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Integrated Circuits and Applications
Voltage Controlled Oscillator
In RC oscillators frequency depends on RC
time constant
In some applications like FM, tone generators, FSK etc. frequency needs to
be controlled by input voltage known as controlled voltage
This is achieved by VCO also known as V to F convertor
eg. NE/SE 566 VCO which provides simultaneous Square and Triangular
wave outputs as a function of input voltage
Voltage Controlled Oscillator
Voltage Controlled Oscillator
VCO contd.
VCO contd.
VCO contd.
R1 and C1 decides the frequency of oscillation and voltage Vc applied to the control terminal 5
The triangular wave is generated by alternately charging C1 by one current source and then linearly discharging by another
The charge- discharge levels are determine by Schmitt Trigger action which also provides square wave o/p
Both o/p waveforms are buffered to improve the o/p impedance
VCO contd.
In typical connection diagram, R1C1 combines the free running frequency and controlled voltage Vc by voltage divider formed with R2 and R3
The initial voltage at terminal 5 must be in the range-
¾(+Vc) ≤ Vc ≤ +V
where +V is the total supply voltage
The modulating signal is AC coupled with C and must be-
< 3Vp-p
the frequency of o/p is approximated by
fo ≈ 2(+V - Vc) / R1C1(+V)
A small capacitor of 0.001µF should be connected between pin 5 and 6 to
eliminate possible oscillations in the controlled current source
Applications of VCO
Used to convert low frequency signals such as electrocardiograms (EKG)
into an audio frequency range
- these audio signals can then be transmitted over telephone lines or
two way communication for diagnostic purpose
- or can be recorded on a magnetic tape for further reference
V to F converters
Teledyne 9400 series can be used as voltage to frequency or frequency to
voltage converter similar to that of SE/NE 566 VCO
These converters have similar internal circuitry but differ slightly in electrical
characteristics
The complete V/F or F/V system can be formed by using two external
capacitors, three resistors and a reference voltage
In addition 9400 series consist of CMOS and bipolar devices that can operate
on dual or single supply voltage
9400 is designed for pulse and square wave o/ps for frequency range of 1Hz
to 100KHz
The i/p can be either current or voltage and the o/p can be interface with most
forms of logic families
It consist of integrator, comparators, delay N/Ws, divide by 2 N/Ws and open
collector o/p transistor
V to F converters contd.
V to F converters contd.
V to F converters contd.
The input current Iin = Vin/Rin is converted to to a charge by the integrating
capacitor Cint and shows up a linearly decreasing voltage Va at the o/p of
OPAMP integrator
VA = -(Iin/Cint)t
The o/p VA of integrator is sensed by comparator
The o/p of comparator is then applied via 3-µS delay N/W
F to V converters
9400 generates an o/p voltage that is linearly proportional to the i/p frequency
waveforms when it is used as F/V converter
The features of 9400 as F/V converter include-
DC to 100kHz operation
Programmable scale factor
High i/p impedance
Capable to accept any voltage wave shape
F to V converters contd.
F to V converters contd.
F to V converters contd.
The i/p frequency is applied to the +ve i/p of the comparator(pin 11)
The i/p signal amplitude must be greater than ± 200mV as the comparator hysteresis voltage is ± 200mV
Each time i/p signal crosses zero in the negative direction, the o/p of comparator goes low
Three microseconds later the Cref charge/discharge control circuit is enabled which suddenly connects the reference capacitor Cref to the reference voltage Vref.
This action charges Cint. Each time with a precise amount of voltage
The charging path is through the o/p terminal of OPAMP, through Cint,, through Cref., and finally through Vref.
Similarly when i/p waveform crosses zero in the positive direction, the o/p of comparator switches high
This disables the Cref. Charge/discharge control circuit and Cref. Is shorted out
Voltage across Cint. Is retained because the only discharge path for Cint. Is through Rint. Which is very large and thus the voltage across Cint. Is the o/p voltage
Analog to Digital and Digital to Analog Converter
Digital systems such as microcomputers use a binary system of one and zeros
The data to be put into the microcomputer have to be converted from analog form
to digital form
The circuit that performs this conversion is called an analog-to-digital converter
While digital-to-analog converter is used when a binary o/p from a digital
system is to be converted to some equivalent analog voltage or current
D /A converter is presented first because it is simpler than A/D and it can be used
to form A/D converter
Digital to Analog Converter
Two types of D/A converters as-
D/A converter with binary weighted resistors
D/A converter with R and 2R resistors