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G8MNY > TECH 15.04.04 21:10l 114 Lines 5666 Bytes #999 (0) @ WW
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Subj: Scope & DMM Calibrator 2/2
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Sent: 040414/2205Z @:GB7CIP.#32.GBR.EU #:52906 [Caterham] $:52906_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (new Graphics Oct 03)
Part 2/2 is about using the calibrator to Scopes & meter tests.
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 75ê 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
2Ä3cm 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.
TO LITTLE HF CORRECT TO MUCH HF
____ _ _____ _ ³\____ _
/ ³ / ³ ³ ³ ³ ³ ³
³ ³ ³ ³ ³ ³ ³ ³ ³
³ \____³ ³ ³_____³ ³ ³ ____³
³/
STEP 2. Switch Y I/P attenuator to next position & 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) & a parallel one (Cstray), this one
seems to do nothing!
Repeat this for the first set (order) of input attenuators.
Typical x10 x100 x1000 Typical x2 x3 or x5
SCOPE Attenuator Attenuator
INPUT o)ÄÂÄ´ÃÄÄÂÄÄÄ>oÄÂÄÄÄÄÄ´ÃÄÄÄÄÄÂÄÄo<ÄÄÄ>oÄÂÄÄÄÄÄ´ÃÄÄÄÄÄÂÄÄ>oÄÄÄ AMP INPUT
BNC ³ AC ³ SW ³ Ctrim ³ SW SW ³ Ctrim ³ SW Input Z
Input Z ÀÄo \ÄÙ ÃÄÄRseriesÄÄÄ´ ÀÄÄRseriesÄÄÄ´ 1Mê //
1Mê // DC ³ ³ ³ 30pF
30pF ===Cstray Rshunt Rshunt
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄ
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. Equalising 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 & 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 & 4 until there is no tweaking needed. Go & adjust Y2!
STEP 5 TIMEBASE CALIBRATION
Make sure the Timebase velocity control is in its "Cal" position. Find the LF
timing preset & 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 & 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".
100%_³ __ __PEAK 100%_³ __________ __RMS = 5V
70.7%_³ /' '\ __RMS +5V ³³ ³
63.6%-³ ³ ³ --MEAN ³³ ³ 1:1
³ ³ ³ ³³ ³ SQUARE
³³ ³ ³³ ³
0% -³³- - - - - ³- - - - - ³ 0% -³³ - - - - -³ - - - - -³
³ ³ ³ ³ ³ ³
³ ³ ³ ³ ³ ³
³ ³ ³ ³ AC ³ ³
³ \.__./ -5V_³ COUPLED ³__________³
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, & will read 5V on a true RMS meter. But a MEAN reading meter which is
scaled x1.11 & will read 5.55V AC.
DC MEAN METERS
The same goes for a square wave, 10V peak 50% of the time (what the square wave
should be!) should read 5V, but 7V RMS. So you can determine if a meter is MEAN
reading DC or not.
100%_³ __________ __PEAK = 10V
³³ ³
³³ ³ 1:1 --RMS = 7.07V
³³ ³ SQUARE
³³ ³
50% -³³ - - - - -³ - - - - -³ --MEAN 5V
³ 50% ON ³ 50% OFF ³
³ ³ ³
³ DC ³ ³
0% _³ COUPLED ³__________³ __0V
DC METER CHECKS
With the Calibrator on DC mode, any reasonably high impedance meter can be
tested for basic accuracy & damaged or burnt out meter calibration Rs detected.
/QSL
73 De G8MNY @ GB7CIP
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