11-07-2011, 10:26 PM
Interesting thread - you can check the Y channel of your scope using a variable DC supply monitored by a DMM, the latter providing more resolution than the cursor of the oscilloscope. With a known DC input you can check the input attenuators of your scopes Y inputs. Square wave testing is used to verify that the scope is free from ringing or overshoot but the input needs to have very fast edges. Next you would use a signal generator with a levelled output so that the frequency response of the Y amps can be verified but even if you do not have the more sophisticated equipment, at least the simple DC supply check will check out your scopes attenuators. If you have a 50 Ohm option then don't have this switched in at this time as you could burn out the termination with a high DC input. All we did in the cal lab was to measure the input of the Y amps when set to 50 Ohms using a DMM set to Ohms to verify that the resistance was OK. The Timebase is a different story of course and we used to use a comb generator. David, I have often toyed with the idea of using a counter edge triggering a monostable. The monostable being set to provide comb pulses varying in width as required depending on the rate the counter is being clocked. I think you can catch my drift on this. I remember spending some considerable time adjusting the trimmers and even altering the leads to the scope X and Y plates to obtain the best responses with regard to rise times and over shoot. I don't recommend anyone trying this unless they have a really fast scope and besides, for general radio work, it probably is not worth pursuing. Even checking Trigger levels can be achieved with just a simple DC power supply, bring it up from zero, either positive or negative, watching for the scope to trigger and noting the voltage indicated on the monitoring DMM when it occurs. Of course we did lots of other things like using a phase matched 'T' piece cable so that phase errors between the two Y channels could be measured but before anything was done....All of the power rails were checked for the right levels and freedom of excessive ripple. The last thing you want to do, on any equipment really, is struggle to get it working if only reasonably, then find the problem was due to a supply rail being out of spec. In some of the HP and TEK equipments, the first task was to tighten the screws on the smoothing capacitors as they sometimes worked loose and caught out the engineer trying to calibrate the item. A good Motto: Always check out the simple things first before diving in deep.
