DIY active probe

DIY active probe
http://www.audiokarma.org/forums/showthread.php?t=216243
Poor Man’s 1-GHz probe
http://elektrotanya.com/files/forum/2009/10/e04a036.pdf

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

Construction The PCB artwork is shown in Fig­ure 2. The board has been designed to allow fitting in a metal tube. All the components are surface mounted (SMD), but assembly by hand is rea­sonably easy with a fine tipped sol­dering iron and tweezers. The parts are assembled on one side of a dou­ble sided 1.6-mm thick PCB. Connec­tions are made between the ground plane on each side of the board with solid wire soldered on each side. Fly­ing leads take the power to the probe and a length of coax ending with a BNC plug take the output to the test instrument. Heat shrink coax is used to strain relieve the leads. The RF and Ground probe are made from steel pins filed to a point. Pins ’borrowed’ from the family’s clothes repair kit are excellent.



Testing and use of the probe After connecting the probe leads to a suitable power supply, the probe should draw between 10 and 30 mA. If all is in order, connect the probe to a spectrum analyzer. Applying an RF signal to the probe should result in an output seen on the spectrum analyzer. To get accurate results it is important that the ground probe contacts an RF ground close to the probed point on the circuit. It is also important to hold the board by the edges to prevent stray effects from fingers on the circuit. If the imped­ance at the probed point is 50 £), then the peak on the spectrum analyzer should be about 20 dB less than the power at that point in the circuit. Commercial probes were notoriously sensitive to electrostatic discharge, but seem more robust these days. Although Tl has internal diodes to protect against ESD it is wise to take normal precautions against unwanted static, while using the probe, as for any sensitive electronics.

Downloads
	PDF Article (U041036.pdf)
	10 Elektor Credits
	PCB layout (040108-PCB.pdf)
	Free download

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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