Heathkit AR-14 - Page 78

Page 68
D3
T6
D4
R27
AUDIO
OUTPUT SIGNAL
l
C35
+
~fb¼Sih&Q
Figure 11
All, or only one of the IF amplifier stages may
operate as limiters. For a very weak signal,
only the fourth IF amplifier Q7 may be limiting,
and transistors Q4, Q5, and Q6 would be
amplifying the IF signal. For a very strong
signal, all four IF amplifiers may be acting as
limiters. This limiting action removes ampii-
tude modulation from the FM signal. Limiting
action is also provided by the self-limiting
characteristics of the ratio detector circuit.
RATIO DETECTOR CIRCUIT
From the collector of transistor Q7, the IF signal
is coupled through resistor R26 and ratio de-
tector transformer T6 to the ratio detector cir-
cuit. This circuit, which separates the audio sig-
nal from the 10. 7 me IF signal, is shown re-
drawn for greater clarity and simplified in Fig-
ure 11. Transformer T6 is represented in this
figure by primary coil Ll, a center tapped sec-
ondary composed of coils L2 and L3, and a third
or tertiary winding, L4. L4 is just a few turns of
wire tightly wrapped around the bottom of pri-
mary Ll. NOTE: In the actual circuit, choke L5
and coil L6 are also connected in series with
coil L4, resistor R31, capacitor C39, and re-
sistor R39.
Consider a separate voltage to be induced by the
primary into each of the windings, L2, L3, and
L4. L4, which is closely coupled to the primary
introduces a voltage that is in series with both
L2 and L3. This voltage across L4 is relatively
constant in amplitude as long as the voltage
across Ll does not change. (Remember, the
voltage across Ll will stay relatively constant
due to the limiting action of transistor Q7 .)
I
Notice that each diode has its own separate loop
through which its current flows (indicated by the
arrows). Current flowing in diode D3 is con-
trolled by the voltage induced in L2 and L4 which
charges capacitor C33. The current flowing in
diode D4
is controlled by the voltage induced
in coils L3 and L4 which charges capacitor C34.
Current flows through L4 in both directions ,
since this coil is common to both current loops .
The two currents flow through capacitors C33
and C34 in the same direction. Electrolyti c
capacitor C35 is connected across both of these
capacitors through resistors R27 and R30.
This large capacitor keeps the total voltage
across these two capacitors from changing , thus ,
any amplitude changes on the IF signal are
damped out by this capacitor.
The audio output signal from the ratio detector
circuit is applied to the base of QB. Note that
the two loop currents are flowing in opposite
directions through coil L4, resistor R31, capa c-
itor C39, and the input resistance of QB. At the
FM IF center frequency of 10. 7 me, the diode
currents are equal, thus they cancel each other
out and no voltage appears across the input re-
sistance of QB.
When the IF frequency deviates from 10. 7 me
due
to FM modulation (audio signal), the curren t
in one diode loop increases while the current in
the other loop d&creases. These changes are
caused by a change in phase relationship in th e
signal current across coils L2 and L4 , and L3
and
L4. Now current flows through the input
resistance of QB in the direction of the large r
signal, and an output voltage is develope d