4/8-line scan converter to expand oscilloscope capabilities

4/8-line scan converter to expand oscilloscope capabilities

Function of the circuit

In logic circuits, it is sometimes necessary to observe multiple signals at the same time. It is certainly convenient to use a logic display, but this circuit uses an ordinary oscilloscope to scan 4 or 8 signals. It is a conversion circuit that displays logic levels. In order to make the waveform still on the CRT display, a synchronization signal is required, and any signal from 1 to 8 can be used for synchronization.

How the Circuit Works

The switch SW1 at the input end is a frequency selection switch. Any channel can be connected as the synchronous input of the oscilloscope, such as connecting CH-2 as the synchronization signal. Resistors R1~R5 are diode current limiting protection resistors. If the resistance value is too large, time delay will occur due to the input capacitance of the IC. R9~R16 are pull-down resistors that make the input end "L" level when the input is open.

IC1 is an 8-channel digital data selector. It selects the control data of the channel from A, B, and C inputs, and can select 1 to 8 signals in turn and output them through YO. In addition, an oscilloscope is used to scan 4 or 8 signals, so an R-2R 3-bit DA converter is added to the output end of the selector, so that each channel has an average 0.1V step voltage superimposed on the YO output signal.

When YO is 5V, the display deflection voltage is more suitable to be 50MA, so R19 and VR1 are used to attenuate the YO output. The maximum open circuit voltage of the 3-bit DA converter is 5*(7/8)=4.375V. Because of the load R27, the voltage must drop.

The counter uses 3-bit binary, and here the A~C outputs of IC, 40H161P are used to drive the digital data selector and as the input signal of the DA converter.

The clock oscillation frequency is 300KHZ. If it is higher than this frequency, the display screen will be bright and difficult to see because of the channel scanning shift. The oscillation frequency accuracy or stability is allowed to fluctuate, so a C-MOS multivibrator circuit is used.

The power supply is 5V, 2.5MA, connected with a wire clip so that it can be supplied by the printed circuit power supply being measured.

Component Selection

The IC uses Toshiba's high-speed C-MOS40H series products, and 74HC series products can also be used, but a high-speed acceleration capacitor C2 may be required.

Resistors R10~R27 depend on the voltage accuracy, and usually metal film resistors with an accuracy of ±1% can be used.

Adjustment

First, open the input terminal and use an oscilloscope to observe the MUX output signal. Set the voltage sensitivity of the oscilloscope to 0.1V/DIV, input a maximum step voltage of 0.7V, and if there are 7 levels, it is normal. If you cannot accurately move 7DIV, you can adjust R27. Then input appropriate pulses from CH-8, synchronize with CH-8, scan 4 or 8 channels of signals, and in order to prevent the scanning of CH-7 and CH-8 from being too close, VR1 should be adjusted so that there is 0.5DIV between the two.