Elenco AM Radio Kit Manuel d'utilisateur télécharger pdf (Page 19)

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TP4 - Test Point Pin

(see Figure E)

T2 - IF Coil

(White Dot)

Q3 - 2N3904 Transistor NPN

(see Figure G)

R10 - 470Ω Resistor

(yellow-violet-brown-gold)

R7 - 39kΩ Resistor

(orange-white-orange-gold)

R9 - 10kΩ Resistor

(brown-black-orange-gold)

C7 - .02μF Discap (203)

or .022μF Discap (223)

C8 - .02μF Discap (203)

or .022μF Discap (223)

SECTION 3

Figure 17

.707

452kHz 458kHz

455kHz

The purpose of the SECOND IF AMPLIFIER is to

increase the amplitude of the intermediate frequency

(IF) and at the same time provide SELECTIVITY.

Selectivity is the ability to “pick out” one radio station

while rejecting all others. The second IF transformer

(T3) acts as a bandpass filter with a 3dB bandwidth

of approximately 6kHz. The amplitude versus

frequency response of the second IF amplifier is

shown in Figure 17.

Both IF amplifiers are tuned to a frequency of

455kHz and only need to be aligned once when the

radio is assembled. These amplifiers provide the

majority of the gain and selectivity needed to

separate the radio stations.

The gain at 455kHz in the second IF amplifier is fixed

by the AC impedance of the primary side of

transformer T3, and the DC current in Q3. The

current in Q3 is set by resistors R7, R9 and R10.

Both C7 and C8 bypass the 455kHz signal to ground,

making Q3 a common emitter amplifier. The signal is

coupled from the first IF amplifier to the second IF

amplifier through transformer T2. The IF

transformers not only supply coupling and selectivity,

they also provide an impedance match between the

collector of one stage and the base of the next stage.

This match allows maximum power to transfer from

one stage to the next.

SECOND IF AMPLIFIER

THEORY OF OPERATION

ASSEMBLY INSTRUCTIONS - SECOND IF AMPLIFIER

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 31 32

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Elenco AM Radio Kit Manuel d'utilisateur Page 19

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