IFR FM/AM-500, Maintenance Manual

Page: 242 / 320

7-13 VARIABLE TONE GENERATOR
7-13-1 THEORY OF OPERATION (Refer to Figure 7-15 and Variable Tone
Generator Circuit Schematic in Section 10)
A. GENERAL
The Variable Tone Generator consists of an adder system, a
crystal oscillator circuit, a ROM/DAC signal generator to pro­
duce the audio frequency sinewave, and an output amplifier/
lowpass filter circuit.
The crystal oscillator circuit provides a 256 kHz frequency to
clock the adder system, the ROM, and its latch. The VAR TONE
FREQ Thumbwheels set a value for the adders (ten times the
thumbwheel setting) which accumulate with every clock pulse.
The ROM is programmed to produce a ser;es of 128 binary-coded
signals. Each time the adder system reaches or exceeds a count
of 20,000, the ROM advances to the next sequential output code.
Any amount in excess of 20,000 is added into the next accumula­
tion. Thus, the higher the thumbwheel setting, the faster the
adders accumulate to 20,000 and advance the ROM to the next
code. With each low output from the oscillator, the latch
stores the ROM output code, then transfers that code to the DAC
when the oscillator output goes high. The DAC converts the
binary code to an analog voltage which, as a result of the ROM
programming, forms a sinewave at the frequency established on
the thumbwheels.
After amplification, the output of the DAC passes through a
10 kHz lowpass filter. When the frequency selected is below
1 kHz, a 1 kHz lowpass filter is switched in series with the
10 kHz filter. The signal is further ampljfied before being
fed to the Generate Audio Module.
Gates coupled to the frequency selection BCD signals (controlled
by the thumbwheels) provide DC control voltages to the Digital
Module - one when the selected frequency is below 1 kHz and
another below 100 Hz. A third control voltage goes to the
Receive Audio Module if the frequency is below 400 Hz. The
adder system also furnishes a reference signal (at the fre­
quency set on the thumbwheels), to.the Digital Module for audio
frequency comparison.
B. CRYSTAL OSCILLATOR CIRCUIT
Crystal oscillator Y3101 and associated timing capacitors pro­
duce a frequency of 2.048 MHz. NANO gates U3113A and U3113B
and binary counter U3102A divide this frequency to 256 kHz to
clock the latches in the adder system and the sine ROM latch
U3116. NANO gates U3113C and U3113D enable ROM U3114 only
during the required operating time to conserve power.
7-79