digital counter may result in the inability to obtain an
2-12. Frequency Measurement of a Passive
accurate zero beat. This can be eliminated by adjusting
Device
the variable attenuator for the proper input level.
a. Circuit Analysis for Measurements Below
(4) The accuracy of the digital counter is t count
12.4 GHz.
+ time base accuracy, if the operating procedures are
(1) Passive devices, such as LC tank circuits
adhered to. The overall accuracy of the frequency
and cavity resonators, are calibrated by superimposing a
measurement is at least one part in 106.
marker of known frequency on an oscilloscope display of
the frequency response curve of the device.
c.
Circuit Analysis for Measurements Above 12.4
(2) The frequency response curve of the device
GHz.
being calibrated is obtained as follows: the passive
(1) The rf signal (CW, am, fm, or pulsed) to be
device under test is connected between the main arm
measured is applied through isolator E-1 to the auxiliary
output of directional coupler DC-3 and isolator E-3 (figure
arm of directional coupler DS-3 (figure 2-18). The
leveled rf output from the signal source is applied to the
signal source no. 1 is applied to the passive device
main arm of directional coupler DC-2. The output from
being calibrated. The frequency-sensitive, swept signal
the auxiliary arm of directional coupler DC-2, which is 10
output from the passive device is applied through isolator
db below the input to the directional coupler, is fed
E-3 to crystal detector CR-3. The detected signal, which
through a variable attenuator to the input of the
represents the response curve of the passive device
frequency converter, which is part of the digital counter.
being calibrated, is then applied to channel 1 of the dual
The variable attenuator is used to obtain the proper input
trace oscilloscope.
level to the digital counter. The main output from
(3) The frequency marker in the frequency range
directional coupler DC-2 is coupled through isolator E-2,
from 500 MHz to 12.4 GHz is generated by phase-locked
and the frequency multiplier to the main arm input of
signal source no. 2. This marker frequency is sampled
directional coupler DC-3. Isolator E-2 prevents the
by directional coupler DC-4 and measured by the digital
transmission of any reflected signal from the varactor
counter before being applied through the passive device
frequency multiplier to the rf head. The frequency
under test, isolator E-3, and crystal detector CR-3 to
multiplier is used to extend the range of the digital
channel 1 of the dual trace oscilloscope, where it is
counter beyond 15 GHz.
The difference of the
superimposed on the response curve of the device. By
frequencies applied to the auxiliary and main arms of
varying the frequency of the marker signal, the center
directional coupler DC-3 is coupled through isolator E3,
frequency and the bandwidth of the response of the
detected by crystal detector CR2 and applied to the
passive device can be measured.
channel 1 input of the dual trace oscilloscope. Isolator
(4) In order to obtain an undistorted
E- prevents reflections from crystal detector CR-2 to the
representation of the relative frequency response of the
frequency multiplier.
To perform a frequency
passive device, the power level of the swept signal must
measurement, the frequency of the signal source is
be maintained as constant as possible over the
adjusted so that one of its harmonics is at the same
frequency band of interest. To illustrate, assume an
frequency as the output of the unit under test, as
ideal response curve which is the positive half of a
indicated by a zero beat on the oscilloscope. The
sinusoid, as shown in figure 220. Assume also that the
frequency of the signal being measured is then obtained
maximum permissible power level variation of the input
by multiplying the digital counter indication by the
signal to the passive device is 0.1 db. Under these
appropriate harmonic number. (2) Refer to b above for
circumstances, a relatively undistorted representation of
measurement errors affecting the accuracy of the
the relative frequency response of a passive device can
measurement.
be obtained over a bandwidth A F. The accuracy of the
calibration depends on the degree of distortion
2-22