HP 410C manual Voltage Measurement AT High FRE, Pulse Measurements

Model 410C

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		,
			True RMS Value	Voltmeter Indication
	o
			100	100
10%		2nd	100.5	90	to 110
20%		2nd	102	80	to 120
50%		2nd	112	75	to 150
	l 0 %	3 r d	100.5	90	to 110
	2 0 % 3 r d		102	87	to 120
5 0 %		3 r d	112	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

TM 11-6625-1614-15

Section III

Paragraphs 3-19to 3-27 and Table 3-1

3-23. TO 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:

t1 is the duration of the positive portion of the voltage in microseconds.

t2 is the duration of the negative portion of the voltage in microseconds.

K is a	factor determined from the	expression
R o/t1	and the graph shown as	Figure 3-6,
where Ro is the source impedance of the pulse
generator in kilohms, and tl is the		duration of

the positive portion of the pulse in micro- seconds.

3-3