DETAILED DESCRIPTION, PROBE AND TERMINATIONS

TM 11-6625-524-14-4

a. Probe: The probe's spring-loaded construction guarantees positive contact

with the various threaded accessories. The built-in resistors are epoxy encapsu-

lated and the entire probe and cable assembly are sealed, with the exception of the

diode cartridge assembly. The probe should provide years of dependable service.The

d i o d e cartridge, being the only part subject to electrical damage, is easily re-

placed.

b. D i o d e cartridge: The diode cartridge employs two (2) special, low-capacity,

h i g h - s t a b i l i t y germanium diodes in a full-wave configuration for RF detection. The

measuring capability of the voltmeter covers RF voltages reaching from the square-

law region (100V to 30mV) to the region of linear rectification (1V to 3V). In the

square-law region, the voltmeter is true RMS responding, while in the linear region

i t is peak-responding. I n the 30mV to 1V "transition" r e g i o n , the diode character-

i s t i c s gradually change from RMS to peak. This non-linear characteristic is true

o f all diode rectifiers, regardless of design and manufacture. The voltmeter is

calibrated in terms of RMS on all 8 ranges.

The diodes have characteristics the user of the voltmeter should be aware of:

1. The shunt resistance of the diodes (probe input impedance) varies in-

v e r s e l y with both signal level and frequency. (see figure 4-3)

2. T h e capacity of the diodes varies inversely with signal level. Probe in-

put capacity varies from approximately 1pf at 3V to 2.2pf at 10mV. (see figure 4-4)

c . Type BNC 50-ohm termination adapter: This is the most widely used termination

with a VSWR of less than 1.2:1 to 600MHz. It is not recommended for accurate

measurements above this frequency. (see figure 4-5) The termination has a 5 watt

power rating.

d. High impedance probe tip: This tip allows high impedance direct measure-

ments up to 250MHz.

Probe input impedance vs. level and frequency.

4-5