Poor Man’s 1-GHz
David Jewsbury
DIY saves a
pretty penny
When it is required to make a measurement at a node of an
RF circuit, connecting to the circuit using a normal oscilloscope
probe, even on the x10 setting can change the behaviour of
the circuit. For those difficult cases, you need a special probe
like the one described here.
Any probe presents extra impedance _in this issue), but costing over a£1000 Admittedly the commercial probe, with
forthe circuit to drive, usually consist- they are too expensive for amateuruse. 0 dB loss, is more convenient to work
ing of some resistance and stray Thisarticle describes a probe that can _with, but for most applications a home-
‘capacitance, resulting in reduction in _be constructed at home, for very litle brew probe is no disadvantage.
gain, or in extreme cases, causing money and has usefl performance.
instal
z. Circuit Description
‘Tho loading effect of the resistance SPecifications The circuit is shown in Figure 1. Its
and stray capacitance can be largely This probe has some compromises in hard to imagine anything simpler
romoved by using an active probe. The performance, as you would expect. in _Adual gate MOSFET, Tt, is used in
major manufacturers in the oscilo: ‘Table itis compared toa commonly a source follower configuration, This
scope market all offer suitable models availabla commercial probe, the type provides a low output impodanco to
(eee also ‘Scope for Scopes‘ elsewhere @SO24A fom Agilent. dive the conx cable and test equip-
36 hon 4/2004