Oscilloscope Tester

This device lets you test the basic accuracy and frequency response of a scope's I/P attenuator and allow accurate adjustment of the compensation trimmers.

The design gives accurate DC or 1KHz square wave voltages with better than 200nS rise time, outputs from 10V p-p down to 1mV p-p in 10,5,2,1 steps. It can also be used for checking other DVMs or other high input Z meters.

CIRCUIT
Based on a simple PNP astable, reference voltage source, and O/P attenuator...

POWER

TR1 (BC214) provides a limited current source (Approx 5mA) controlled by the 1M ohm base resistor, to put current into an 11V Zener Z1 and the rest of the circuit. Although this current source varies a little with the supply and temperature it is much better than just a resistor, and hence the zener reference voltage (eg 10.8V + or - 10mV) is kept fairly accurate!

If the +10V range is not wanted the circuit can be worked with a 5.6V zener, a PP3 battery and suitable attenuator changes!

When the DC mode is selected D1 (1N4148) powers the circuit, while D2 inhibits TR2 from conducting and hence T3 must conduct in the normal way providing an identical output voltage to that when the circuit oscillates. D1 voltage drop should be of no consiquence due to the constant current current from TR1.

ASTABLE

The TR2 and TR3 (BC214) make up the normal if up side down astable circuit. Diode D3 isolates the charging up of T2 base capacitor from the T3 output, so keeping the edge turn on very fast, rise time better than 200nS.

The frequency is determined by both sets of C and R (4n7 and 150K) time constants so the Cs should be polyester type for best thermal stability. Any in-balance of the square wave (not 1:1 ratio on a scope) can be adjusted by trimming either of the 4n7 caps or 150Ks with paralleled components. Using a common 5K ohm preset to trim the joint bias will allow the frequency to be trimmed to exactly 1KHz on a frequency counter.

10V CALIBRATION

This is done using a accurate DVM ( >1M I/P Z). Set the mode switch to DC and set the O/P to switch to 10V, adjust the 1K Cal pot for 10.000V. Check that this DC is the same value as the 1KHz square wave rises to on an oscilloscope.

ATTENUATOR

The attenuator resistance values are made up so that 10mV/1K ohm gives easy numbers to work with. But the 5K=10K//10K, 3K=3.3K//33K etc. need 2 Rs to make the exact value from the E12 resistor series. All the Rs are soldered around the switch. I used a screened 2 bank 13 step switch, other switches will do with the Rs made up the values to suit.

The 6 positions out of the 1st 7 positions on the 1st wafer bank are paralleled up to the last 6 positions, which sees the 2nd wafer bank switch in a 100:1 attenuator, which uses high enough values not to significantly load the 1st attenuator values. Open wire low capacity wiring is better for all the wiring in this area, NO "neatly tied up wiring forms"!

For accurate work (1% or better) the R values can be trimmed with much higher value Rs to ground or 10V using an accurate DVM as for the initial 10V calibration.

If the 5V O/P impedance is too high (2.5K=5K/5K) for some loads, put a 5K6 from the 5V point to ground and change the 10K//10K for 2K7//68K, the 68K value needs to be selected on test to give 5V once the 10V CAL is setup again.

For a good square wave on the low signals end of the attenuator, a screening plate is required between the dual bank switch, and possibly a trimming "Ct" to ground across the 1K to over come crosstalk from the 10V to 1mV circuits.

Part 2 - How to use it - Click here

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