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G8MNY > TECH 24.08.03 20:07l 146 Lines 7711 Bytes #999 (0) @ WW
BID : 12100_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Scope & DMM Calibrator
Path: DB0FHN<DB0RGB<DB0FSG<DB0PV<DB0MRW<OK0PPL<DB0RES<ON0BEL<CT4DK<CT2GWY<
GB7YKS<GB7YFS<GB7CIP
Sent: 030824/1601Z @:GB7CIP.#32.GBR.EU #:12100 [Caterham] $:12100_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
This device lets you test the basic accuracy & frequency response of a scope's
I/P attenuator & allow accurate adjustment of the compensation trimmers.
The design gives better than 1% accurate DC or 1KHz square wave with better
than 200nS rise time, outputs from 10V p-p down to 1mV p-p in 10,5,2,1 steps.
It can also be used for checking DVMs or other high input Z meters.
CIRCUIT
Based on a simple PNP astable, reference voltage source, & O/P attenuator...
WAFER 2 WAFER 1
O/P-->o--->o------------.
10V ! 5K !
5V o o-!---f ! 10V
! 3K 1K CAL
2V o o-!---e ! PRESET OFF
! 1K ! o
1V o o-!---d ! DC MODE
! 500 ! __________________o SWITCH
500mV o o-!---c ! ! ! <------- ---- +12-18V
! 300 ! --- --- o \ /e @ 5mA
200mV o o-!---b ! D2 \ / D2 \ / ! SQUARE ---
! 100 ! --- --- ! ! TR1
100mV o o------a ! BC214 ! ! ! !
100K ! ! ----------(-------------------- !
50mV o o-f ! ! ! TR3 ! ! ! ! !
! 100 ! e\!__ !___!/e ! ! 1M
20mV o o-e ! !___/! ! ! !\ TR2 ! ! !
! ! ! ! ! ! ! ! !
10mV o o-d ! --- ! ! ! 100u!+ ! !
! ! D3 \ / !----)-4n7-! === ! !
5mV o o-c ! --- ! ! ! ! ! !
! ! !--4n7--)----! ! ! !+ !
2mV o o-b ! ! 150K 150K ! ! ---\ !
! ! ! !____! ! ! / \ !
1mV o o-a ! ! ! ! ! --- Z1 !
1K Ct ! 10K 5K 4K7 ! ! 11v !
GND ! ! ! !PRESET ! ! ! ! -VE
----------------------------------------------------------------------------
ATTENUATOR ASTABLE REF CURRENT LIMIT
POWER
TR1 (BC214) provides a limited current source (Approx 5mA) controlled by the 1M
base resistor to put current into Z1 the 11V Zener & the rest of the circuit
via the mode switch. Although this current source varies a little with the
supply it is much better than just a resistor, & hence the zener reference
voltage (eg 10.8V +/-10mV) is kept fairly accurate!
If the +10V range is not wanted the circuit can be worked with a 5.6V zener, a
PP3 battery & suitable attenuator changes!
When the DC mode is selected D1 (1N4148) powers the circuit, while D2 inhibits
TR2 from conducting & hence T3 must conduct in the normal way providing an
identical output voltage to that when the circuit oscillates.
ASTABLE
The TR2 & TR3 (BC214) make up the normal if up side down (PNP) astable circuit.
Diode D3 isolates the charging up of T2 base capacitor from the T3 output, so
keeping the edge turn on very fast, rise time better than 200nS.
The frequency is determined by both sets of C & R (4n7 & 150K) time constants
so the Cs should be polyester type for best thermal stability. Any in-balance
of the square wave (not 1:1 ratio on a scope) can be adjusted by trimming
either of the 2 caps or 150Ks with paralleled components. Using a common 5K
preset to trim the joint bias will allow the frequency to be trimmed to exactly
1KHz on a frequency counter.
10V CALIBRATION
This is done using a accurate DVM ( >1M I/P Z). Set the mode switch to DC & set
the O/P to switch to 10V, adjust the 1K Cal pot for 10.000V. Check that this DC
is the same value as the 1KHz square wave rises to on an oscilloscope.
ATTENUATOR
The attenuator resistance values are made up so that 10mV/1K ohm gives easy
numbers to work with. But the 5K=10K//10K, 3K=3.3K//33K etc need 2 Rs to make
the value. All the Rs are soldered around the switch. I used a screened 2 bank
13 step switch, other switches will do with the Rs made up the values to suit.
The 6 positions out of the 1st 7 positions on the 1st wafer bank are paralleled
up to the last 6 positions, which sees the 2nd wafer bank switch in a 100:1
attenuator, which uses high enough values not to significantly load the 1st
attenuator values. Open wire low capacity wiring is better for all the wiring
in this area, NO "neatly tied up wiring forms"!
For accurate work (1% or better) the R values can be trimmed with much higher
value Rs to ground or 10V using an accurate DVM as for the initial 10V
calibration.
If the 5V O/P impedance is too high (2.5K=5K/5K) for some loads, put a 5K6 from
the 5V point to ground & change the 10K//10K for 2K7//68K, the 68K value needs
to be selected on test to give 5V once the 10V CAL is setup again.
For a good square wave on the low signals end of the attenuator, a screening
plate is required between the dual bank switch, & possibly a trimming "Ct" to
ground across the 1K to over come crosstalk from the 10V to 1mV circuit.
CALIBRATING A SCOPE I/P ATTENUATOR.
Connect the calibrator with a very short 75ohm coax cable. With the scope at
max Y1 gain setting, but without any added gain multiplier (as these reduce
bandwidth), select a suitable square wave signal level to give a 2-3cm display
using timebase to show 0.2cm/mS to give a large steady trace.
STEP 1. Adjust Y1 preset X1 gain for correctly calibration, adjust preamp C/R
for best (ideal) square rising edge response. Check the X10 (or whatever) gain
option is accurate, if there are separate C/Rs for X10 gain 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 I/P attenuator to next position & up the I/P 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 for best (ideal) square rising edge response, there may be 2 Cs a
parallel one as well that seems to do nothing! Repeat this for the first set
(order) of input attenuators.
STEP3. Higher order attenuators "see the 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 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 square wave. Adjust the probe 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 for the same shape. Remove the scope probe.
Now repeat step 3 & 4 until there is no tweeking needed. Go & adjust Y2!.
If the timebase is uncalibrated find the LF timing preset & adjust (sometimes
it is the X gain).
AC 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 peak, compared to the RMS value of 0.707 of peak,
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) this is a 5V peak
square wave which actually has the same RMS value as a 5V. But scaled by 1.11
will read 5.55V AC.
/QSL
73 De G8MNY @ GB7CIP
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