TM 11-6625-524-14-2
5-2. Rf Probe
perature characteristics of the rf diodes and are
switched into the circuit through sections C and
The rf probe (fig. 5-2) contains a diode cartridge
E.
assembly. The cartridge assembly contains a
d. Sections F and G switch in both resistors
coupling capacitor, two matched diodes, a pair of
and potentiometers for correcting the nonlinearity
filter capacitors and a pellet resistor. The sealed
of the rf diodes.
section of the rf probe housing contains two filter
resistors connected in series with the output
terminals of the diode cartridge, and a tern-
The input filter consists of RC networks and a
perature sensing thermistor network connected in
tuned parallel T-network providing an effective ac
the dc feedback loop of the amplifier. The ther-
f i l t e r with a minimum of 60-db rejection at
mistors are connected to the individual ranges by
powerline. frequency.
means of sections C and E of the range selector
switch 3S2. The individual pins of connector 3J1
5-5. Dc Modulator
are bypassed to ground by filter capacitors and
A 94-Hz printed circuit plug-in dc modulator
thus prevent any possible error introduced by
(3G1), is employed. It modulates the dc input
stray rf pickup in the probe cable.
signal, and the resultant ac component is im-
pedance matched to the narrow band pream-
a. The range selector switch consists of sec-
plifier.
tions A through H. Sections A and B (range
5-6. Narrow Band Preamplifier
attenuator) contain resistive dividers supplying
The narrow band preamplifier features a low
the full or partial voltage to the input of the
amplifier. This voltage, when modulated, am-
noise, high impedance input achieved by the use
plified, and demodulated, will indicate on the
of a high quality field effect transistor 13A2Q9).
meter a reading corresponding to the value of the
Diodes 3A2CR3 and 3A2CR4 form a clamp circuit
input signal within the accuracy rating of the
to protect the input transistor from excessive
range being used.
overload. A three-stage conventional RC-coupled,
common-emitter amplifier, consisting of 3A2Q10,
b. Section D disconnects 3R55 to provide
Q11, and Q12, with the input transistor 3A2Q9,
additional gain on the two 10 west ranges.
develop a gain of approximately 80 db. A tuned
c . Sections C and E, in conjunction with
parallel T-network in a negative feedback loop
section H, form part of the dc feedback loop.
from the collector of 3A2Q12 to the base of
S e c t i o n H contains individual resistors and
3A2Q10 narrows the amplifier bandwidth and
potentiometers to control the main feedback loop
t h u s improves the signal-to-noise ratio. The
o f each midrange control. The thermistors,
supply voltage is held at +8.5 volts by Zener
located in the rf probe, sense and correct for any
diode 3A2VR1.
thermal errors occurring as a result of the tem-
