Fuzzy Oscilloscope reading

[Steve Benz]

I've recently renewed my hobby of building electronic stuff, and I talked myself into buying an old oscilloscope, a Tektronics 453.  For somebody with my skill level, it's more than good enough for me.  Still, I've been putting it through its paces and I noticed something, and I wonder if any old hands will know what to make of it.

I get crisp readings when I set the scope to >20mv/div, but when I get lower than that, the scope loses resolution - that is, instead of a clean line, I get a fuzzy blur, more or less centered where the real reading should be, that grows proportionally with the volts/div setting.  That is, at 5mv/div, the cloud of readings is a full div.  At 10mv/div, it's a half a div, and at 20mv/div, it's a quarter (meaning that it's pretty much accurate enough at that level).  Focus is set correctly - e.g. if I set the oscilloscope to GND, the line is crisp.

If I clip the probe and ground together, I get a clean line.  If I simply clip a 1K resistor between the probe and its ground, I get a clean line.  If I clip a 25K resistor, the fuzz starts to come back and at 1Meg, it's fully there, so I don't believe it's noise in my power supply that's doing this.

I haven't come across this in reading the manual (that I got from the Boat Anchor Manual Archive).  There's talk about "Astigmatism Adjustment", but from what I read elsewhere, that doesn't seem like what's going on here.

Honestly, I won't cry if I just have to accept this as a limitation of the equipment.  What do you want for a $60 'scope?  Almost everything I do is such that 20mv/div is more than enough anyway.  But if it's a fixable thing, I'd be glad to know it.

Thanks!

[Skimask]

http://www.reprise.com/host/scopes/

[tautech]

Things that come to mind:
Electrically noisy environment and the scope is picking this up at it's more sensitive settings. 
Dicky probe/s 

However without connection to an actual DUT the probes act as antennae picking up all sorts if EMI rubbish from your surrounds.

More use will determine just what the problem is.
Keep us posted.

[Andy Watson]

Where is the variable gain set? The manual indicates that the variable gain can multiply the signal by 2.5 - so on the 5mV setting you could be looking at the equivalent of 2mV per division, i.e. you have about 2mV peak-peak noise. This is probably just Johnson noise - there's nothing you can do - it's an artefact of measuring any signal that has a resistive source. The fact that it reduces when you lower the resistance at the input confirms this.

Try putting your resistance, bandwidth and room temperature into the following formula:
$$ Vn_{rms} = \sqrt{ 4 \times k \times T \times B \times R} $$

This will give you the expected R.M.S. noise.voltage for a resistor.
\$k\$ is  Boltzmann's constant \$1.38 \times 10^{-23} \$
\$T\$ is temperature in Kelvin
\$B\$ is the bandwidth (50,000,000 ?)
\$R\$ is the resistance of at the input.

Note that that formula gives the RMS noise and only accounts for the resistive noise source.

[voltz]

Its sounds likes noise being picked up by the probe. Short out the probe and try your tests again. If its flat, then there is no fault and the scope is working correctly.

With the probe not shorted out, ie. high impedance, you are seeing RF pickup as the probe is acting as an antenna. Shorting it makes it low impedance and should stop the pickup. And thats with any scope.

[Andy Watson]

External noise, i.e. picked-up by the probe is likely to be mains and should correlate with the displayed signal if the trigger is set to "line".

[Steve Benz]

Thanks all for the advice.  Sounds like there's solid consensus on the probe being the likely problem.  I wonder if that hunk of it that looks like it's been bitten by a cat and then taped up has anything to do with it...  Idunno, call it a working theory 

Still, it's working well for me.  Maybe someday I'll come across a nice probe on the cheap.

[tautech]

Do yourself a favour and get a pair of these:
http://www.ebay.com/itm/Two-P6100-DC-100MHZ-Oscilloscope-Scope-Clip-Probe-100MHz-For-Tektronix-HP-2Y-/281785013454?hash=item419bb214ce:g:nqEAAOSwLVZV4E5c

[Andy Watson]

Sounds like there's solid consensus ...

Oh!