I had an 555 astable circuit breadboarded as shown, running at 300 kHz. My scope showed the amplitude of the output at pin 3 to be about 1V, while Vcc was 9V.

Is this expected due to the high frequency? Stray inductance?

Lee

 

What is the make and model of your scope?
What is the bandwidth of the scope?

 

It's a bit of a no-name chinese DSO, 25 MHz.

I just worked it out though.... the probe was set to 10x.... after I put it back on 1x it was all ok.

 

It is the preferred practice to operate your scope probe at the x10 setting.
Make sure your scope is also set for x10 setting.

 

As MrChips noted, the preferred probe setting is X10 to minimize the loading effect of the scope on the circuit, particularly at higher frequencies. Use X1 only if you need the added sensitivity at low signal levels and only for low frequency signals where the 50pF or more capacitance of the X1 probe setting will not adversely affect the signal.

 

Thanks for the advice.
I'd better delve into the manual further then.... the first thing the book said was - set the probe to 1x, then use the in-built square wave to check it.

 

Thanks for the advice.
I'd better delve into the manual further then.... the first thing the book said was - set the probe to 1x, then use the in-built square wave to check it.

That's fine for a preliminary check. You also want to set the probe to 10x and look at the square-wave. You then may need to adjust the probe compensation (usually a small adjustment screw, either on the probe body or the connector body) to get the best square-wave with no undershoot or overshoot on the leading edge.

 

1. Connect your scope probe to 9V. What does it read? (This will check the scaling)
2. At pin 3, do you see square waves? (This will check the bandwidth.)
If you read 9 volts correctly and see square wave the problem is not in your scope. Check your circuit.