5. The lesser of the two frequencies being
compared is applied to the deflecting
plates of the CRO through the resistance-
capacitance phase shifter.
The higher frequency applied to the
control grid of the CRO.
If the voltage across capacitor and
resistor is equal, the pattern is circle.
But if voltage is unequal, elliptical pattern
is obtained.
6. As a result, the pattern appears as a series
of alternate bright and dark spots.
The ratio of high to low frequencies is given
by the number of bright spots.
R=1/wc where w=2πf. The voltage across C
lags behind the voltage across R by 90.
When the grid bias of the CRT reaches the
cut-off potential, there is no electron stream
and fluorescence at the screen cannot occur.
This property is used for the comparison of
two frequencies, provided their ratio is an
integer.
10. When the unknown frequency is
determined by multiplying the known
frequency by the number of teeth in
the pattern is called Gear wheel
method.
11.
12. In the figure, a phase shift network RC
introduces a 90 phase shift between the
horizontal and vertical channels of the
oscilloscope.
This is needed to produces a ring or
circle pattern with the known frequency fv.
A voltage from the unknown frequency
source modulate this ring.
13. When the voltage across R and C are
unequal, the pattern is elliptical. If they are
equal, the pattern is a circle.
The unknown frequency must be higher
than the known frequency.
The amplitude of the unknown signal
must be below than that of the known to
prevent distortion.
14. Set up the oscilloscope. Switch off the
internal and sync control. Connect the
circuit as in above figure and temporarily
switch off signal B.
Switch on signal source A and adjust R to
obtain a ring pattern on the screen. Adjust
the horizontal and vertical gain controls to
speed the ring over the screen as much as
possible.
15. Switch on signal source B, noting that the ring
becomes toothed by the signal.
Adjust the known frequency fv to stop the ring
from rotating.
Adjust the amplitude of fh to obtain a distinct
teeth pattern.
The unknown frequency is then given by fh=nfv,
where fh is an integral multiple of fv.
The wheel rotates or spin if fh is not an exact
multiple of fv ,Therefore, adjust the known
frequency to stop the wheel from rotating.
16. This pattern is also known as Gear wheel
or toothed wheel because of its shape as
shown in the figure below.
