1. Assume that the center frequency of the IF signal in an FM receiver is 9.1
MHz.
If one of the IF tuned circuits has a Q of 100, it will pass a band of
frequencies 91 kHz wide. If the circuit is adjusted to pass efficiently a band of
frequencies 150 kHz wide, it must have a Q of approximately
a. 10.
c.
60.
b. 30.
d.
140.
SITUATION
Figure 153 of TM 11-668 is the schematic diagram of a military double-conversion
superheterodyne FM receiver. Assume that: Bandpass of the tuned circuits = 150 kHz
(that is, the response is 3 db down at 75 kHz to either side of the center
frequency).
Q of primary = Q of secondary (transformer T2)
1st IF = 4.3 MHz
2d IF = 455 kHz
Exercises 2 and 3 are based on this situation.
2. Assume that the receiver in figure 153 is receiving a signal that introduces
significant IF side frequencies of 380 kHz and 430 kHz to the second IF strip.
Compared with the signal power for these frequencies at the plate of V5, the signal
power at the grid of V9 will be reduced by approximately
a. 6 db.
c.
24 db.
b. 18 db.
d.
32 db.
3. The response characteristic of a transformer-coupled amplifier depends on the
Q of the circuit and the degree of coupling between the transformer windings. The
coefficient of coupling required to provide the desired 150-kHz bandpass through
transformer T2 is approximately
a. 0.015.
c.
0.035.
b. 0.025.
d.
0.045.
4. Assume that an FM receiver with an IF of 4.7 MHz is tuned to a station with a
carrier frequency of 108 MHz and a frequency deviation of 75 kHz. An oscilloscope
is to be connected across the output of the final IF amplifier to determine the
frequency response of the entire IF section. If the receiver has an ideal response
to this signal, the oscilloscope presentation will appear
a. high at 4.7 MHz, and gradually dropping to zero at 4.625 MHz and 4.775
MHz.
b. high at approximately 4.650 MHz and 4.735 MHz, with a slight dip at 4.700
MHz.
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