ment, expressed in terms of reflection coefficient, is
system accuracy is maintained constant for all rf bands.
+0.01 if the operating procedures are adhered to.
A block diagram of the attenuation measurement setup
c.
Magnitude of Reflection Coefficient
is shown in figure 225.
Measurement.
b. Signal source No. 1 is operated at the frequency
(1) Above 1 GHz, the modified reflectometer method is
source no. 2 is used as a local oscillator and is operated
at a frequency 30 MHz removed from the measurement
The test setup diagram for this measurement is shown in
frequency Each of the two signals is applied through an
The 1000-Hz square-wave-modulated
isolator and a variable attenuator to one to the two inputs
leveled rf signal is obtained directly from the signal
of the balanced mixer. The resulting output of the mixer
source in the frequency ranges from 1 to 2 GHz and 12.4
is a 30-MHz intermediate frequency (IF) signal which is
to 40 GHz. In the frequency range from 2 to 12.4 GHz,
coupled through the matching network to the input of the
the signal source is operated in the CW mode and is
externally modulated with a PIN modulator-modulator
in adjusting the intermediate frequency. Initially the
combination which is inserted at the main-line output of
unknown attenuator is not connected in the setup and
directional coupler DC-1. The modulator is adjusted to
the two variable attenuators are adjusted to obtain both a
obtain a 1000-Hz squarewave-modulated rf signal at the
null indication on the microwave receiver and a high ratio
output of the PIN modulator. The modulated signal,
of the local oscillator signal level (signal source no. 2) to
obtained either directly from the signal source or from the
the signal source no. 1 signal level. When these
PIN modulator, is applied through isolator E-1, the
conditions are met, the unknown attenuator is inserted in
variable attenuator, tuner T-1, directional coupler DC-2,
the measurement line and the reference attenuator,
and tuner T-2 to the unit under test. The variable
located in the microwave receiver, is adjusted to re-
attenuator and the two tuners serve to calibrate the
establish the null indication. The value of the unknown
system, using the sliding termination and sliding short.
attenuator is computed by subtracting the original
The signal reflected from the unit under test is fed
reference attenuator setting (usually zero) from the
through tuner T-2 to directional coupler DC-2. The
setting necessary to re-establish the null indication after
output from the auxiliary arm of DC-2 is applied through
the unknown attenuator has been inserted.
isolator E-2 to crystal detector CR-2, where it is detected
c.
The microwave receiver, a simplified block
and applied to the SWR meter, which indicates the
difference between an established reference point and
component in this measurement setup. The 38-Hz
coaxial switch alternately samples the 30-Mhz inputs
(2) The measurement accuracy of the magnitude of
from the matching network and from the below-cutoff
reflection coefficient is within +0.01 (expressed in
reference attenuator and applies it to the IF amplifier and
reflection coefficient) if the operating procedures are
synchronous detector. When the alternately sampled
adhered to.
signals are of different amplitudes, an error signal is
produced in the synchronous detector. The output of the
2-15. Attenuation Measurement
synchronous detector is applied to the null meter, which
a. In this test set the IF substitution method is
indicates the magnitude of this error signal.
d. The major sources of error encountered in making
at the detector is maintained at a constant level by
attenuation measurements are discussed in the following
adjusting a calibrated reference attenuator to
paragraphs.
compensate for the unknown attenuation. The standard
attenuator may be inserted in any point of the system,
of reference attenuator. The below-cutoff at-
however to eliminate its frequency dependence, it is
placed in the IF line where the frequency is constant. In
this
manner
the
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