25 October 2002

Build an oscilloscope amplifier linearity tester

With this oscilloscope tester you can test your amplifiers on your scope

By Craig Kendrick Sellen
Birch Hills Residence
25 Reservoir Street
Simpson, PA 18407-1300

The circuit shown in Fig. 1 was designed to test any oscilloscope to see if its amplifiers are linear in operation. This is accomplished by producing an ever-expanding circle on the oscilloscope screen.

In the design, a function generator that supplys a saw-tooth waveform. This saw-tooth or ramp voltage is applied to the sweep input potentiometer R1, which is connected through resistor R8. The CA3080E integrated circuit IC1 serves as a voltage-controlled amplifier that slowly ramps up the signwave obtained from the AC line is applied to the integrated circuit through capacitor C4. The level of the signwave voltage, or circuit size, is controlled by potentiometer R9. The output of IC1 is applied to the resistor and capacitor bridge network made up of R15, R16, C9, and C10. This phaseshift network shifts apart the output phase by 90°. This 90° phaseshift is buffered by the LF353 dual op-amps, IC3-a and IC3-b, and is applied to the vertical and horizontal inputs of the oscilloscope under test.

If the amplifiers of the scope under test are working properity, and they are linear, a circle will appear on the screen--starting small and expanding ever larger to the outermost edges of the screen.

The 741 single op-amplifier, IC2, functions as a voltage-source reference for IC1 and IC3.

There is one adjustment to be made on the tester. Place a voltmeter across resistor R11 and temporarily disconnectcapacitor C4 from resistor R9 (this connection point is shown as an "X" in the schematic).Now adjust potentiometer R6 (the "DC offset control") so there is 0 volts DC across resistor R11on the voltmeter. Now reconnect C4 to R9 and place it back in the circuit. You should get a sinewave voltage across resistor R11 in proportion to the setting of the meter.

Please note that for the bridge network to work property it is important to match capacitors C9 and C10 and resistor R15 with R16 (the value of 121.2K ohms needed can be obtained by series connecting precision 120K-and 1.2K-ohm resistors together). Also note that capacitors C11 and C12 should be good Mylar or polystyrene type capacitors.

 

 

Parts List

RESISTORS ARE 1/4 Watt 5% unless otherwize noted
R1-500K linear taper potentiometer front panel mount
R2, R11, R13, R14, R26, R27-10,000 ohms
R3, R4, R5-N.C.
R6-100,000-ohms printed circuit board trimmer
R7-47,000 ohm
R8-100,000 ohms
R9-500-ohm linear taper potentiometer front panel mount
R10, R12-4,700 ohms
R15, R16-121.200 ohms (see text) 121.2K
R17, R18, R19, R121-1 megohm
R20, R22-10 megohm
R23-750-ohms
R24-1,000 ohms

CAPACITORS ARE 25 working volts or better 10% unless otherwize noted
C1,C2,C3-N.C.
C4-0.22uF Mylar
C5-500uF electroletic
C6-100uF electroletic
C7, C8-10uF electroletic
C9, C10-0.022 uF Mylar or Polystyrine
C11, C12-0.01uF Mylar or Polystyrine
C13, C14-0.1uF Mylar

INTEGRATED CIRCUITS
IC1-LM3080N chip or CA3080E chip
IC2-LM741P chip
IC3-LF353N chip
U1-78L06 voltage reg.
D1, D2-1N4001 diode
LED-LED red