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G8MNY > TECH 14.04.04 08:30l 107 Lines 5547 Bytes #999 (0) @ WW
BID : 52739_GB7CIP
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Subj: Scope & DMM Calibrator 1/2
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Sent: 040413/2256Z @:GB7CIP.#32.GBR.EU #:52739 [Caterham] $:52739_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (new Grpahics Jun 03)
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 accurate DC or 1KHz square wave voltages 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 other 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/PÄÄ>oÄÄÄÄ>oÄÂÄÄÄÄÄÄÄÄÄÄ¿
10V ³ | 5K ³
5V o | oÄÅÄÄÄf ³ OFF
³ | 3K ³ 10V o
2V o | oÄÅÄÄÄe 1K CAL DC MODE
³ | 1K ³ POT ÚÄÄÄÄÄÂÄÄÄÄÄÄÄo SWITCH
1V o | oÄÅÄÄÄd ³ _³_ _³_ <ÄÄÄÄÄÄÄÄÄ. .ÄÄÄ +12Ä18V
³ | 500ê ³ D2 \ / \ /D1 Úo \ /e @ 5mA
500mV o | oÄÅÄÄÄc ³ ÄÂÄ ÄÂÄ ³ SQUARE ÄÂÄ
³ | 300ê ³ ³ ³ ³ WAVE ³ TR1
200mV o | oÄÅÄÄÄb ³ BC214 ³ ³ ³ ³ PNP
³ | 100ê ³ ÚÄÄÄÄÄÄÄÄÄ(ÄÄÄÄÄÁÄÂÄÄÄÄÁÄÄÂÄÄÄÄ¿ ³
100mV o | oÄÁÄÄÂÄa ³ ³ ³ ³ ³ ³ ³
100K | ³ ³ e\³TR3 ³ ³/e ³ ³ ³
50mV o | oÄf ³ ³ PNPÃÄÄ¿ ÃÄÄÄÄÄ´ PNP ³ ³ 1M
³ | 100ê ³ /³ ³ ³ ³\ TR2 ³ ³ ³
20mV o | oÄe ³ ÃÄÄÙ ³ ³ ³ ³ ³ ³
³ | ³ _³_ ³ ³ ³ 100u³+ ³ ³
10mV o | oÄd ³ D3 \ / ÃÄÄÄÄ)ÄÄ´ÃÄÄÄ´ === ³ ³
³ | ³ ÄÂÄ ³ ³ 4n7 ³ ³ ³ ³
5mV o | oÄc ³ ÃÄÄ´ÃÄÄÄ)ÄÄÄÄ´ ³ ³ ³ ³
³ | ³ ³ 4n7 ³ ³ ³ ³ _³_+ ³
2mV o | oÄb ³ ³ ³ ³ ³ ³ / \' ³
³ | ³ 10K 150K 150K ³ ³ ÄÂÄ Z1 ³
1mV o | oÄa ³ ³ ÀÄÂÄÄÙ 4K7 ³ ³ 11v ³
ÚÁ¿ | ³ ³ ³ ³ ³ ³ ³
1K Ct| ³ ³ 5K Preset ³ ³ ³ ³ Äve
GNDÄÄÁÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ
ATTENUATOR ASTABLE REF CURRENT LIMIT
POWER
TR1 (BC214) provides a limited current source (Approx 5mA) controlled by the
1Mê base resistor, to put current into an 11V Zener Z1 & the rest of the
circuit. Although this current source varies a little with the supply &
temperature it is much better than just a resistor, & hence the zener reference
voltage (eg 10.8V ñ10mV) 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. D1 voltage drop
should be of no consequence due to the constant current from TR1.
ASTABLE
The TR2 & TR3 (BC214) make up the normal if up side down 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 4n7 caps or 150Ks with paralleled components. Using a common 5Kê
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 exact value from the E12 resistor series. 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 circuits.
Part 2/2 is how to use it.
/QSL
73 De G8MNY @ GB7CIP
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