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Model 492A/494A
Section III
Paragraphs 3-31 to 3-36
G-l-7
TIM E
DESIRED
OUTPUT (Ftl
FR EQU E N CY
+-__-'---_----'__
---A._ _
--L.._---l
- - - - - - - - 1 - - -
INPUT FREQUENCY
(Fl
UNDESIRED
OUTPUT ( F )
FREQUENCY
2
HIGH
Figure 3-8. Offset Frequency Produced by
Sawtooth Modulation of the Helix
LOW
SAWTOOTH
MODULATION
APPLIED
TO
HELIX
3-31. In practical applications, a constant amplitude
linear-slope sawtooth generator is used to produce
the sawtooth waveforms.
If
the amplitude 8f the saw-
tooth voltage is adjusted to produce a 360 shift, one
cycle of the cw signal will be added or subtracted
during each sawtooth, and the frequency shift pro-
duced in the output will be equal to the sawtooth repe-
tition rate. Sawtooth voltages having negative slopes
(see figure 3-7) produce a decrease in the output fre-
quency (delay in phase). Conversely, sawtooth volt-
ages of the opposite slope cause an increase in output
frequency.
of the input signal can be shifted by a predetermined
amount. Unlimited phase deviation is effectively sim-
ulated by continuously repeating exact 360
0
phase
deviations.
This is accomplished by modulating the
traveling wave tube helix wiSh a sawtooth waveform,
each sawtooth producing 360
phase shift, as shown
in figure 3-7.
An rf output frequency that is shifted
in relation to the input frequency is thus produced, as
shown in figure 3-8.
O·
HIGH
SAWTOOTH MODULATION
APPLIED TO HELIX
LOW
PHASE DIFFERENCE
BETWEEN INPUT AND
OUTPUT
WAVES
frequency shift with its relatively small power content
would be rejected by most narrow band circuits. In
systems where the undesired frequency shift falls
within the pass band of the equipment under test, a
negative pulse can be applied to the GRID MOD. con-
nector to cut off the twt beam current during the fly-
back time.
This method reduces the undesired fre-
quency shift although it produces some small tran-
sients and leaves small time intervals during which
there is no signal output.
Practical applications of
offset frequencies include the measurement of ex-
tremely high swr's accurate calibration of attenu-
ators over wide amplitude ranges (paragraph 3-34),
frequency shifting of microwave radio relay channels
for retransmission, production of mixer frequencies
for radar and other microwave receivers, etc.
G-l-I
TI ME
. .
3-34. HOMODYNE DETECTION.
Figure 3-7. RF Phase Shift Produced by
Helix Modulation
3-32. With sawtooth modulation, the desired output
frequency shift (F
in figure 3-8) occurs during the
sawtooth formatioJ time, and is proportional to the
rate of change of voltage.
During the sawtooth fly-
back time the output phase is shifted in the opposite
direction producing an undesired frequency shift (F2
in figure 3-8).
If
the flyback time is made extremely
short, this frequency is far removed from the desired
frequency since the degree of frequency shift is in-
versely proportional to the flyback time. In addition
to being far removed from the desired frequency the
power in the undesired frequency is very small since
it is proportional to the ratio of flyback time to saw-
tooth time.
3-33. In a typical case involving a desired 50-kc fre-
quency shift, a I-microsecond flyback time would
produce a I-megacycle frequency shift in the opposite
direction and would contain only 5% of the total power
in the output wave.
In practice, this undesired
3-35. The ability of a twt to produce an offset fre-
quency that is stable with respect to the signal source
makes it an ideal instrument to use in a homodyne
(linear) detection system.
The difference frequency
will be dependent upon the stability of the sawtooth
generator used to modulate the twt helix, a problem
of no consequence at the low modulating frequencies
involved.
3-36. A typical linear detector system suitable for
calibrating microwave attenuators is illustrated in
figure 3-9.
The signal generator supplies a signal
(f) both to a crystal mixer and to a traveling wave
tube amplifier. The traveling wave tube amplifier is
sawtooth modulated to produce an offset frequency
(f-f ) which is applied to the attenuator to be cali-
brated. The output signal from the attenuator is then
combined with the original signal (f) in the mixer to
produce a beat frequency (f ) whose amplitude is di-
rectly proportional to the ahtplitude of (f-f
j
)
so long
as the amplitude of (f-f
j
)
remains within the square-
law region of the cryStal.
This beat frequency is
amplified by the tuned amplifier and the output is in-
dicated by an ac voltmeter.
The lower sensitivity
01144-2
3-7
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