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RCA Crystal-Cal ibrated Marker Generator
WR·99A
Figure 10. Response curve for fm-sound
detector.
Symmetrical
Figure 12. FM-detector curve showing 600-cps modulation. Mod-
shape indicates correct alignment.
ulation causes wide trace or waviness on base line. When center
frequency of detector is tuned exactly to the sound intermediate
frequency, modulation will disappear.
Figure 11. Sound-if response curve. Marker shows center of pass
band and is the sound intermediate frequency.
If a discriminator
detector is used, the stage will be
preceded by a limiter. The over-all response of the if
amplifier
is checked by connecting
the oscilloscope
across the resistor in the grid circuit of the limiter, In
some receivers, the time constant of the grid circuit
may be large enough to cause distortion of the pattern
when the scope is connected. If the pattern is distorted,
the difficulty may be eliminated by temporarily
remov-
ing the capacitor from the rf circuit or by shunting the
resistor with another resistor of a value determined by
experimentation.
The response of the sound-if amplifier alone in a
system using a discriminator-type
detector
may be
checked by using the same procedure as for checking
an amplifier using a ratio- type detector. Set the CALI
MOD control to "4.5 Mc & 600 CPS". The sound-if
point should appear at the intersection of the response
curve and the zero-reference
line. Tune the detector
transformer
as indicated in the manufacturer's
service
notes.
If the detector is not correctly aligned, the 600 cps
will
modulate the "S" curve, as shown in Figure 12.
When the detector is set to exactly 4.5 Mc, the modula-
tion will be cancelled out.
Use of WR·99A as Heterodyne Frequency
Meter
The WR-99A may be used with good accuracy to de-
termine the frequency of an external signal between
20 and 260 Mc. In general, the procedure consists of
feeding
the signal of unknown
frequency
into the
WR-99A, mixing it with the vfo signal from the WR-
99A, and, by use of the zero-beat method and inter-
polation, reading the frequency directly from the WR-
99A dial. This feature is particularly
useful in checking
the oscillator frequency in intercarrier
receivers. Pro-
cedure is as follows:
1. Connect a lead from the rf output of the external
signal source to the RF IN connector.
2. Set the tuning dial of the WR-99A to read the
approximate estimated frequency of the external signal.
3_ Calibrate
the WR·99A at the I-Mc or 10-Mc
check point nearest the frequency of the external sig-
nal, as described under "Calibration".
4. If the unknown signal is of very low level, turn
the AF GAIN control fully clockwise. It may also be
necessary to connect a ground lead from the external
signal source to the GND terminal of the WR-99A.
5. Turn the tuning dial of the WR·99A until zero
beat is obtained with the external signal.
6. Determine the frequency of zero beat by observ-
ing the dial setting and interpolating,
as described un·
der "Calibration".
• 19 .
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