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SUMMER CIRCUITS
COLLECTION
032
Economical Timebase Calibrator
pulse generator with accurate
time intervals between pulses
i.e. the pulse repetition fre-
quency. If the pulse width is
made relatively small compared
to the repetition rate and the
pulse edges are steep then the
output signal will look like a
series of illuminated dots. These
can be conveniently used to
measure time periods on the
screen just as you use the grad-
uation marks on a ruler to mea-
sure length.
The circuit diagram shown in
Figure 1
uses five commonly
available ICs (excluding the
power supply). A 1 MHz crystal provides an accurate time
base for the oscillator circuit built around IC1A. Resistor R3
governs the switching threshold while trimmer C1 alters
the loading on the crystal and allows its frequency to be
F. Hueber
An external timebase calibrator is a useful accessory for
any oscilloscope it provides precise, visible time markers
on the scopes horizontal sweep. Basically the circuit is a
IC6
1
R3
10M
+5V
D1
7805
IC1.A
&
IC1.B
&
K2
1N4001
1
2
4
5
3
6
14
16
16
16
14
C10
C12
C18
C11
C17
C13
C14
C15
C16
R2
IC1
IC2
IC3
IC4
IC5
220µ
25V
100n
100n
47µ
16V
100n
100n
100n
100n
100n
7
8
8
8
7
X1
R1
C1
C2
+5V
1MHz
IC1 = 74HC00
IC2 = 74HC390
IC3 = 74HC390
IC4 = 74HC390
100p
33p
R5
C3
10p
IC2.A
IC3.A
IC4.A
C4
CTR
CTR
CTR
33p
6
2
2
2
CT=0
CT=0
CT=0
5
C5
S1.B
100p
4
3
1
3
1
3
1
3
DIV2
DIV2
DIV2
+
+
+
C6
5
5
5
0
0
0
1n
2
DIV5
DIV5
DIV5
4
6
4
6
4
6
+
CT
+
CT
+
CT
IC1.C
&
C7
1
7
7
7
9
10
2
2
2
10n
8
C8
IC2.B
IC3.B
IC4.B
22n
CTR
CTR
CTR
14
14
14
CT=0
CT=0
CT=0
IC5
11
10
CX
RCX
15
13
15
13
15
13
DIV2
DIV5
DIV2
DIV5
DIV2
DIV5
+
+
+
11
11
11
6
0
0
0
1
3
4
&
IC1.D
&
12
10
12
10
12
10
+
+
+
CT
CT
CT
12
13
9
9
9
11
2
2
2
S3
R7
5
S2
R
1
*
see text
+5V
zie tekst
*
voir texte
74121
R6
R8
S1:
1 = 100 ms
2 = 10 ms
3 = 1 ms
4 = 100 µs
5 = 10 µs
6 = 1 µs
*
*
siehe Text
*
3
4
2
5
1
6
S1.A
994092 - 11
64
Elektor Electronics
7-8/2001
SUMMER CIRCUITS
COLLECTION
‘pulled‘ slightly which is neces-
sary when calibrating the circuit.
IC1B buffers the oscillator from
the rest of the circuitry and R1
cleans up the square wave out-
put by reducing any overshoot
on the clock edges. The output
signal is connected to five cas-
caded decade counters type
74HC390 (IC2 to IC4A) each
counter divides its input fre-
quency by 10. Switch S1A
selects one of the frequencies or
time intervals from 1 MHz (1
X1
C12
C1
R5
H2
S1
C8
IC2
IC6
C11
C17
K2
C7
C16
+
C6
IC3
C5
0
C4
C18
D2
IC4
C3
H4
S2
S3
994092-1
F
2
S)
to 10 Hz (100 ms) to route it to a
pulse generator formed by IC5.
The second half of counter IC4
is used to provide a divide-by-
two function, this can be
bypassed by switch S2. In total
this gives 12 possible pulse rep-
etition rates from 1
S to 200 ms.
The output timing pulse is
generated by IC5. This is a stan-
dard TTL monostable type 74121. Standard TTL devices can
be interfaced directly with HC devices without any prob-
lem. The output pulse width of the monostable is a function
of the resistor/capacitor value at pins 10 and 11. As the rep-
etition rate is changed by switching the counter outputs with
S1A so the second half of the switch (S1B) also switches dif-
ferent R-C components to the monostable. This ensures that
the marker pulses shown on the oscilloscope screen will be
the correct width for each selected range. The output stage
of a standard TTL IC does not drive symmetrically so IC1D
is used as a buffer to give a better output performance.
