an air core coil, with no explicit cap, is self
resonating at 550,000 Hz then yes, a change of
30 pF will likely swing the observed resonance.
>Different scopes of different internal capacities also
>interpret these hf signals differently.
_change_ the signals differently.
Length of ground lead will affect stray capacitance,
add some odd micro/nano/whatever henry inductance.
best
dwp
...the net of a million lies...
Vernor Vinge
There are Many Web Sites which Say Many Things.
-me
...for he has read everything, and written nothing...
A J Raffles