3-14
polarity with respect to the center tap of the winding. Because the winding is inductive, the voltage across
it is 90 degrees out of phase with the current through it. Because of the center-tap arrangement, the
voltages at each end of the secondary winding of T1 are 180 degrees out of phase and are shown as e1 and
e2 on the vector diagram.
The voltage applied to the anode of CR1 is the vector sum of voltages ep and e1, shown as e
3
on the
diagram. Likewise, the voltage applied to the anode of CR2 is the vector sum of voltages ep and e
2,
shown as e4 on the diagram. At resonance e3 and e4 are equal, as shown by vectors of the same length.
Equal anode voltages on diodes CR1 and CR2 produce equal currents and, with equal load resistors, equal
and opposite voltages will be developed across R3 and R4. The output is taken across R3 and R4 and will
be 0 at resonance since these voltages are equal and of appositive polarity.
The diodes conduct on opposite half cycles of the input waveform and produce a series of dc pulses
at the rf rate. This rf ripple is filtered out by capacitors C3 and C4.
OPERATION ABOVE RESONANCE.—A phase shift occurs when an input frequency higher
than the center frequency is applied to the discriminator circuit and the current and voltage phase
relationships change. When a series-tuned circuit operates at a frequency above resonance, the inductive
reactance of the coil increases and the capacitive reactance of the capacitor decreases. Above resonance
the tank circuit acts like an inductor. Secondary current lags the primary tank voltage, ep. Notice that
secondary voltages e
1
and e2 are still 180 degrees out of phase with the current (iS) that produces them.
The change to a lagging secondary current rotates the vectors in a clockwise direction. This causes el to
become more in phase with ep while e2 is shifted further out of phase with ep. The vector sum of ep and e2
is less than that of ep and e1. Above the center frequency, diode CR1 conducts more than diode CR2.
Because of this heavier conduction, the voltage developed across R3 is greater than the voltage developed
across R4; the output voltage is positive.
OPERATION BELOW RESONANCE.—When the input frequency is lower than the center
frequency, the current and voltage phase relationships change. When the tuned circuit is operated at a
frequency lower than resonance, the capacitive reactance increases and the inductive reactance decreases.
Below resonance the tank acts like a capacitor and the secondary current leads primary tank voltage ep.
This change to a leading secondary current rotates the vectors in a counterclockwise direction. From the
vector diagram you should see that e2 is brought nearer in phase with ep, while el is shifted further out of
phase with ep. The vector sum of ep and e2 is larger than that of e
and e1. Diode CR2 conducts more than
diode CR1 below the center frequency. The voltage drop across R4 is larger than that across R3 and the
output across both is negative.
Disadvantages
These voltage outputs can be plotted to show the response curve of the discriminator discussed
earlier (figure 3-10). When weak AM signals (too small in amplitude to reach the circuit limiting level)
pass through the limiter stages, they can appear in the output. These unwanted amplitude variations will
cause primary voltage ep [view (A) of figure 3-11] to fluctuate with the modulation and to induce a
similar voltage in the secondary of T1. Since the diodes are connected as half-wave rectifiers, these small
AM signals will be detected as they would be in a diode detector and will appear in the output. This
unwanted AM interference is cancelled out in the ratio detector (to be studied next in this chapter) and is
the main disadvantage of the Foster-Seeley circuit.
Q-23.
What type of tank circuit is used in the Foster-Seeley discriminator?
Q-24.
What is the purpose of CR1 and CR2 in the Foster-Seeley discriminator?
Q-25.
What type of impedance does the tank circuit have above resonance?
