APPLICATIONS
(1) The attenuator can be used with a Type 874-YR or -VRL Volt-
meter Rectifier and a Type 874-VI Voltmeter Indicator to provide the level-
monitor and variable attenuator of a signal generator, when combined with
a General Radio Oscillator. The Types 1362, 1363 and 1218 are recom-
mended. A block diagram of a typical setup is shown in Figure 5.
In this case the stub is adjusted asindicated in a previous paragraph
and the adjustable line1 (Type 874-LAL) is adjusted to obtain a maximum
indication
from the Type 874-VI Voltmeter Indicator. The voltage indi-
cated with the load connected is the effective open-circuit output voltage,
and the source impedance is accurately 50 ohms. The output can be reduced
by adjusting the attenuator; the actual voltage at any attenuator setting is
the indicated number of db below the original calibrating value.
Since the source impedance is the same as the characteristic imped-
ance of the line, the same effective open-circuit voltage and source im-
pedance are obtained at the end of any length of 50-ohm line connected to
the output of the voltmeter rectifier.
If the line is not lossless, the ef-
fective open-circuit voltage will be reduced by the loss in the line.
(2)
The attenuator can also be used to measure the attenuation of a
network by the substitution method, as indicated by the block diagram in
Figure 6.
Readings of the detector output and attenuator setting are first made
without the circuit under test connected.
The circuit is then inserted and
the Type 874-GAL Adjustable Attenuator is readjusted to give the same
detector indication.
The attenuation of the unknown circuit is then the
difference in the attenuator readings. The pads indicated are used to
make the source and load impedances very close to 50 ohms.
One or both
can be omitted if the source and detector impedances are matched.
(3)
For monitoring purposes the input line of the attenuator can be
connected in series with the line under test without introducing an appre-
ciable reflection or loss.
The changes in level in the main line can be
measured accurately using the attenuator and an uncalibrated detector which
may be 1) a Type 874-VRL Voltmeter Rectifier with a dc microammeter or
an audio amplifier such as the General Radio Type 1232-A Tuned Ampli-
fier and Null Detector if the signal is modulated or 2) if the signal is not
modulated,
the General Radio Type 1241 Heterodyne Detectors may be
‘At the lower frequencies,
it may be necessary
to add
lengths
of Type
874-L30
Air Line in series with the adjustable
line to obtain the maximum output.
4 GENERAL RADIO
