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FUNCTIONAL DESCRIPTION
The FG-500 is a function generator integrated circuit capable of producing high quality sine, triangle, and square
waves of high stability and accuracy. A picture of each waveform is shown below:
Sine Wave
THEORY OF OPERATION
The heart of the FG-500 Function Generator is the
XR-2206 monolithic function generator integrated
circuit. The XR-2206 is comprised of four main
functional blocks as shown in the functional block
diagram (Figure 1). They are:
• A Voltage Controlled Oscillator (VCO)
• An Analog Multiplier and Sine-shaper
• Unity Gain Buffer Amplifier
• A set of current switches
The VCO actually produces an output frequency
proportional to an input current, which is produced by
a resistor from the timing terminals to ground. The
current switches route one of the currents to the VCO
to produce an output frequency. Which timing pin
current is used, is controlled by the FSK input (pin 9).
In the FG-500, the FSK input is left open, thus only
the resistor on pin 7 is used. The frequency is
determined by this formula:
f
= 1/RC Hz
o
where f
is the frequency in Hertz
o
R is the resistance at pin 7 in Ohms
C is the capacitance across pin 5 and 6 in Farads
Note that frequency is inversely proportional to the
value of RC. That is, the higher the value of RC, the
smaller the frequency.
The resistance between pins 13 and 14 determine the
shape of the output wave on pin 2. No resistor
produces a triangle wave. A 200Ω resistor produces a
sine wave.
Triangle Wave
FUNCTIONAL BLOCK DIAGRAM
1
AM Input
Sine/Saw
2
Output
3
Mult. Out
4
V+
5
Timing
Capacitor
6
7
Timing
Resistor
8
-11-
Square Wave
+1
Multiplier
and
Sine
Shaper
VCO
Current
Switches
Figure 1
16
Symmetry
Adjust
15
14
Waveform
Adjust
13
12
Ground
Sync
11
Output
10
Bypass
FKS
9
Input
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