Velleman Instruments PCS500 free download

File name PCS500_operation.pdf

PC-OSCILLOSCOPE PCS500 Analog and digital circuit sections Description of the operation Operation of the analog section This description concerns only channel 1 (CH1) input stages. The operation of CH2 is similar. The signal is applied to the input attenuator either via relay contact RY1 or via capacitor C4. The relay is used to select the type of input connection, AC or DC. If DC connection is selected the input signal is directly connected to the input attenuator. This position is probably the mainly used input connection type. Both DC and AC components of the signal can be measured and displayed on the screen. If AC coupling is selected the input signal passes through a 47 nF capacitor C4 to the input attenuator. The capacitor blocks DC component of the input signal to pass to the input circuit. Also AC signals at very low frequencies are blocked. The low frequency limit is about 5 Hz (at -3dB). The AC position is useful if low amplitude AC signals over a rather high DC voltage have to be measured. If, for example, an AC ripple voltage of a DC power supply must be measured. Next to the input coupling selector there is a frequency-compensated voltage divider (R6, R12, R13). To adjust the frequency response of the attenuator there are two trimmer capacitors CV5 and CV6. A square wave obtained from the test connector J9 (at the rear of the scope) can be used to help the adjustment of the trimmers. The attenuation ratios are 1:1, 1:10 and 1:100. The attenuator output is selected by the relays RY2, RY3 and RY4. Relay RY5 is used to connect the input of the following amplifier to ground. When input selector position "GND" is selected all of the relays RY2, RY3 and RY4 are open and the input of the amplifier is grounded via RY5. Overvoltage protection of the input amplifier is made with resistor R30 and the diodes D10 and D14. This circuit limits the maximum voltage at the gate of the FET T1A to the level of +/-5.5V. Fet T1B provides a constant current source for T1A. Trimmer RV3 is used to adjust the constant current to the level that the voltage across R4 is exactly same as the gate-source voltage of T1A plus the offset voltage of amplifier IC2A. When this adjustment is correctly done then the gain change of the amplifier IC2A doesn't cause any jump to it's output voltage if zero voltage is connected to the input connector of the scope. The gain of the amplifier IC2A is set by the relay RY6. The gains are x1 and x10. The gain of the second amplifier stage IC2B is set by the relay RY7. The gains of this stage are x0.833 and x 2.5. The Y-position of the trace is adjusted by feeding DC current from the collector of the transistor T8 to the inverting input of IC2B. The current is set by the output voltage OFFS_1 of the D/A converter IC26. Finally the signal is fed to the A/D converter IC16 via the emitter follower T10 and R115. Transistor T3 provides a constant current source for T10. The emitter follower transistor T10 has two functions. First, it sh