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BUILD THIS
LOW-CAPACITANCE
SCOPE PROBE
By HERB FRIEDMAN
What's that - your budget won't extend to a
low-capacitance x10 scope probe? You can
build this one using junk-box parts.
Even if you're measuring well
down into the bass audio frequencies, an oscilloscope's display of a
complex waveform - such as a
square wave - will not be accurate unless the input to the scope
is made through a low-capacitance
test probe.
In the June 1989 issue of SILICON
CHIP, we published an in-depth
discussion on the why's and
wherefore's of low-capacitance
oscilloscope probes so there's no
sense in repeating what's already
been said. Suffice to say that, as a
general rule, all AC scope
measurements should be made using a low-capacitance xl0 test
probe.
Also, as a general rule, you're
probably best off with a low-C xlO
scope probe that's specifically
designed for your scope. But what if
you can't afford to buy one? Simple!
You build it using readily available, budget-priced parts.
From input to output, the prototype low-C xlO probe shown here
can cost as little as $10. And by
building your own, you get the extra
advantage of precisely matching
your scope's vertical input connector: anything from a modern BNC
connector to a somewhat older
banana plug, to an ancient
microphone-type plug connector.
Whatever your scope needs, just
hang it on the end of the probe's
cable.
The circuit of a low-capacitance
c,
PROBE
,:.
[!J
. ..... T
~
PLUG
COAX
CABLE
=----e
1
CLIP~
LOW CAPACITANCE PROBE
FIG.I: ALTHOUGH A LOW-CAPACITANCE probe is a simple device,
it plays a vital role in obtaining accurate oscilloscope traces.
76
SILICON CHIP
FIG.2: A PLASTIC HANDLE, a metal
shield, a probe tip and a small piece
of perforated board are required to
make the scope probe. Matching
holes in the handle and metal shield
provide access to Cl.
FIG.3: THE RESISTOR SHOULD be
placed to the side of CL Do not run
the resistor across the top of the
capacitor.
FIG.4: THE POINTER INDICATES
where the wiring board's solder lug is
tack-soldered to the inside of the·
shield tube.
probe, shown in Fig.1 , is certainly
simple enough because it essentially consists of three components: Rl,
Cl and PL1. Rl can be any kind of
¼ W or ½ W resistor with a value
of about 9 megohms. The actual
value isn't all that critical; it can be
9.2Mfl or 8.8Mfl if you happen to
have one of these values in your
parts box.
Trimmer capacitor Cl can be
anything that physically fits inside
the handle. The unit specified in the
parts list is cheap and readily
available. As already mentioned,
PLl is whatever connector is need-
ed to match your scope's vertical
input.
The problem is, however, to
assemble Rl and Cl in a shielded
probe - one that does not pick up
extraneous hum and noise because
of capacitance-coupling to the
user's hand. On the other hand,
even though the probe is shielded, it
should be insulated from the user's
hand to ensure safety. So you will
need some form of plastic handle, a
probe tip for the handle, an internal
shield, a perforated wiring board,
and a solder lug that can be connected as a common ground.
Be sure to use perforated wiring
board (the stuff without the copper
strips). Don't use Veroboard or
stripboard as the copper strips
have too much stray capacitance.
Just where you scrounge all the
parts is up to you. The internal
shield, for example, could be easily
make up from a discarded tin can
and should be about 10mm in
diameter and about 80mm long. The
PARTS LIST
1 9Mfl resistor, see text
1 trimmer capacitor (Jaycar
Cat. RV-5706 or RV-5708 ,
DSE Cat. R-2930)
1 plug connector to match
scope input, see text
1 piece of pert board
1 metal shield, see text
1 plastic handle
1 small alligator clip
1 solder lug
1 probe tip assembly, see text
1 2-metre length of coaxial
cable, RG-58A/U or
RG-58C/U
FIG.5: YOU MUST CUT A NOTCH in
the plastic handle s6 that the ground
test lead from the shield tube can exit
from the front of the probe.
FIG:6: THE BNC CONNECTOR at the
top provides shielding all the way
into the scope. On the other hand, a
banana plug causes a break in the
shield which exposes the inner
conductor to possible hum and noise
pickup.
perforated wiring board should be
trimmed so that it slides neatly inside the shield.
Construction
Holes must be drilled in both the
handle and the shield to allow access to trimmer Cl. Take extreme
FIG.7: A FULLY SHIELDED CABLE will provide the rock-steady square wave display
shown in (a). A break in the display, such as caused by a banana plug, can cause
the trace smearing shown in (b).Closer examination of the smear in (c) shows a
60Hz (USA) mains noise signal superimposed on the desired signal.
AUGUST 1989
77
FIG.8: THE PARTS FOR THE CRO probe could be fitted into this commercial
probe case which is available from Jaycar for $12.95. Alternatively, a suitable
case could be fashioned from a short piece of plastic conduit.
FIG.9: THE TWO HALVES OF the Jaycar probe case simply clip together.
There's more than enough room to accommodate all the parts.
care when drilling the holes in the
handle and the shield because they
are prone to snag on the drill bit.
You must create some kind of safe
drilling jig when working with the
handle and the shield.
Fig.2 shows the probe-assembly
components after the handle and
the shield are drilled. A 6mm hole
should be drilled in the metal shield
about 8mm from one end and a matching hole drilled about 18mm from
the front of the plastic handle.
