Tenma 72-455 - Circuit Description; Summary; Wien Bridge Oscillator Circuit; Square Wave Shaping Circuit

CIRCUIT DESCRIPTION
1. Summary
When reading the following descriptions, refer to the
block diagram (Fig. 1) and the schematic diagram.
The sine-wave signal generated by the Wien bridge
oscillator circuit is fed through the WAVE FORM selector
switch set at the "~,, position to the OUTPUT control,
by means of which it is adjusted to any desired voltage.
If the WAVE FORM switch is in the" Jl.. "position,
the sine-wave signal is shaped into the square wave and the
voltage is also adjusted by the OUTPUT control.
The signal voltage thus adjusted is applied to the out-
put circuit, where its impedance is appropriately converted,
and then del ivered through an output attenuator to the
output terminal. The attenuator provides selectable
attenuations of OdS through -50dS in 10dS steps at 600n
of output impedance.
2. Wien Bridge Oscillator Circuit
The Wjen bridge oscillator circuit elements consist
of the resistance elements, which may be switched over
for 5 ranges by the FREO. RANGE switch, and the variable
capacitor controlled by the FREQUENCY dial.
These elements provide means to vary the oscillating
frequency continuously over 10 times its frequency on one
range, thus determining any desired frequency within
the entire frequency range from 10Hz to 1MHz.
The amplifier circuit for the oscillator circuit is
composed of a 2-stage differential amplifier and an output
stage, employing an DC amplifier circuit. The first stage
is a ·high input impedance circuit with FET while the driver
stage
is a wide band, high amplification type circuit with
-3-
PNP transistors featuring high cut-off frequency. The out·
put stage is a SEPP circuit uSing complementary transistors.
The output voltage is fed back with positive polarity
through the oscillator elements to form an oscillating
circuit, while it is also fed back with negative polarity
through the non-linear thermistor to stabilize the
amplitude.
3. Square Wave Shaping Circuit
The square wave shaping circuit is a Schmit-trigger
circuit in which the sine wave signal from the oscillator
circuit is shaped into a square wave. It is composed of an
emitter coupled Schmit-trigger circuit and a buffer
amplifier, thus providing sufficient rising and falling charac·
teristics.
4. Output Circuit
The output circuit converts the impedance of signal
from the OUTPUT control and feeds the signal to the
output attenuator at a low impedance. It is a SEPP-OCL
circuit employing complementary transistors to provide
sufficiently low output impedance characteristics over the
range from DC to lMHz.
5. Output Attentuator
The 6-position output attenuator selects attenuations
of OdS to -50dS in 10dS steps. At the OdS position with
the OUTPUT control turned fully clockwise, the output
voltage (sine wave at no-load time) is more than 7V rms.
The output impedance is rated for 600n and the
attenuation accuracy is as high as ±1.0dS at a 600n load.