HP 8414A Operating And Service Manual page 70

Model 8414A
Figure 8-8. Horizontal and Vertical Vectors
Required to Deflect CRT Beam
that can be displayed on the CRT. This is done by
producing an illuminated spot on the CRT where
the arrowhead of the vector would appear. (See
Figure 8-8.) This can be interpreted into amplitude
(A) and phase (0) by the use of the built-in polar
graph on the face of the CRT.
To produce the illuminated spot on the CRT, the
amplitude and phase of the polar signal must be
resolved into X (horizontal) and Y (vertical) rec¬
tangular components as shown in Figure 8-8. These
X and Y signals are applied to the horizontal and
vertical deflection plates producing a spot on the
CRT representing the arrowhead of the vector.
Figure 8-9. Vector Graph of a Typical Test Signal
Conversion from the two signals applied to the
input of the Polar Display to the rectangular X-Y
signal is accomplished by a 90 degree phase-shifter
and two phase-detector circuits. To understand the
operation of these circuits a brief discussion of the
trigonometry used is presented. Figure 8-9 shows a
vector diagram in the form of a right triangle of the
signals discussed. From trigonometric relationships
in a right triangle:
^ _ Adjacent Side _ X Component.
OOS
0
_ _ . » 9
Hypotenuse A
therefore;
X Component = A Cos 0.
Also,
Opposite Side Y Component;
Sin 0 = —p-— - - ---
Hypotenuse A
therefore,
Y Component = A Sin 0.
The sin
0
detector circuit produces a voltage with
the value of A sin 0. This voltage is amplified and
applied to the Y-axis or vertical deflection plates.
The cos
0
detector similarly produces a voltage
that corresponds to the value of A cos
0
which is
amplified and applied to the X-axis or horizontal
deflection plates.
The foregoing discussion explains the method of
developing a spot on the CRT that represents the
behavior of a device under test at one frequency.
During swept-frequency operation, the device
under test reacts differently as the frequency is
changed. This causes a continuous trace to be pro¬
duced on the CRT. This trace may be interpreted
as follows. Amplitude is proportional to distance
from the center of the CRT, phase can be read
directly from the built-in graticule and frequency is
indicated by marker pips superimposed on the
trace through the intensity-modulator circuit.
Marker signals applied to this circuit from a Sweep
Oscillator cause a bright pip on the trace. In this
manner specific frequencies may be located on the
CRT display.