Switch S3 allows the polarity of the output pulse to be
switched and resistor R7 provides short circuit protection
for the output buffer. Unfortunately in combination with the
capacitance of the output lead, this resistor also forms a low
pass filter that has the effect of rounding off the sharp edges
of the output signal. Socket K2 is used for connection of an
external 9 V mains unit to power the circuit and IC6 regu-
lates this to 5 V for use on board. Current consumption is
only a few milliamps so a heatsink is unnecessary.
Fitting the PCB into a case is greatly simplified by
mounting the single-sided PCB directly to the back of the
front panel switches.
Mounting the components on the board is begun by first
soldering the six wires bridges and the smaller compo-
nents to the board. It’s worth taking a little care here to
ensure that the polarised capacitors and diode are cor-
rectly fitted. This design will produce RF interference so it
is advisable to fit the unit inside a metal case or at least a
screened plastic case, the screen or case should be con-
nected to the power supply ground.
To test the circuit, first check that 5 V is available from
COMPONENTS LIST
C11,C13-C18 = 100nF
C12 = 47
F 16V radial
Resistors:
R1,R6 = 680
Ω
R2 = 5k
Semiconductors:
D1 = 1N4001
IC1 = 74HC00
IC2,IC3,IC4 = 74HC390
IC5 = 74121
IC6 = 7805
6
R3 = 10M
Ω
R5 = 6k
8
R7 = 220
Ω
R8 = *
Miscellaneous:
S1 = rotary switch, 2 poles,
⋅
6 contacts
X1 = 1MHz quartz crystal
S2,S3 = toggle
switch,1
Capacitors:
C1 = 100pF trimmer
C2,C4 = 33pF
C3 = 10 pF
C5 = 100pF
C6 = 1nF
C7 = 10nF
C8 = 22nF
C10 = 220
µ
change-over
contact
K2 = 2-way PCB terminal
block, lead pitch 5mm
F 25V radial
the power supply. Next, connect a frequency counter to
resistor R1 and adjust trimmer C1 until 1.000 MHz is
achieved. If there is insufficient adjustment in C1 then try a
different value for C2. If you do not have access to a fre-
quency counter then just set the trimmer to mid-position or
replace it with a 56 pF fixed capacitor.
The output of the calibrator can be connected to the
scope input channel via a short length of 50-
coax cable.
An output series resistor (R8) is used to dampen ringing on
the output pulses introduced by the cable capacitance. R8
can be fitted directly to the output BNC socket and its value
will be in the range of 220
.
The best output pulses will be produced by hooking the
tip of a 10x scope probe directly on the output pin of the
to 470
7-8/2001
Elektor Electronics
65
SUMMER CIRCUITS
COLLECTION
3
graticule. Use the horizontal position adjustment to place
the pulses exactly under the graticule lines Check carefully
that the pulses occur exactly at each graticule line inter-
section across the full width of the screen. This will not
always be the case with budget priced oscilloscopes!
If you have a two-channel scope it is also possible to use
the calibrator to perform quick and easy frequency mea-
surements so that in many cases you will not need a fre-
quency counter at all. First of all connect the signal to be
measured to channel A of the scope input and the calibra-
tor output to channel-B input. Adjust the scope timebase
generator so that one whole period of the unknown fre-
quency is displayed on the screen. With the scope trigger
mode set to ’alternating’ adjust the vertical positions of the
channels until they are superimposed and the edge of one
of the pulses coincides exactly with a point on the channel
A waveform (see (1) in
Figure 3
). Now to find the fre-
quency just count the number of pulses that occur until the
channel-A waveform has completed one complete period
(2). In the screen shot shown here there are 12.3 intervals of
1.0
BNC connector, most scope probes will be able to manage
this without any problem. A useful addition to the front
panel next to the BNC output would be a solder/test point
connected to the circuit earth. This provides a convenient
parking spot for the scope probe earth clip.
To check the horizontal timebase of an oscilloscope first
make sure that and variable time base controls are set to
the ‘calibrate‘ position then select a sweep speed so that
each output pulse corresponds to one square of the screen
s therefore the frequency is given by
f = 1/ 12.3
10
-6
s = 81.3008 kHz.
These are only two applications of this versatile circuit, no
doubt you will find many more.
(994092-1)
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