Poor Man’s 1-GHz Active Probe: DIY Saves a Pretty Penny （1）

Published:2011/8/3 3:20:00 Author:Amy From:SeekIC

By David Jewsbury

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 behavior 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.

Specifications

This probe has some compromises in performance, as you would expect. In Table 1 it is compared to a commonly available commercial probe, the type 85024A from Agilent.

Admittedly the commercial probe, with 0 dB loss, is more convenient to work with, but for most applications a home­brew probe is no disadvantage.

Circuit Description

The circuit is shown in Figure 1. It is hard to imagine anything simpler.



A dual gate MOSFET, Tl, is used in a source-follower configuration. This provides a low output impedance to drive the coax cable and test equipment. The signal at the probe tip is applied to gate 1. The impedance at gate 1 is a very high resistance shunted by a few picofarads of capacitance. The choice of MOSFET used in the circuit is not critical, any one of the types listed in Table 2 and housed in a SOT143 case can be used with impunity. Be sure how­ever to steer clear of ’-R’ suffix devices because they have a different pinout and will not work on the proposed PCB.

Capacitor CI has a value of about 0.5 pF, and is made by patches of cop­per on each side of the board. The gain of the buffer itself is a little less than one, but because of the voltage divider action of CI and the input capacitance of Tl, the overall loss of the probe is approximately 20 dB, or the input voltage is divided by 10.

IC1 regulates the supply voltage to astable 5 volts. Dl protects the probe in the event of the supply leads being reversed.

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