HP 410C, Организационное, ДС, ГС и ремонтное руководство на уровне базы, Страница: 17 / 64

Model 410C
TM 11-6625-1614-15
Section III
Paragraphs 3-19 to 3-27 and Table 3-1
second maY be made without loss of accuracy by re-
moving the. plastic nose on the Model 11036A and using
in its place a 0.25 microfarad blocking capacitor in
series with the exposed contact of the probe.
THE GRAY INSULATING MATERIAL
AROUND THE AC PROBE IS POLY-
STYRENE, A LOW-MELTING POINT
MATERIAL. IT IS NOT POSSIBLE TO
SOLDER TO THE CONTACT WHICH IS
EXPOSED WITH THE PROBE NOSE IS
REMOVED WITHOUT DESTROYING
THE POLYSTYRENE.
Table 3-1.
Possible Error When Measuring Voltage
of Complex Waveforms
,
% Harmonic
o
10% 2nd
20% 2nd
50% 2nd
l 0 % 3 r d
2 0 % 3 r d
5 0 % 3 r d
True RMS Value
100
100.5
102
112
100.5
102
112
Voltmeter Indication
100
90 to 110
80 to 120
75 to 150
90 to 110
87 to 120
106 to 150
3-19. VOLTAGE MEASUREMENT AT HIGH FRE-
QUENCIES. At frequencies above 100 megacycles
the distance between the point of voltage measure-
ment and anode of the probe diode must be made as
short as possible. If feasible, substitute a small disc
type capacitor of approximately 50 picofarsds for the
removable tip on the probe. Solder one terminal of
the button capacitor to the measurement point in the
circuit and not to the probe contact. The probe con-
tact ( with tip removed ) can then contact the other
terminal of the capacitor for the measurement.
3-20. At frequencies above 100 megacycles consid-
erable voltage may be built up across ground leads
and along various part of
a
grounding piane.
Con-
sequently, to avoid erroneous readings when measur-
ing medium and high frequency circuits, use the
ground clip lead on the shell of the probe to connect
the circuit ground. In some cases at the higher fre-
quencies it maybe necessary to shorten the grounding
lead on the probe.
3-21. For all measurements at higher frequencies,
hold the molded nose of the probe as far from the ex-
ternal ground piane or from object at ground potential
as can conveniently be done. Under typical conditions,
this practice will keep the input capacitance several
tenths of a picofarad lower than otherwise.
3-22. For measurements above approximately 250
megacycles
it is almost mandatory that measurements
be made on voltages which are confined to coaxial
transmission iine circuits. For applications of this
type, the Model 11036A Probe is particularly suitable
because the physical configuration of the diode and
probe is that of a concentric line, and with a few pre-
cautions it can be connected to typical coaxial trans-
mission line circuits with little difficulty.
01556-2
3-23. T
O
connect the probe into an existing coaxial
transmission line, cut the line away so the center con-
ductor of the line is exposed through a hole large
enough to clear the body of the probe. The nose of the
probe should be removed for this type of measurement.
Connect one terminal of a button-type capacitor of ap-
proximately 50 picofarads to the center conductor of
the coaxial line so that the other terminal of the oapa-
citor will contact the anode connection of the probe.
A close-fitting metal shield or bushing should be ar-
ranged to ground the outer cylinder of the probe to the
outer conductor of the transmission line.
This type of
connection is likely to cause some increase in the
standing wave ratio of the line at higher frequencies.
The Model 11042A Probe T Connector is designed to
do this job with SWR or less than 1.1 at 500 Mc (see
Paragraph 1-11).
3-24. EFFECT OF PARASITIC ON VOLTAGE
READINGS . At frequencies above 500 megacycles,
leads or portions of circuits often resonate at fre-
quencies two, three, or four times the fundamental
Of the voltage being measured. These harmonics may
cause serious errors in the meter reading. Owning to
the resonant rise in the probe circuit at frequencies
above 1000 megacycles, the meter may be more sen-
sitive to the harmonics than to the fundamental. To
make dependable measurements at these frequencies,
the circuits being measured must be free of ail para-
sitics.
3-25. EFFECT OF DC PRESENT WITH AC SIGNAL.
When measuring an AC signal at a point where there
is a high DC potential, such as at the plate of a vacuum
tube, the high DC potential may cause small leakage
current through the blocking capacitor in the tip of the
Model 11036A AC Probe. When the AC signal under
measurement is small, the error introduced into the
reading can bes significant. To avoid leakage, an addi-
tional capacitor with a dielectric such as mylar or
polystyrene which has high resistance to leakage is
required. (Use 5 picofarads or higher, and insert the
capacitor between the point of measurement and the
probe tip.)
3-26. PULSE MEASUREMENTS
3-27. POSITIVE PULSES. The Model 11036A AC Probe
is peak-above-average responding and clamps the
positive peak value of the applied voltage. This per-
mits the probe to be used to measure the positive-
voltage amplitude of a pulse, provided the reading ob-
tained is multiplied by a factor determined from the
following expression:
t
1
t
2
K
is the duration of the positive portion of the
voltage in microseconds.
is the duration of the negative portion of the
voltage in microseconds.
is a factor determined from the
expression
R
o
/
t
1 and the graph shown as
Figure 3-6,
where R
o
is the source impedance of the pulse
generator in kilohms, and t
l
is the duration of
the positive portion of the pulse in micro-
seconds.
3-3