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and cw operation this reduction will not exceed the low-speed limit
im-
posed by mechanical standards.
Once a flat horizontal line representing the oscillator output-volt-
age characteristic
is
obtained, the oscilloscope vertical input can be
transferred to the device under test with the assurance that the output
voltage
is
being regulated by the Type 1263-B.
4.3
INTERNAL AMPLITUDE MODULATION.
In addition to the toggle switch that selects either CW or 1-KC
nJ
MOD, there are two controls for internal modulation. If a 1-kc fixed-fre-
quency selective audio amplifier is used with a demodulating detector,
adjust the modulating frequency for maximum response with the ADJ MOD
FREQ control. For a symmetrical modulation. waveform, monitor the de-
modulated output on an oscilloscope and adjust the duty ratio with the
INCREASE DUTY RATIO thumb-set control on the left-hand
side
of the
Type 1263-B.
If
a gate voltage
is
desired for keying a synchronous de-
modulator or for other synchronizing purposes, connect to the GATE out-
put with a standard telephone plug.
4.4
EXTERNAL AMPLITUDE MODULATION.
Although the Type 1263-B Amplitude-Regulating Power Supply was
designed to operate oscillators with cw or internal1-kc square-wave mod-
ulated output only, a limited amount of amplitude-modulating voltage may
be applied to the BLANKING terminals when the Type 1263-B
is
set for
cw.
Connect a blocking capacitor in series with the lead to the high
(red) BLANKING terminal, and set the output voltage to 1 volt. At a 1-
volt output, the input impedance
is
about 5000 ohms, and a 20-pJ blocking
capacitor
is
suitable at frequencies above 20 cps. If the blocking capac-
itor
is
an electrolytic, connect its negative terminal to the high (red)
BLANKING terminal. At outputs smaller than 1 volt, the voltmeter diode
nonlinearity causes greater modulation nonlinearity and lower input im-
pedance. Therefore, it is recommended that if lower outputs are desired,
the required
attenua~ion
be added at the output of the Type 874-VR.
Figure 5 shows the limit of the resulting modulation as a function
of frequency.
This is
not a curve of over-all frequency response, but
rather a curve representing the response-speed limitation of the Type
1263-B.
Owing to the large amount of feedback in the system, the fre-
quency response
is
flat below the overload points represented by the
curve. To determine the maximum voltage that can safely be applied at
any frequency, slowly increase the modulating voltage while observing
the output-voltage meter on the Type 1263-B. Set the modulation voltage
just below the point where the meter starts to drop, since this voltage
12
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