Section
II
Pa ragraph 2-6
E.
EF FECT OF PROBE ON CiRCUIT
BEING
MEASURED.
The probe adds an inductance of
less than
0.05
microhenries to the circuit under test.
With almost all conditions this small an inductance
will not affect the measurement.
However, if the measurement is made in a very low
capaci tance and very high impedance circuit, with
high ac voltages, and at high frequencies , the shun t
capacitance of the probe to ground must be taken into
consideration.
A
capacitanc e is added from th e meas
ured w ire to the grounded shield inside the 456A probe
when the probe is clamped around the wire. This
capacitance typically has a value anywhere from 1 pf
to
5
pf, depending upon wire size, insula tion and
location w ithin the probe aperture . Th is capacitance
has two effects:
1) The measu red wire is slightly "loaded" with a
capacitance to ground of around 3 pf.
2) The capacitive current which flows to ground
th rough th is wire-to-probe capacitance can be meas
ured by the probe, although it a lmost a lways adds
a negligihlc amount to th e reading of the actual cur
rent in the wire .
To test if this effect shou ld be con sidered, solder one
end of a shorr, stiff, piece of insulated wire to the
circuit at the point where the meas urement is to be
made. Leave the other end of this wire unconnected.
This wire will have VOltage on it but no current th rough
it. Clip th e probe over this wi re and read the mete r .
Reverse the direction that the probe is clipped on the
wire (point arrow on probe in other direction) and read
meter aga in. If there is no rea ding on the meter in
either of these positions the probe capacitanc e has no
effect.
If th ere is a reading proceed to the ne xt
para graph.
The shunt capacitance introduced by the probe is 1 to
S
pf (typica lly 3 pf). Assuming the worst case of
5
pf, a ma ximum error current of:
I
=
0.03 rna
X
volts
X
megacycles
can flow through the 5 pf to ground. That is, a 1
me
v()l tage ()f 1 vol t impressed ac ross this shunt capaci
tance will cause a reading of only 0.03 rna by the
456A.
However, at higher frequencies the effect will
inc rease proportionally.
Even with this effect you may choose which side of the
circuit to measure the current.
See figure 2-5.
Note in figure 2-5 the probe measures the current
flowing in the circuit on the same side as the wide
side of th e arrow on the probe.
Occasionally a t h igh frequencies (above 15 me) a
greatly magnified voltage effect exists a t a particular
frequency. This effect is caused by a resonance be
tween excessive external ground lead l engths and
stray capacities . The following steps are sugges ted
to el iminate tllis rype of problem:
2-4
Model 456A
THE
PRO BE
AND WIRE BEING MEASURED
HAS AN EQUIVALENT
CI
RC
U
IT O F:
THIS CURRENT
MEASURED
/-
-
I
THIS
CURRENT
!!QI
MEASURED
I
,
\
\
"
........
----
-- <:...
-
......
I TO
5
PF
PRO BE
MEASURES
CURRENT IN
CIRCUIT
ON
THE
S
A
ME
SIDE OF PRO BE AS
SIDE
OF
ARROW,
F igure 2-5. Illustrating Direc tional Effect
of Current Measurement
"-
RO
\
\
I
/
I) Ground oscilloscope or voltmeter to equipment
ground with as shorr leads as possibl e.
2) Connect special res istive ground lead (supplied)
to the closest ground to the conductor being measured.
3) S
p
a
c
e
456A cabinet
aw ay
[rom groundedcondu
tors
(steel
t
a
b
l
e
s,
e
t
c)
.
4) Ground 456A to oscilloscope or voltmeter with a
standard (non-resistive) short clip lead (not furnished).
2-6.
SPE CIAL MEASURING T ECHNI QUES .
Since the probe is effectively a current transformer,
it ha s th e property that it will algebra ically sum the
ins tantaneous value of the currents in two or more
conduc tors it may be clipped around. This property
makes the probe a valuable and easily-applied rool
in applications in which it is desired to equalize or
balance ac currents. For exampl e , in th e class C
amplifier shown in figure 2-
6
it is possible to u s e
this summing property ro examine the plate current
pulses exclusive
of
the current component flowing
through the capacity of the tube . This arrangement
will allow you to measure the true angle of conduc tion
of the class C amplifier .
The method used to obtain the bucking c urrent is indi
ca ted in figure 2-6. The probe wa s clipped a round
the plate lead of the tube, but at the same time a l ead
from an exter na l varia ble capacitor was connected to
the pla te lead and passed th rough the probe as shown.
By suitably adju sting the variable capac itor, a capa
citiv e current equal ro but in opposite phase to the
capacitive current flowing at the plate can be applied
to the probe.
00395-1
