Heathkit IG-18 User Manual

CIRCUIT DESCRIPTION
Refer to the BlockDiagram on Page 65 and to the
Schematic
Diagram (fold-out from Page 75) while
reading this Circuit Description.
The circuit of the Sine-Square Audio Generator
includes three principal sections: The Sine Wave
Generator (including the meter and output attenu-
ator circuits), the Square Wave Generator and
the Power Supply. Each of these Sections will be
described separately.
SINE WAVE GENERATOR
The sine wave oscillator circuits consists of
differential amplifier transistors Ql and Q2;
voltage amplifier transistor Q3; power amplifier
transistors Q4 and Q5; and the positive and
negative feedback loops.
Positive (regenerative)
feedback comes from the common emitter output
. of transistors Q4 and Q5, and is coupled to the
base of transistor Q2 through the lamp Ll, the
arm of feedback control R7, and resistor R6.
Negative (degenerative) feedback
comes through
the notch filter and is directly coupled to the
base of transistor Ql.
Oscillation occurs due to the positive feedback.
Without negative feedback,
the circuit would
oscillate at some indeterminate frequency; how-
ever, the notch filter, which passes all fre-
quencies except the one to which it is tuned,
provides the negative feedback to the base of
transistor Ql. This negative feedback prevents
oscillation at all frequencies except the one that
is not passed, permitting the system to oscil-
late at only the selected frequency.
HEATHKIT®
The tuned frequency of the RC notch filter
circuit may be calculated by the general formula:
F=
1
2 rr RC
where F is frequency in hertz, R is resistance
in ohms, and C is capacitance in farads. How-
ever, since this notch filter is a specialized RC
network in which there are two resistances and
two
capacitances, the formula for this network
then becom es:
1
F=
2rr~2 -oJ C
1
C
2
•
Since Rl and R2 will always be equal, the for-
mula simplifies to:
F=
1
63

Heathkit IG-18 Specifications

General

Brand	Heathkit
Model	IG-18
Category	Test Equipment
Language	English