Oscilloscope help with probe

[gjpollitt]

I have a crappy scope I borrowed from work

When measuring a crystal rated 2.368 Mhz or something like that I measured 86Hz.

Obviously this is incorrect as the circuit works fine so I took the scope home and tried to measure frequency of basic astable 555

Problem is I cannot measure this, even when setting the coupling to dc and i measure the battery voltage to the circuit (9v) I read around .2vdc.

Is the scope buggered or am I doing something wrong? I have 3 different probes and they all do the same. The time and voltage settings are set correctly too. Someone mentioned to me about calibrating the probe. What does this mean?
The scope has a .5v cal terminal on the front but this only measures .250mv, strange.

Thanks

 

[bmcculla]

A scope probe has a 1MOhm resitor to ground. Its quite possible that adding the scope probes resistance to the oscillator circuit would screw up the oscillation. Try looking at a buffered output from the microcontroller that the crystal is attached to.


Scopes usualy have a setting for 10X and 1 X probes which might be part of the problem if the probe and setting don't match.

The probe calibrations only affects AC signals so thats probably not the probelm (it's a good idea to calibrate your probes though)

Hope this helps
Brent