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4
Accessories
We have examined a variety of oscilloscopes in the previous
chapters, both simple and advanced. All are capable of examin-
ing waveforms as they stand: simply connect the circuit whose
waveform you wish to examine to the Y input and the
waveform will appear on the screen (assuming the controls are
suitably set).
Actually, it is not quite that simple. Although the Y input of an
oscilloscope has a very high input impedance, in many cases its
effect upon the circuit to which it is connected is not entirely
negligible. The standard Y input resistance is 1 M~ and the input
capacitance is usually in the range 15-40 pF depending upon the
particular make and model. With such a high input impedance,
hum pick-up on the input lead would often be a problem when
examining small signals in high impedance circuits unless a
could easily add another 50-100pF to the oscilloscope's input
capacitance; on the other hand, trying to connect the circuit
under test directly to the input connector of the scope with
negligible lead lengths is always tedious and often impossible. The
usual solution to this problem is a passive divider probe, and this
is the first accessory at which we shall look.
Passive divider probes
Experience shows that to connect an oscilloscope to a circuit
convenient, screened to avoid hum pick-up when working on
high-impedance circuits.
Even a low-capacitance cable has a capacitance of about 60 pF/
metre, so a metre of cable plus the input capacitance of the scope
would result in about 100 pF of input capacitance. The purpose of
a 10:1 passive divider probe is to reduce this effective input
capacitance to around 10 pF. This is a useful reduction, bearing in
mind that at even a modest frequency like 10 MHz, the reactance
of 100 pF is as low as 160~.
34 Oscilloscopes
Figure 4.1(a) and (b) show the circuit diagram of the
traditional type of scope probe, where Co represents the oscillo-
scope's input capacitance, its input resistance being the standard
value of 1 M~. The capacitance of the screened lead plus the
input capacitance of the scope form one section of a capacitive
probe
tip
earth
connect Ion ~
r
cc+Co ]
ICT= g I
I
I
I l
I '
I1 9M _~
.•/'jf__
_ _~u~ coax ial
/17- plug
(a)
CT
s
Z%~l
I
v
I
cable capacitance I
probe I scope
A
I R o
1M
typical
equivalent
~nput c,rcu~t
}
T
30p
(b)
r 1
I I
c T
I I
I I
IJ~ RA, J~ I
I 9M
L__
{c)
--~E-
l
Figure 4.1 (a) Circuit diagram of traditional 10"1 divider probe. (b) Equivalent
circuit of probe connected to oscilloscope. (c) Modified probe circuit with trimmer
capacitor at scope end (courtesy
Practical Wireless)

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