Circuit Descriptions

TM 11-6625-922-15

automatic gain control (agc) of the signal level. The

f. Dual flip-flop (SN7473N) (D, fig. 5-6). This

signal is fed to tuned-amplifier stages Q9, Q10, and to

integrated circuit contains two flip-flops, and is used in

Q11, Q12, Q13, Q14 which comprise a compound

the divide-by-4 mode (C, fig. 5-7).

emitter follower. Transistor Q15 is a squaring circuit,

g. Decade Counter (SN7490N) (E, fig. 5-6). This

which converts the sine wave to a square wave, with

integrated circuit contains four flip-flops, and is used for

voltage levels between ground and + 5 volts.

divide-by-5, divide-by-2, or divide by-10 (Mode D, fig.

b. Variable-Frequency Oscillator A2 (fig. 9-5).

5-7). The counter can be reset to 0's by applying a

The variable-frequency oscillator (vfo) is a modified

logical 1 to an R0 input.

Colpitts, with an inductor, split capacitors, and tuning by

h. Decoder/Driver (SN7441N) (F, fig. 5-6). This

means of a variable capacitor across the tuned tank.

integrated circuit takes the outputs from an SN7490N

Inductors L1 and L2 in series form the inductor,

decade counter, decodes the input for 1 out of 10, and

capacitors C2 and C3 are the split capacitors, and the

then drives an indicator lamp. The selected output will

variable capacity is switched in by front panel controls.

be a low voltage (near ground). The circuit operates

Transistor Q1 is a common-base amplifier for the

according to the following chart:

oscillator section. Feedback is from emitter to base.

The output signal from the collector is a rectangular

Input

wave, and is fed to emitter follower Q2. A negative

Output selected

voltage applied to varactor diodes CR1 and CR2 is used

to vary the capacitance of the circuit.

c. Sweep Drive A16 (fig. 9-7). This circuit is a

free-running oscillator, generating a negative triangle

wave, with a constant base line voltage of about -0.5

volt, and independently variable amplitude and

frequency. Positive current source Q15 and negative

current source Q16 are alternately turned on and off to

charge and discharge capacitor C8.

The charge-

discharge rate is determined by the SWEEP TIME

control. The voltage across the capacitor is fed to

compound emitter follower Q1, Q2, then applied to

j. Linear Amplifier (uA702C) (G, fig. 5-6). This

Schmitt trigger Q5, Q7. The generated triangle wave

integrated circuit is an operational amplifier, with high

has a dc voltage that is between the two threshold

gain, dc, differential operation. Pin 2 is the inverting

voltages of the Schmitt trigger. Stages Q9, Q11, and

input (a positive input causes a negative output). Pin 3

Q12 are switches for controlling the current sources.

is the noninverting input.

When positive source Q15 is conducting, its emitter is

k. Physical Configuration (fig. 5-8). All logic

tied through a resistor to +12 volts; Q16 is not

circuits are in the dual in-line package. The linear circuit

conducting because its emitter is connected to ground.

is in a modified TO-5 package. Viewed from the top, pin

When the upper threshold voltage of the Schmitt trigger

numbers run counterclockwise.

is reached, the switching circuits change and Q15 is cut

5-7. Circuit Descriptions

off (emitter connected to ground) and Q16 is turned on

a. 2-MHz Oscillator A4 (fig. 9-3). Crystal Y1

(emitter connected to -12 volts). The capacitor voltage

controls the frequency of the oscillator stage composed

now heads down toward the lower threshold voltage,

of transistors Q1, Q2, and Q3. Q2 is a common-base

and the process is repeated. Transistors Q3, Q6, Q8,

amplifier, and Q1 and Q3 are used as a compound

and Q10 comprise a dc operational amplifier. Resistor

emitter follower. The crystal determines the frequency

R1, in conjunction with variable resistor R4, provides a

by being in the direct feedback loop. Varactor diode

dc level shift, so that the baseline voltage applied to the

CR1 is a voltage-variable capacitor in series with the

amplifier (at the junction of R1 and R6) can be 0 volt.

crystal, and can be used to vary the frequency slightly

Transistors Q3 and Q6 are a differential amplifier,

by the application of a dc back-bias to the diode through

transistor Q8 is an amplifier with a gain of 3, capacitor

pin H. The 2-MHz signal is amplified by stages Q6 and

C2 is used to reduce gain at high frequencies to insure

Q7, rectified and voltage doubled by CR3, and CR4,

stability, and transistor Q10 is an emitter follower. A

and C14, and this dc level is amplified by Q8. The

variable feedback resistor is between pins C and J,

output of Q8 controls the bias of Q4 and QS, which are

varied by the SWEEP WIDTH control, which varies the

used as diodes with a variable impedance, to provide an

amplitude of the output triangular wave.

5-9