c. Input Impedance.
input impedance decreases at higher frequencies (above 1 kHz).
decrease is due to the capacitive loading effect of the amplifier input.
Thus when a signal source of fixed impedance is connected to the input, a
meter at the signal source will not indicate the correct input level. To
avoid this error, monitor the input level with an RF Voltmeter to ensure a
satisfactory input level.
The Model 5261A is composed of four functional circuits; a
preamplifier, attenuator, video amplifier and output amplifier.
circuits combine to amplify ac signals as small as 1 millivolt and provide a
usable output level to the counter input circuit.
operate the Model 5261A are obtained from the counter via the plug-in
circuit (figure 4) consists of a
nuvistor cathode follower (Q1) driving an
emitter follower (Q2) to provide a
high impedance input and a low impedance
output. Diodes CR1 and CR2 limit
input signal peaks to prevent damage to
Resistor R4 limits Q1 gate
current during overload. Capacitors C2,
b. Attenuator. The resistive attenuator (figure 5) reduces the input
of attenuation inserted is controlled by the Sensitivity switch.