Fig.3 shows how the ground lug is
bolted to one end of the board. Bend
this lug upwards through 90° so
that the board can slide through the
metal shield. Position the lug so that
it is inside the tube and exactly
flush with one end. Mark the opposite end of the board at the end of
the metal tube and cut the board
exactly on the line.
Install Cl on the board so that it
is centred 8mm from the cut edge.
You will have to enlarge the ex78
SILICON CHIP
isting board holes so that the trimmer's lugs just barely squeeze
through.
Using Fig.3 as a guide, install a
PC pin on each side of Cl; then install Rl between the pins so that it
lies alongside Cl. Do not position Rl
across the top of Cl. Notice from
Fig.3 that' one of Rl 's leads at the
front PC pin is left at fu]l length.
This extra length will pass into the
probe's tip during final assembly.
Connect Cl to the two PC pins on
the bottom of the board using short,
direct lengths of wire.
The connecting cable is 1-2
metres of conventional coax. To
prevent constant flexing from snapping the centre conductor, use one
of the cable types having a stranded centre conductor, such as
RG-58A/U or RG-58C/U. Avoid
RG-59 cable because its centre conductor is solid.
The probe's ground is the solder
lug on the wiring board. Unbraid
the cable's shield, twist the strands
tightly into a pigtail and solder the
pigtail to the lug. This done, connect
the cable's centre conductor to the
nearest PC pin that supports Rl.
Finally, slip the shield tube over
the board until the back end is exactly flush with the solder lug.
Force or bend the lug against the inside of the shield, then rotate the
shield so that it's hole is exactly opposite Cl and tack solder the lug to
the inside of the shield. Fig.4 shows
the details.
Using a knife or a 3mm drill bit as
a router, cut a slot for the grounding test lead in the front of the
plastic handle, 180° opposite to the
6mm hole. This done, solder a 15cm
length of stranded insulated wire to
the front of the metal shield. Once
again, this should be diagonally opposite to the adjustment hole.
Now slide the assembly into the
handle so that the holes for Cl are
in alignment and the ground test
wire flows out the slot. Terminate
the ground lead with a small
alligator clip. The next step is to
assemble the probe tip.
It's up to the constructor to
devise a suitable probe assembly.
For example, you could use parts
from a discarded multimeter test
probe. The probe tip assembly
should be mounted on a plastic end
piece and the resistor lead soldered
directly to the probe tip. This end
piece can be secured to the handle
using small retaining screws.
Alternatively, you could house
the entire project in a ready-made
probe case which is currently
available from Jaycar (Cat. HB6400; $12.95).
FIG.to: ADJUST Cl USING an
insulated tool for optimum squarewave scope display.
banana jacks for the vertical input
connections then PLl must be a
banana plug.
Adjustment
(b)
(a)
FIG.11: IMPROPER ADJUSTMENT of Cl will produce the rounded leading edge
shown in (a) or overshoot of the leading edge as shown in (b).
Ideally, PLl should be a BNC connector but some scopes may require
the use of a banana plug connector.
Bear in mind that, as shown in
Fig.6, a BNC connector provides
full shielding all the way from the
probe to the scope's input, while a
banana plug actually breaks the
shield at the connector and will
allow some noise pickup.
For example, on a square wave
signal, a BNC connector gave a
rock-steady trace as shown in
Fig.7a. In Fig.7b, PLl was a banana
Garbage Reminder
ctd from page 52
To set the unit, all you need to do
is insert the batteries at the time
you want the LED to begin flashing
on your garbage collection day or
days. The LED will flash for 7 hours
and 12 minutes unless stopped using the BIN OUT switch. Normally
the reminder would be set to start
the LED flashing at about 5pm or
6pm.
It does not matter which day the
batteries are inserted since the
days are selected using the DIP
switches. Day 1 is the day that the
Reminder is set, day 2 the next day
and so on.
For example, if the Reminder has
its batteries inserted on Wednesday at 5pm, day 1 is Wednesday
and day 7 is Tuesday. If your garbage night is Sunday and Wednesday, the required settings are switches 1 and 5 on. All other switches
are off. Got it? Good.
Now get that garbage out!
~
plug that was connected to a BNCbanana adaptor. Notice that noise
(hum) pickup has caused the trace
to "smear" . Fig.7c is the same trace
as Fig.7b but a faster camera shutter was used to show that the
"smear" is actually another signal
(60Hz mains signal leaking through
the broken shield at the banana
plug) that is superimposed on the
square wave.
So if possible, maintain shielding
throughout the entire assembly. Of
course, if your scope only has
To adjust the probe, set the
scope's vertical input for DC. Then,
using any known good square wave
as the signal source - lkHz is
almost a universal standard - use
an insulated alignment tool to adjust Cl for the precise square wave
shown in Fig.7a. Improper adjustment of Cl will result in a rounding
of the signal's leading edge
(Fig. 11 a) or overshoot of the
signal's leading edge (Fig.llb).
Refer to the scope probe article
in the June issue to learn why Cl 's
adjustment does what it does.
Finally, be sure to allow for the
x 10 factor when using the low-C
xlO probe. For example, if the
scope indicates that a signal is 1.1
volts p-p, then the actual value is 11
volts p-p.
~
Copyright 1989, Gernsback Publications. Reprinted with permission from
January 1989 Radio-Electronics.
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AUGUST 1989
79
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