Poor Man's 1-GHz Active Probe
DIY saves a pretty penny
Published in issue 336, October 2004
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 for the circuit to drive, usually consisting of some resistance and stray capacitance, resulting in reduction in gain, or in extreme cases, causing instability. The loading effect of the resistance and stray capacitance can be largely removed by using an active probe. The major manufacturers in the oscilloscope market all offer suitable models (see also ‘Scope for Scopes’ elsewhere in this issue), but costing over a £1000 they are too expensive for amateur use. This article describes a probe that can be constructed at home, for very little money and has useful performance.
All resistors and capacitors: SMD, ‘0805’ case
Resistors:
R1 = 10MOhm
R2 = 4kOhm 7
R3 = 6kOhm 8
R4 = 47Ohm
Capacitors:
C1 = PCB capacitor
C2,C4 = 1nF
C3 = 100nF
C5,C6 = 470nF
Semiconductors:
D1 = 1A diode, SMD
T1 = BF998 in SOT143 case (see Table 2)
IC1 = 78L05 in SO-8 case
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Click below to download a PDF copy of this article from Elektor magazine. Please note. In view of the complexity of international markets, Elektor cannot guarantee the availability of components for this project.
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