CALIBRATING A SCOPE I/P ATTENUATOR.

This assumes you are OK with working inside live scopes with HIGH VOLTAGES!

Connect the calibrator with a very short 75 ohm coax cable. With the scope at maximum Y1 gain setting, but without any added gain multiplier (as these reduce the bandwidth), select a suitable calibrator square wave signal level to give a 2-3cm display, using timebase controls to show 0.2cm/mS to give a large steady trace.

STEP 1. Adjust Y1 preset gain (not high gain X10) for correctly calibration height, adjust preamp C/Rs tweeks for best (ideal) square rising edge response. Check the X10 (or whatever) gain option is also accurate adjust that high gain preset, if there are separate C/Rs for X10 also adjust. N.B. some scopes have HF C/Rs that will not be adjustable with a 1KHz square wave but need faster 1MHz or RF frequency sweeps to set up.

STEP 2. Switch Y I/P attenuator to next position and up the calibrator level to suit, check defection calibration (do not adjust scope calibration unless you have suitable replacement scope attenuator Rs!), adjust the correct Y attenuator series C (Ctrim) for best (ideal) square rising edge response. There may be two Cs, a series one (Ctrim) and a parallel one (Cstray), this one seems to do nothing!

Repeat this for the first set (order) of input attenuators.

Both Attenuators banks are straight though wires for x1.

STEP3. Higher order attenuators (x10, x100, x1000) must "see the same I/P capacitance" of the first order attenuators so they can only be adjusted once the sensitive ranges have been done.

STEP4. Equalizing the scope I/P capacitance (Cstray) across the ranges is best done with a x10 scope probe. Set the Y attenuator gain attenuator to max sensitivity, set the scope probe to x10 and view a suitable sized square wave. Adjust the probe's trimmer for the best waveform. Now step up the scope Y attenuator over the ranges with suitable input levels, if the square wave shape changes, adjust the unused (did nothing before) parallel Y caps (Cstray) for the same shape. Remove the scope probe.

Now repeat step 3 and 4 until there is no tweeking needed. Go and adjust Y2!

STEP 5 TIMEBASE CALIBRATION
Make sure the Timebase velocity control is in its "cal" position. Find the LF timing preset and adjust (sometimes it is the X gain) so that the 1KHz waveform fits a 1mS/Dev range. Check on some of the other ranges for a mean calibration. For HF timebase calibration you will need an RF sig gen.

AC RMS SCALED and TRUE RMS METERS
Using a square wave to calibrate RMS meters is not as complex as it may seem. On AC most meters are MEAN reading, but calibrated in RMS for sine wave use. The sine wave mean is 0.636 of the peak, compared to the RMS value of 0.707 of the peak, so the readings are scaled higher. The error is exactly 1.11 which is called the "form factor".

So back to the square wave of 10V p-p (DC component removed with a series cap if needed) this is a 5V peak square wave, which actually has the same RMS value as 5V DC, and will read 5V on a true RMS meter. But a MEAN reading meter which is scaled x1.11 and will read 5.55V AC.

DC MEAN METERS
Same goes for square wave, 10V peak 50% of the time (what he square wave should be!) should read 5V. So you can determine if a meter is MEAN reading DC or not.

DC METER CHECKS
With the Calibrator on DC mode, any reasionably high impedance meter can be tested for basic accuracy and burnt out meter calibration Rs detected.

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