Heathkit IG-4505 - Page 48

Page 48
Gee gurarHKiT?
CIRCUIT DESCRIPTION
Transistor Q1, the crystal Y1 and the associated circuitry
form
a
4
MHz
oscillator.
This
oscillator,
the
heart
of
the
calibrator circuit, assures an accurate and stable time
reference signal.
Integrated circuit IC1 contains two separate flip-flop
circuits. Each flip-flop is wired to form a divide-by-two
circuit. The frequency of the output signal of this type of
circuit is exactly one-half the frequency of the input signal.
Also, a useful characteristic of a flip-flop circuit is its
symmetrical square wave output.
The 4 MHz signal is first divided by two by IC1A and
coupled to the Multiplier switch and to another divider
circuit 1C2. This divider is a decade divider wired to produce
two separate outputs, a 1 MHz output (+2) and a 400 kHz
output (+5). Both of these signals are also connected to the
Multiplier switch. The output signal of the Multiplier switch,
either 2 MHz, 1 MHz, or 400 kHz, provides the 1-2-5
sequence of the output signal. [C1B again divides the signal
by 2 (+2) to obtain a symmetrical output signal.
Integrated circuits IC3, IC4, IC5, IC6, and IC7 are decade
dividers. The period of the output signal of a decade divider
is ten times the period of its input signal. Thus, the output
signal of 1C1B is successively divided by ten by IC3, 1C4,
IC5, 1C6, and IC7. This produces .1 s to 1 us output signals
in a 1-2-5 sequence,
The Time Out switch (SW1) selects one of the scaled signals
and couples it to the base of transistor Q3 in the Volts Out
circuitry, and to the base of transistor 4 in the Time Out
circuitry.
Resistor R24 limits the current through transistors 04 and
Q5 to prevent either of them from saturating. Since they can
only operate in their active region, they can switch (turn on
and off) very quickly. Transistor Q4 is turned on and off by
the square wave signal from the Time Out switch. When the
signal is low {logic 0) transistor Q4 turns on and when the
signal goes high (logic 1) transistor 04 turns off. Since the
emitters of Q4 and Q5 are coupled together, Q4 can contro!
Q5. Transistor Q5 is normally biased on by resistors R27
and R28. When O4 turns on, it starves current from Q5,
causing
OS
to
turn
off
very
quickly.
Then
as
O4
turns
off,
current is again available to Q5 and it turns on very quickly.
This turn on and off of Q5 occurs very rapidly producing a
fast rise time (<4 ns) output signal.
Resistors R26, R29, and termination resistor R104 provide
the proper impedance to drive and terminate the output
cable with a minimum of ringing and reflections.
Transistors Q2 and Q3 form a totem-pole switch driven by
the signal from the Time Out switch. The collector voltage
of Q3 is either at near zero volts or at 110 volts: Calibration
control R14 is adjusted so the voltage across R15 through
R22 is 100 volts. The Volts Out switch (SW2) selects one of
six output voltages (1 mV to 100 V).
Transistor Q101, zener diode ZD102, and the associated
power supply circuitry forms a series regulator circuit to
operate the logic circuits. The voltage at test point 2 (TP2),
which is 5 volts +1%, is used only during calibration as a
comparison or reference voltage. Calibration contro} R14 is
adjusted during calibration so the voltage at TP1 is also 5
volts +1%.